Table of Contents
Introduction
Why Go in Production?
If you are reading this book, we assume you are interested in running Go in a production environment. Maybe you dabble in Go on your side projects, but are wondering how you can use it at work. Or perhaps you’ve read a company blog post about converting their codebase to Go, which now has 3 times less code and response times one tenth of what they were before. Your mileage will vary when it comes to gains in productivity and efficiency, but we have generally found a switch to Go to be more than worthwhile. Our goal in writing this book is to provide the knowledge to write a production-ready service in Go. This means not only writing the initial implementation, but also reliably deploying it, monitoring its performance, and iterating on improvements.
Go is a language that allows for fast iteration, which goes well with continuous deployment. Although it is a statically typed language, it compiles quickly and can often be used as a replacement for scripting languages like Python. Many users report that when writing Go, once a program works, it continues to “just work”. This is due to the relatively simple design of the language, and the focus on readability rather than clever constructs.
In one project, we replaced existing APIs in PHP with equivalent functionality in Go. We saw performance improvements, including an order of magnitude reduction in response times, which led to both higher user retention and a reduction in server costs. We also saw developer happiness increase, because the safety guarantees in Go meant we could deploy changes regularly and safely.
This book is not meant for beginner programmers. We expect our audience to be knowledgeable of basic computer science topics and software engineering practices. Over the years, we have helped ramp up countless engineers who had no prior experience writing Go. Ideally this will be the book that people recommend to engineers writing Go for the first time, and who want to better understand the “right way” to write Go.
We hope this book will help guide you on your journey to running Go in production. It will cover many important aspects of running a production system, including topics not covered by most books on the language, like profiling the memory usage of a Go program, deploying and monitoring apps written in Go, and writing tests for web applications.
Feel free to skip around to chapters that seem more relevant to your immediate concerns or interests. We will do our best to keep the chapters fairly independent of one another in order to make that possible.
Installing Go
Installation
The Go Downloads page contains binary distributions of Go for Windows, Apple macOS, and Linux.
You can also find instructions for installing Go from source on their Installing Go from source page. We recommend installing a binary release first, as there are extra steps necessary to install from source. You only need to install from source if you’re planning to contribute changes upstream to the Go language.
Also, if you use Homebrew on macOS, you can install with brew install go.
Once installed, test the command by running:
and you should see something like:
Writing Code
You can write Go code anywhere on your filesystem. Let’s assume that you have a folder in your home directory called ~/code. Let’s also assume that you use GitHub for hosting your source code. Make a new folder in ~/code called myapp, then run:
go mod init github.com/[yourusername]/myapp.
You should have a go.mod file now with your package name in it. Now make a main.go file with the following contents:
1 package main
2
3 import "fmt"
4
5 func main() {
6 fmt.Println("Hello, world")
7 }
and run it:
go run main.go
You should see the “Hello, world” output.
Next, let’s add a dependency with go get.
When you retrieve a package using the go get command, the source is downloaded into your $GOPATH/pkg/mod dir (note: if you haven’t set the $GOPATH environment variable explicitly, Go will use $HOME/go as your $GOPATH). A line for that import is also added to your go.mod file. For example, if you want to download the popular URL router library gorilla/mux, you can run:
The go get command downloads the pacakge’s source into your $GOPATH/pkg/mod dir (note: if you haven’t set the $GOPATH environment variable explicitly, Go will use $HOME/go as your $GOPATH). A line for that import is also added to your go.mod file. For example, if you want to download the popular URL router library gorilla/mux, run:
go get github.com/gorilla/mux
and Go will download the source into (assuming the latest version is 1.8.0) $GOPATH/pkg/github.com/gorilla/mux@v1.8.0. You can now import that library in your code:
1 package main
2
3 import (
4 "fmt"
5 "net/http"
6
7 "github.com/gorilla/mux"
8 )
9
10 func HomeHandler(w http.ResponseWriter, r *http.Request) {
11 fmt.Fprintf(w, "Hello, world")
12 }
13
14 func main() {
15 r := mux.NewRouter()
16 r.HandleFunc("/", HomeHandler)
17 http.Handle("/", r)
18 http.ListenAndServe(":8080", r)
19 }
and you can run go run main.go again and curl localhost:8080 should respond with Hello, world.
Your go.mod should now look like this:
1 module github.com/[yourusername]/myapp
2
3 go 1.15
4
5 require github.com/gorilla/mux v1.8.0
If you prefer to vendor your dependencies, you can run go mod vendor and Go will create a vendor/ directory for you with the appropriate dependencies. When the Go toolchain recognizes that you have a vendor/ directory it will choose to use that during compilation.
Editor Integrations
Since there are so many editors and IDEs out there, we will only briefly cover three common editors: Goland by JetBrains, vim and Sublime Text. We will link to relevant information for other popular editors at the end of the section.
For all editors, we recommend running goimports on save. Like the gofmt command, goimports formats your Go code, but it also adds or removes imports. If you reference mux.NewRouter in your code but have not yet imported github.com/gorilla/mux, goimports will find the package and import it. It also works for the standard library, which is especially useful for functions like fmt.Println. It is worthwhile to ensure your editor has the goimports plugin: it will speed up your Go development process.
Goimports can be installed with go get:
1 go get golang.org/x/tools/cmd/goimports
We now discuss some useful plugins for three common editors, in (arguably!) decreasing order of complexity.
GoLand
GoLand is a powerful and mature IDE for Go. It features code completion, the ability to jump to variable and type definitions, standard debugging tools like run-time inspection and setting break points, and more. A free 30-day trial can be downloaded from https://www.jetbrains.com/go/
Once installed, we recommend installing goimports on save. To do this, go to File -> Settings -> Tools -> File Watchers. Click the Plus (+) icon, and select “goimports”. Press OK. When you now save a Go file in the project, it should get formatted according to Go standards automatically.
Sublime Text
For Sublime Text 3, we recommend installing a package called GoSublime. Sublime Text package control can install this package. Install package control with the commands provided at https://packagecontrol.io/installation. To open package control, press Ctrl+Shift+P on Windows and Linux, or Cmd+Shift+P on OS X. Then type “install package” and select “Package control: install package”. Now type “GoSublime” and choose the matching option. Finally, open the GoSublime settings by going to Preferences -> Package Settings -> GoSublime -> Settings-User. Make sure that GOPATH matches the path you configured earlier. Here are some typical settings:
1 {
2 // you may set specific environment variables here
3 // e.g "env": { "PATH": "$HOME/go/bin:$PATH" }
4 // in values, $PATH and ${PATH} are replaced with
5 // the corresponding environment(PATH) variable, if it exists.
6 "env": {"GOPATH": "$HOME/Code/go", "PATH": "$GOPATH/bin:$PATH" },
7
8 "fmt_cmd": ["goimports"]
9 }
vim
For vim, you will want to install vim-go. Instructions can be found on that page for the various vim package managers.
Once you have installed vim-go, you can add the following line to your .vimrc file in order to run goimports on save:
let g:go_fmt_command="goimports"
(Optional) Linters and Correctness Tooling
Linters and other types of tooling can help you catch common mistakes in Go code more quickly. By tightening the feedback loop between writing code and finding a bug, having such tools part of your regular workflow will also accelerate your journey to becoming an experienced Go programmer.
To install a suite of linters and tools to run on your code in one simple step, we recommend golangci-lint. For installation instructions in your local environment, see https://golangci-lint.run/usage/install/#local-installation.
golangci-lint allows you to run a variety of linters and tooling and conveniently combines all of their output into a standard format. We’ve found the deadcode, ineffassign, staticcheck and misspell (disabled by default, enable with --enable=misspell) linters to be particularly useful.
To run the default set of linters, run:
$ golanci-lint run
You may also want to disable all but a specific linter, as in the following example where we run only errcheck:
1 golangci-lint run --disable-all -E errcheck
While you may not want to require all the linters to pass in your build, one tool we recommend requiring is go vet. Using golangci-lint, it can be run like this:
1 golangci-lint run --disable-all -E govet
Go vet is a tool concerned with code correctness. It will find problems in your code such as using the wrong string formatting verb in a call to Printf:
1 package main
2
3 import (
4 "fmt"
5 )
6
7 func main() {
8 fmt.Printf("%d", "test")
9 }
This code compiles and runs, but if you do actually run it you’ll see this:
(string=test)%
because %d is meant for printing integers, not strings. If you were to run go vet on the above code (or equivalently through golangci-lint run), you would see this warning:
"test" for printf verb %d of wrong type: string
exit status 1
Another common issue vet will catch for you is the use of printf verbs inside a call to Println:
1 package main
2
3 import (
4 "fmt"
5 )
6
7 func main() {
8 fmt.Println("%d", "test")
9 }
Again this will compile and run fine, but the output would be this:
test
Calling go vet on this code will tell you:
in Println call
exit status 1
Basics
This chapter gives a quick run-through of Go’s basic syntax, and the features that differentiate it from other languages. If you have never programmed in Go before, this chapter will give you the knowledge to start reading and writing simple Go programs. Even if you have programmed in Go before, we recommend that you still read this chapter. Through the examples, we will highlight common pitfalls in Go programs, and answer some questions that even experienced Go programmers might still have.
Program Structure
Go code is organized in packages containing one or more Go source files. When building an executable, we put our code into a package main with a single func main.
As mentioned in the Installation chapter, our Go code lives in GOPATH, which we’re saying is $HOME/go. Let’s say we want to write our first Go command, which randomly selects an item from a list and outputs it to stdout. We need to create a directory in our GOPATH, with a main.go file containing a package main and single main function:
In this directory we’ll add our main.go with the following code:
1 package main
2
3 import (
4 "fmt"
5 "math/rand"
6 "time"
7 )
8
9 func main() {
10 dinners := []string{
11 "tacos",
12 "pizza",
13 "ramen",
14 }
15 rand.Seed(time.Now().Unix())
16 fmt.Printf("We'll have %s for dinner tonight!\n", dinners[rand.Intn(len(dinn\
17 ers))])
18 }
We can run our code with go run main.go. The other option is to build our code as an executable and run that. To do that, we first run go build to make sure that the code compiles. Then we run go install:
We can also run these commands from outside of our GOPATH, like so:
If we want to expose this functionality in a package so we can reuse it in other places, we need to add this functionality to a package. Let’s say we want to return the name of the dinner, rather than the whole string, like “pizza”, “ramen”, etc. For convenience, let’s reuse the same github.com/prodgopher/dinner directory. Remove the main.go file and create a new dinner.go file that looks like this:
1 package dinner
2
3 import (
4 "math/rand"
5 "time"
6 )
7
8 func Choose() string {
9 dinners := []string{
10 "tacos",
11 "pizza",
12 "ramen",
13 }
14 rand.Seed(time.Now().Unix())
15
16 return dinners[rand.Intn(len(dinners))]
17 }
Now, somewhere outside of our dinner package directory (let’s use our home folder), we’ll invoke our new functionality in a file called main.go:
1 package main
2
3 import (
4 "fmt"
5
6 "github.com/prodgopher/dinner"
7 )
8
9 func main() {
10 fmt.Println(dinner.Choose())
11 }
Now we have a convenient package for randomly selecting what to eat for dinner.
Variables and Constants
There are multiple ways to declare variables in Go. The first way, declaring a var with a given type, can be done like so:
var x int
With this type of declaration, the variable will default to the type’s zero value, in this case 0.
Another way to declare a variable is like this:
var x = 1
Similar to the above method, but in this case we can declare the specific contents. The type is also implied.
Lastly, the short-hand variable declaration:
x := 1
This is probably the most common way, and the type is also implied like the above. Sometimes the var declaration method is used stylistically to indicate that the variable will be changed soon after the declaration. For example:
1 var found bool
2 for _, x := range entries {
3 if x.Name == "Gopher" {
4 found = true
5 }
6 }
One key difference between var and := declarations is that the shorthand version (:=) cannot be used outside of a function, only var can. This means that variables in the global scope must be declared using var. This is valid:
var to declare variable in global scope1 package main
2
3 import "fmt"
4
5 var a = 1
6
7 func main() {
8 fmt.Println(a)
9 }
but this will not compile:
1 package main
2
3 import "fmt"
4
5 a := 1 // this is invalid, use var instead
6
7 func main() {
8 fmt.Println(a)
9 }
Running the above, we get the following error:
The other subtle difference between var-declarations and shorthand-declarations occur when declaring multiple variables. The following code is valid,
1 var a, b = 0, 1 // declare some variables
2 b, c := 1, 2 // this is okay, because c is new
3 fmt.Println(a, b, c) // Outputs: 0, 1, 2
but this is not valid:
1 var a, b = 0, 1 // declare some variables
2 var b, c = 1, 2 // this is not okay, because b already exists
3 fmt.Println(a, b, c)
The second example, when placed into a program, fails to compile:
This is because the shorthand := may redeclare a variable if at least one of the variables to its left is new. The var declaration may not redeclare an existing variable.
Basic Data Types
Basic Types
Go supports the following basic types:
- bool
- string
- int8, int16, int32, int64, int
- uint8, uint16, uint32, uint64, uint
- float32, float64
- complex64, complex128
- byte (alias for uint8)
- rune (alias for int32)
Booleans
The bool type represents a boolean and is either true or false.
1 package main
2
3 import "fmt"
4
5 func main() {
6 a, b := true, false
7 c := a && b
8 d := a || b
9
10 fmt.Println("a:", a)
11 fmt.Println("b:", b)
12 fmt.Println("c:", c)
13 fmt.Println("d:", d)
14
15 // Output: a: true
16 // b: false
17 // c: false
18 // d: true
19 }
In the above example we first create a and b, and assign them the values true and false, respectively. c is assigned the value of the expression a && b. The && operator returns true when both a and b are true, so in this case c is false. The || operator returns true when either a or b are true, or both. We assign d the value of a || b, which evaluates to true.
Note that unlike some other languages, Go does not define true or false values for data types other than bool.
Strings
The string type represents a collection of characters. When defined in code, a string is a piece of text surrounded by double quotes ("). Let’s write a simple program using strings.
1 package main
2
3 import "fmt"
4
5 func main() {
6 sound := "meow"
7 sentence := "The cat says " + sound + "."
8 fmt.Println(sentence)
9 }
The example demonstrates that strings support the + operator for concatenation. The variable sentence contains a concatenation of the strings "The cat says ", “meow”, and ”.”. When we print it to the screen, we get The cat says meow.`
This only scratches the surface of strings in Go. We will discuss strings in more depth in the chapter on Strings.
Integers
The integer types can be divided into two classes, signed and unsigned.
Signed integers
The signed integer types are int8, int16, int32, int64, and int. Being signed, these types store both negative and positive values, but up to a maximum half the value of its uint counterpart. int8 uses 8 bits to store values between -128 and 127 (inclusive). int16 stores values in the range -32,768 to 32,767. int32 stores values in the range -2,147,483,648 to 2,147,483,647. int64 stores values in the range -263 to 263-1, which is to say, between -9,223,372,036,854,775,808 and 9,223,372,036,854,775,807.
Unlike the other signed integer types, the int type does not explicitly state its width. This is because it acts as an alias for either int32 or int64, depending on the architecture being compiled to. This means that it will perform optimally on either architecture, and it is the most commonly used integer type in Go code.
Go does not allow implicit type conversions. When converting between integer types, an explicit type cast is required. For example, see the following code:
1 package main
2
3 import (
4 "fmt"
5 )
6
7 func main() {
8 var i32 int32 = 100
9 var i64 int64 = 100
10
11 // This will result in a compile-time error:
12 fmt.Println(i32 + i64)
13 }
This results in a compile-time error:
1 $ go run type_mix.go
2 type_mix.go:12:18: invalid operation: i32 + i64 (mismatched types int32 and i\
3 nt64)
To fix the error, we can either use the same types from the start, or do a type cast. We will discuss type casts again later in this chapter, but here is how we might use a type cast to solve the problem:
1 package main
2
3 import (
4 "fmt"
5 )
6
7 func main() {
8 var i32 int32 = 100
9 var i64 int64 = 100
10
11 fmt.Println(int64(i32) + i64)
12 // Output: 200
13 }
A nice feature of the Go compiler is that it warns us if a constant value overflows the integer type it is being assigned to:
1 package main
2
3 import (
4 "fmt"
5 )
6
7 func main() {
8 var i int8 = 128
9 fmt.Println(i)
10 }
Here we try to assign a number that is one bigger than the maximum value of int8, and the compiler prevents us from doing so. This is neat, but as we will see in the next section on Unsigned integers, the compiler will not save us from calculations which result in over- or underflow.
Unsigned integers
Under unsigned integers, we have uint8, uint16, uint32, uint64, and uint. uint8 uses 8 bits to store a non-negative integer in the inclusive range 0 to 255. That is, between 0 and 28-1. Similarly, uint16 uses 16 bits and stores values in the inclusive range 0 to 65,535, uint32 uses 32 bits to store values from 0 to 4,294,967,295, and uint64 uses 64 bits to store values from 0 to 264-1. The uint type is an alias for either uint32 or uint64, depending on whether the code is being compiled for a 32-bit architecture or a 64-bit architecture.
uints are useful when the values to be stored are always positive. However, take special care before deciding to use uint. Go strictly enforces types, so uint requires a cast to be used with int. Built-in slice functions, like len, cap, and almost all library functions, return int. So using those functions with uint will require explicit type casts, which can be both inefficient and hard to read. Furthermore, underflow is a common enough problem with the uint type that it’s worth showing an example of how it can happen:
1 package main
2
3 import (
4 "fmt"
5 )
6
7 func main() {
8 var p, s uint32 = 0, 1
9 fmt.Printf("p - s = %d - %d = %d", p, s, p-s)
10 // Output: p - s = 0 - 1 = 4294967295
11 }
Running this, we get:
As this example illustrates, if we are not careful when subtracting from the uint type, we can run into underflow and get a large positive value instead of -1. Be aware of the limitations before choosing unsigned integer types.
Floating point numbers
For floating point numbers we have two types, float32 and float64. A float32 represents a 32-bit floating-point number as described in the IEEE-754 standard, and float64 represents a 64-bit floating-point number.
An integer can be converted to a floating-point number with a type conversion:
1 x := 15
2 y := float64(x)
This will be especially useful when using the math package, as the functions of that package typically work with float64 (for example, math.Mod and math.Abs both take float64).
Complex numbers
Complex numbers are expressed by the types complex64 and complex128. A complex64 number is a float32 with real and imaginary parts, and a complex128 is a float64 with real and imaginary parts. Creating a complex number is done like so:
1 x := complex(1.0, 2.0)
2 fmt.Println(x)
1 (1+2i)
There are built-in functions to extract the real and imaginary parts of a complex number:
1 x := complex(1.0, 2.0)
2 fmt.Println(real(x))
3 fmt.Println(imag(x))
1 1
2 2
You can express an imaginary literal by appending i to a decimal of float literal:
1 fmt.Println(1.3i)
1 (0+1.3i)
Structs
A struct is a collection of fields, which can be declared with the type keyword:
1 package main
2
3 import "fmt"
4
5 type Person struct {
6 Name string
7 Email string
8 }
9
10 func main() {
11 p := Person{"Robert", "robert@example.com"}
12
13 fmt.Println(p.Name)
14 }
Struct fields are accessed with a dot, so our above example should print:
Since structs are commonly used for reading and writing data formats such as JSON, there are struct tags which define how you would like your fields to be decoded or encoded in that data format. Here is an example of JSON struct tags:
1 package main
2
3 import (
4 "encoding/json"
5 "fmt"
6 "log"
7 )
8
9 type Person struct {
10 Name string `json:"name"`
11 Email string `json:"email"`
12 }
13
14 func main() {
15 p := Person{"Robert", "robert@example.com"}
16 b, err := json.Marshal(p)
17 if err != nil {
18 log.Fatal(err)
19 }
20
21 fmt.Println(string(b))
22 }
Running this code will output:
If we didn’t have the struct tags, then we would have:
since it isn’t very common to see the first letter of a field capitalized like that in JSON, we use the struct tags to define how we want our struct fields to be encoded.
One important note about struct field names and JSON: field names must be exported (first letter of field name must be capitalized) in order for encoding to JSON to work. If our struct field names were name and email, we would get an empty JSON object when marshalling.
Golang also supports anonymous structs, which can be commonly found in table-driven tests for example. You can see some examples in our Testing chapter, but here is a quick (not testing-related) example to show how it works:
1 package main
2
3 import (
4 "encoding/json"
5 "fmt"
6 "log"
7 )
8
9 func main() {
10 p := struct {
11 Name string `json:"name"`
12 Email string `json:"email"`
13 }{
14 Name: "Robert",
15 Email: "robert@example.com",
16 }
17 b, err := json.Marshal(p)
18 if err != nil {
19 log.Fatal(err)
20 }
21
22 fmt.Println(string(b))
23 }
This will print the same as our “Struct tags example” example above.
Operators
Operators in Go are very similar to other languages in the C-family. They can be divided into five broad categories: arithmetic operators, comparison operators, logical operators, address operators and the receive operator.
Arithmetic Operators
Arithmetic operators apply to numeric values. From the Go specification:
Arithmetic operators apply to numeric values and yield a result of the same type as the first operand. The four standard arithmetic operators (+, -, *, /) apply to integer, floating-point, and complex types; + also applies to strings. The bitwise logical and shift operators apply to integers only.
The following table summarizes the different arithmetic operators and when they may be applied:
Comparison Operators
Comparison operators compare two operands and yield a boolean value. The comparison operators are:
In any comparison, the first operand must be assignable to the type of the second operand, or vice versa. Go is strict about types, and it is invalid to use a comparison operator on two types that are not comparable. For example, this is valid:
var x int = 1
var y int = 2
// eq will be false
var eq bool = x == y
but this is not valid, and will result in a compile-time type error:
var x int = 1
var y int32 = 2
// error: mismatched types int and int32
var eq bool = x == y
Logical Operators
Logical operators apply to boolean values and yield a boolean result.
As in C, Java, and JavaScript, the right operand of && and || is evaluated conditionally.
Address Operators
Go has two address operators: the address operation, &, and the pointer indirection, *.
&x returns a pointer to x. The pointer will be a pointer of the same type as x. A run-time panic will occur if the address operation is applied to an x that is not addressable.
1 var x int
2 var y *int = &x
3
4 // Print the memory address of x,
5 // e.g. 0x10410020
6 fmt.Println(y)
When x is a pointer, *x denotes the variable pointed to by x. If x is nil, an attempt to evaluate *x will cause a run-time panic.
1 var x *int = nil
2 *x // causes a run-time panic
The Receive Operator
The receive operator, <- is a special operator used to receive data from a channel. For more details on this, see channels.
Conditional Statements
We’ve seen some simple if statements in previous sections’ code snippets. Here we’ll cover some other common uses of conditional statements in Go.
An if can contain a variable declaration before moving on to the condition. This can often be seen in tests, like in this example from the Go source code (bytes/reader_test.go):
1 if got := string(buf); got != want {
2 t.Errorf("ReadAll: got %q, want %q", got, want)
3 }
Variables declared in the condition are restricted to the scope of the if statement - meaning that in the example above, we cannot access the got variable outside of the if statement’s scope.
An else statement is done as follows (this example is taken from Go’s time package, in time/format.go):
1 if hour >= 12 {
2 b = append(b, "PM"...)
3 } else {
4 b = append(b, "AM"...)
5 }
It is also quite common to see switch statements used in lieu of if/else statements. Here is an example from Go’s source code (net/url/url.go), of a switch statement:
1 func ishex(c byte) bool {
2 switch {
3 case '0' <= c && c <= '9':
4 return true
5 case 'a' <= c && c <= 'f':
6 return true
7 case 'A' <= c && c <= 'F':
8 return true
9 }
10 return false
11 }
This switch statement has no condition, meaning it is functionally equivalent to switch true.
A switch with a condition looks like this (taken from Go’s fmt/print.go):
1 func (p *pp) fmtBool(v bool, verb rune) {
2 switch verb {
3 case 't', 'v':
4 p.fmt.fmt_boolean(v)
5 default:
6 p.badVerb(verb)
7 }
8 }
and we can also declare a variable in the condition and switch on that:
1 switch err := err.(type) {
2 case NotFoundError:
3 ...
4 }
Arrays
An array of a specific length can be declared like so:
1 var x [3]int
In Go, however, arrays are rarely used unless you have a specific need for them. Slices are more common, which we’ll cover in the next section.
Slices
While arrays have a fixed size, slices are dynamic. To create a slice of integers for example, we can do:
1 x := []int{1, 2, 3, 4, 5}
Slices are abstractions on top of arrays. A slice contains a pointer to an array, its length, and its capacity. We can get the length and capacity of a slice with the built-in len and cap functions, respectively. We’ll call “slicing” the act of creating a new slice which points to a range of contiguous elements in the original array. We can “slice” arrays as well as slices - in which case the new slice will point to the underlying array. For example:
1 x := []int{1, 2, 3, 4, 5}
2 y := x[0:3]
3 fmt.Println(y)
will give us:
We can also leave out the 0:
1 y := x[:3]
and the result will be the same. Likewise for the high bound, we can leave that out and it will default to the length of the slice:
1 x := []int{1, 2, 3, 4, 5}
2 y := x[3:]
3 fmt.Println(y)
and we get:
A slice’s zero value is nil:
1 var x []int
2 fmt.Println(x == nil)
To append to a slice, we use the builtin append function:
1 x := []int{}
2 x = append(x, 1)
3 x = append(x, 2, 3)
4 y := []int{4, 5}
5 x = append(x, y...)
6 fmt.Println(x)
Notice the y... on line 5: append is a variadic function. The first argument is a slice, and the second is 0 or more arguments of the same type as the slice’s values. Running the above code will give us the following output:
Use copy to copy the contents of a slice:
1 x := []int{1, 2, 3}
2 y := make([]int, 3)
3 copy(y, x)
4 fmt.Println(x, y)
Note that we’re using make to create the slice y, with a size argument of 3. This is to ensure that y has enough capacity to copy x into it. If we had used an empty y with 0 capacity, for example, our y would have remained empty:
1 x := []int{1, 2, 3}
2 y := []int{}
3 fmt.Println("y capacity:", cap(y))
4 copy(y, x)
5 fmt.Println(x, y)
We can sort a slice with the sort.Slice function. All we have to do is provide the slice and a less argument which serves as a comparator function in which we define how one element in the slice is considered “less” than another when sorting:
1 package main
2
3 import (
4 "fmt"
5 "sort"
6 )
7
8 type Country struct {
9 Name string
10 Population int
11 }
12
13 func main() {
14 c := []Country{
15 {"South Africa", 55910000},
16 {"United States", 323100000},
17 {"Japan", 127000000},
18 {"England", 53010000},
19 }
20
21 sort.Slice(c,
22 func(i, j int) bool { return c[i].Name < c[j].Name })
23 fmt.Println("Countries by name:", c)
24
25 sort.Slice(c,
26 func(i, j int) bool { return c[i].Population < c[j].Population })
27 fmt.Println("Countries by population:", c)
28 }
Running this, our output should be:
Maps
Maps are a necessary and versatile data type in any programming language, including Go. Here we’ll go over some ways to use maps, and cover some idiosyncrasies in their usage.
First, as we know from earlier, there are a couple of ways to instantiate variables in Go, and this goes for maps as well. Let’s look at some of them:
1 var m = map[string]string{}
2 m := make(map[string]string)
3 m := map[string]string{}
The var declaration could be used in the top-level (or “package block”) scope:
1 package main
2
3 var m = map[string]string{}
4
5 func main() {
6 }
But otherwise these are all basically functionally equivalent.
If you’re familiar with maps in other programming languages, you can probably pick up on using maps in Go pretty quickly. Here is an example that is self-explanatory:
1 package main
2
3 import "fmt"
4
5 func main() {
6 m := make(map[string]string)
7 m["en"] = "Hello"
8 m["ja"] = "こんにちは"
9 // loop over keys and values of map
10 for k, v := range m {
11 fmt.Printf("%q => %q\n", k, v)
12 }
13
14 // nonexistent key returns zero value
15 fmt.Printf("zh: %q\n", m["zh"])
16
17 // check for existence with _, ok := m[Key]
18 if _, ok := m["ja"]; ok {
19 fmt.Printf("key %q exists in map\n", "ja")
20 }
21
22 delete(m, "en")
23 fmt.Printf("m length: %d\n", len(m))
24 fmt.Println(m)
25 }
Running this code, we should get this output:
The first output is from the loop, then we check a nonexistent key “zh”, then check for the existence of “ja”, print the length of the map, then print the map itself.
Another important note is that the map type is not safe for concurrent use. If you need to use a map in a concurrent way, take a look at sync.Map.
Also, the iteration order of maps is not guaranteed, so you can’t rely on any specific order when looping over your map.
Lastly, the following will make a nil map, which will panic when writing to it:
var m map[string]string
So avoid using this declaration style when making maps.
For further reading, although it is slightly outdated as it doesn’t mention sync.Map, check Go maps in action on the Go blog.
Loops
Loops in Go are done with the for construct. There is no while in Go, but you can achieve the same effect with for:
1 package main
2
3 import (
4 "fmt"
5 )
6
7 func main() {
8 x := 1
9 for x < 4 {
10 fmt.Println(x)
11 x++
12 }
13 }
The above code outputs:
A more traditional version that you may be familiar with is also available:
1 package main
2
3 import (
4 "fmt"
5 )
6
7 func main() {
8 for i := 0; i < 3; i++ {
9 fmt.Println(i)
10 }
11 }
This will output:
An infinite loop can be expressed with an empty for:
1 package main
2
3 func main() {
4 for {
5 }
6 }
To loop over a slice, we can do the following, where i is the index and x is the value at that index:
1 package main
2
3 import "fmt"
4
5 func main() {
6 nums := []int{1, 2, 3}
7 for i, x := range nums {
8 fmt.Printf("nums[%d] => %d\n", i, x)
9 }
10 }
The above code will output:
If we don’t need the index, we can leave it out with _:
1 package main
2
3 import "fmt"
4
5 func main() {
6 nums := []int{1, 2, 3}
7 for _, x := range nums {
8 fmt.Println(x)
9 }
10 }
and this will output:
We can also range over the keys and values of a map like so:
1 package main
2
3 import "fmt"
4
5 func main() {
6 m := map[int]string{
7 1: "一",
8 2: "二",
9 3: "三",
10 }
11 for k, v := range m {
12 fmt.Printf("%d => %q\n", k, v)
13 }
14 }
This will output:
Functions
Functions are declared with the func keyword. They can take zero or more arguments, and can return multiple results.
Functions are first-class citizens, and can be stored in variables or used as values to other functions.
Exported Names
In a Go package, a name is exported if it starts with an uppercase letter. Take the following code for example:
1 package countries
2
3 import (
4 "math/rand"
5 "time"
6 )
7
8 type Country struct {
9 Name string
10 Population int
11 }
12
13 var data = []Country{
14 {"South Africa", 55910000},
15 {"United States", 323100000},
16 {"Japan", 127000000},
17 {"England", 53010000},
18 }
19
20 // Random returns a random country
21 func Random() Country {
22 rand.Seed(time.Now().Unix())
23
24 return data[rand.Intn(len(data))]
25 }
When importing this package, you would be able to access the countries.Country type, as well as countries.Random() function, but you cannot access the countries.data variable because it begins with a lowercase letter.
Pointers
Declaring a variable with * before the type indicates a pointer:
var p *int
The zero-value of this is nil. To generate a pointer to an existing variable, use &:
x := 100
p = &x
Now we can dereference the pointer with *:
fmt.Println(*p)
and this results in:
100
Lastly, there is no pointer arithmetic in Go. The authors decided to leave it out for reasons such as safety, and simplifying the implementation of the garbage collector.1
Goroutines
Goroutines are functions that run concurrently in a Go program. They are lightweight, and cheap to use. Prefixing a function with the go keyword will run the function in a new goroutine:
1 package main
2
3 import (
4 "fmt"
5 "time"
6 )
7
8 func hello() {
9 fmt.Println("hello from a goroutine!")
10 }
11
12 func main() {
13 go hello()
14 time.Sleep(1 * time.Second)
15 }
(Note that we have a call to time.Sleep in the main function. This is to prevent the main from returning before our goroutine completes. We’re using a sleep to show a small example of a goroutine; it is not a valid way of managing goroutines.)
It’s also common to see a goroutine used with an anonymous function:
1 go func() {
2 fmt.Println("hello from a goroutine!")
3 }()
One way to synchronize goroutines is to use sync.WaitGroup:
1 package main
2
3 import (
4 "fmt"
5 "sync"
6 )
7
8 func main() {
9 var wg sync.WaitGroup
10 wg.Add(3)
11 go func() {
12 defer wg.Done()
13 fmt.Println("goroutine 1")
14 }()
15 go func() {
16 defer wg.Done()
17 fmt.Println("goroutine 2")
18 }()
19 go func() {
20 defer wg.Done()
21 fmt.Println("goroutine 3")
22 }()
23
24 wg.Wait()
25 }
In this example, we instantiate a sync.WaitGroup and add 1 to it for each goroutine. The goroutines then call defer wg.Done() to signify that they’ve finished. We then wait for the goroutines with wg.Wait().
When using sync.WaitGroup, we must know the exact number of goroutines ahead of time. If we had called wg.Add(2) instead of wg.Add(3), then we would risk returning before all of the goroutines finished. If we had called wg.Add(4), the code would have panicked with the following error:
fatal error: all goroutines are asleep - deadlock!
Another way to manage goroutines is with channels, which we’ll discuss in the next section.
Channels
Channels are used for sending and receiving values of a specific type. It is common to see them used inside of goroutines. Channels must be created with their specific type before use:
1 ch := make(chan int)
We can create a buffered channel like so:
1 ch := make(chan int, 5)
This means that sending to the channel will block when the buffer is full. When the buffer is empty, receives will block.
To send to a channel, we use the <- operator:
1 ch <- 5
And to receive a value from a channel, we do:
1 v := <-ch
Let’s see what happens when we send too many integers to a buffered channel of ints:
1 package main
2
3 func main() {
4 ch := make(chan int, 5)
5
6 ch <- 1
7 ch <- 2
8 ch <- 3
9 ch <- 4
10 ch <- 5
11 ch <- 6
12 }
We get an error - all goroutines are asleep - deadlock!. What if we were to read one int off of the channel before sending the final 6?
We’re no longer overfilling the buffered channel, because we read one int off of it before sending a sixth item.
What if we want to know whether a buffered channel is full before sending to it? There are a couple of ways we can do this.
One way would be to check the len of the channel before sending to it again:
1 package main
2
3 import "fmt"
4
5 func main() {
6 ch := make(chan int, 5)
7
8 ch <- 1
9 ch <- 2
10 ch <- 3
11 ch <- 4
12 ch <- 5
13
14 fmt.Println("channel length:", len(ch))
15 switch {
16 case len(ch) >= 5:
17 <-ch
18 case len(ch) < 5:
19 ch <- 6
20 }
21 }
We could also use a select statement with a default that does nothing when the channel is full:
1 package main
2
3 import "fmt"
4
5 func main() {
6 ch := make(chan int, 5)
7
8 ch <- 1
9 ch <- 2
10 ch <- 3
11 ch <- 4
12 ch <- 5
13
14 select {
15 case ch <- 6:
16 default:
17 fmt.Println("channel is full, ignoring send")
18 }
19 }
Interfaces
An interface in Go is a set of methods that another type can define in order to implement that interface. We define an interface type with the interface keyword:
1 type Entry interface {
2 Title() string
3 }
Now any concrete type we define that implements a Title() string method will implement the Entry interface:
1 package main
2
3 import (
4 "fmt"
5 "time"
6 )
7
8 type Entry interface {
9 Title() string
10 }
11
12 type Book struct {
13 Name string
14 Author string
15 Published time.Time
16 }
17
18 func (b Book) Title() string {
19 return fmt.Sprintf("%s by %s (%s)", b.Name, b.Author, b.Published.Format("Ja\
20 n 2006"))
21 }
22
23 type Movie struct {
24 Name string
25 Director string
26 Year int
27 }
28
29 func (m Movie) Title() string {
30 return fmt.Sprintf("%s (%d)", m.Name, m.Year)
31 }
32
33 func Display(e Entry) string {
34 return e.Title()
35 }
36
37 func main() {
38 b := Book{Name: "John Adams", Author: "David McCullough", Published: time.Da\
39 te(2001, time.May, 22, 0, 0, 0, 0, time.UTC)}
40 m := Movie{Name: "The Godfather", Director: "Francis Ford Coppola", Year: 19\
41 72}
42 fmt.Println(Display(b))
43 fmt.Println(Display(m))
44 }
Note that the Display function takes e Entry, not a concrete type like Book or Movie. Our concrete types implement the Entry interface, so we’re now allowed to pass implementations of those types into any function that takes an Entry.
The empty interface
We define an empty interface as interface{}, and it can hold a value of any type:
1 var i interface{}
2 i = "こんにちは"
3 fmt.Println(i)
1 こんにちは
If we want to test whether an interface is a certain type, we use a type assertion:
1 var i interface{}
2 i = "こんにちは"
3 s, ok := i.(string)
4 if !ok {
5 fmt.Println("s is not type string")
6 }
7 fmt.Println(s)
1 こんにちは
In our example above, i is a type string, so the second return value from our type assertion is true, and s contains the underlying value. If i had been another type, such as an int, then our ok would have been false and our s would have been the zero value of the type we were trying to assert, or in other words 0.
Nil interfaces
An interface in Go is essentially a tuple containing the underlying type and value. For an interface to be considered nil, both the type and value must be nil. Here is an example:
1 package main
2
3 import (
4 "fmt"
5 )
6
7 func main() {
8 var i interface{}
9 fmt.Println(i == nil)
10 fmt.Printf("%T, %v\n", i, i)
11
12 var s *string
13 fmt.Println("s == nil:", s == nil)
14
15 i = s
16 fmt.Println("i == nil:", i == nil)
17 fmt.Printf("%T, %v\n", i, i)
18
19 }
Note how our s variable is nil, but when we set our interface i to s then check if i is nil, i is not considered nil. This is because our interface has an underlying concrete type, and interfaces are only nil when both the concrete type and the value are nil.
Error Handling
Functions in Go often return an error value as the final return argument. When the function does not encounter any error conditions, we return nil. The error type is a builtin interface type that we can create with functions like errors.New and fmt.Errorf. As an example, let’s make a function that parses a string and returns the boolean value that string represents. This function is inspired by the ParseBool function in the Go standard library’s strconv package:
1 package strconv
2
3 import "fmt"
4
5 func ParseBool(str string) (bool, error) {
6 switch str {
7 case "1", "t", "T", "true", "TRUE", "True":
8 return true, nil
9 case "0", "f", "F", "false", "FALSE", "False":
10 return false, nil
11 }
12 return false, fmt.Errorf("invalid input %q", str)
13 }
Here we are returning a nil error when we’re able to parse the input properly, and using fmt.Errorf to return an error in the case that we cannot parse the input.
We’ll cover more about error handling in the “Style and Error Handling” chapter that follows this one.
Reading Input
You can use a bufio.Scanner to read input from stdin, which by default will split the input line by line. Here is an example of a Go program that reads a file containing one word per line, and keeps a count of every occurrence of each word:
1 package main
2
3 import (
4 "bufio"
5 "log"
6 "os"
7 )
8
9 func main() {
10 m := map[string]int{}
11 scanner := bufio.NewScanner(os.Stdin)
12 for scanner.Scan() {
13 m[scanner.Text()]++
14 }
15 if err := scanner.Err(); err != nil {
16 log.Printf("ERROR: could not read stdin: %s", err)
17 }
18 for k, v := range m {
19 log.Printf("%s => %d", k, v)
20 }
21 }
If we had a file that looked like this:
and we piped it into our program like so:
we would see output like the following (order is not guaranteed when iterating over maps, so the order of our output might change when running more than once):
Writing Output
One way of writing output to a file in Go is to use the *File returned from os.Create. os.Create will create a file for reading and writing. If the file already exists, it will be truncated:
1 package main
2
3 import (
4 "log"
5 "os"
6 )
7
8 func main() {
9 langs := []string{"python", "php", "go"}
10 f, err := os.Create("langs.txt")
11 if err != nil {
12 log.Fatal(err)
13 }
14 defer f.Close()
15 for _, lang := range langs {
16 f.WriteString(lang + "\n")
17 }
18 }
Running this code will give us the following content in a file called langs.txt:
Another utility function we could use is ioutil.WriteFile which will open the file and write our data in one function call.
Style and Error Handling
There are two quite major points about Go that take some getting used to when ramping up on learning the language: style and error handling. We’ll first talk about what is considered “idiomatic” style in the Go community.
Style
Gofmt
Go comes with a tool that formats Go programs called “gofmt”. Gofmt formats your program, and is prominent in the Go community. It would not be a stretch to say that every popular (let’s say > 500 stars on GitHub) open source library uses it. When using gofmt, you are not allowed to add exceptions like you can with tools such as PEP8 for Python. Your lines can be longer than 80 chars without warning. You can of course split your long lines up, and gofmt will format them accordingly. You cannot tell gofmt to use spaces instead of tabs.
You might find such strict formatting (maybe you hate tabs) backwards and annoying, but gofmt likely played a big role in Go’s success. Since everyone’s code looks similar, it takes an element of surprise out of looking at others’ code when debugging or trying to understand it. This made it easier to contribute to the standard library and open source libraries, in turn speeding up the growth of the Go community.
To show a very simple example of gofmt in action, here is some code that hasn’t been run through gofmt:
1 package main
2
3 import "fmt"
4
5 func main(){
6 a := "foo"
7 someVar := "bar"
8
9 fmt.Println(a, someVar)
10 }
This is a valid program and it will run, but it should be run through gofmt to look like this:
1 package main
2
3 import "fmt"
4
5 func main() {
6 a := "foo"
7 someVar := "bar"
8
9 fmt.Println(a, someVar)
10 }
While you may not agree with all of the rules of gofmt, it is so widely used within the community that it has become a requirement. You should always gofmt your Go code.
Many editors have integrations that allow you to run gofmt on save. We recommend running goimports on save. Goimports, according to its godoc, “updates your Go import lines, adding missing ones and removing unreferenced ones. In addition to fixing imports, goimports also formats your code in the same style as gofmt so it can be used as a replacement for your editor’s gofmt-on-save hook.”2
Also, as a bonus, use gofmt -s to automatically simplify some of your code:
1 package main
2
3 type Animal struct {
4 Name string
5 Species string
6 }
7
8 func main() {
9 animals := []Animal{
10 Animal{"Henry", "cat"},
11 Animal{"Charles", "dog"},
12 }
13 }
After gofmt -s, this becomes:
1 package main
2
3 type Animal struct {
4 Name string
5 Species string
6 }
7
8 func main() {
9 animals := []Animal{
10 {"Henry", "cat"},
11 {"Charles", "dog"},
12 }
13 }
Note how the extra Animal struct names are unnecessary in the slice []Animal, and are therefore removed. gofmt -s makes stylistic changes and does not affect the logic of the code at all, meaning it is safe to run gofmt -s all the time.
Short Variable Names
Another contentious topic in the earlier days of Go was the use of short variable names. If you look at the Go source code, you’ll see a lot of code that looks like this:
1 func Unmarshal(data []byte, v interface{}) error {
2 // Check for well-formedness.
3 // Avoids filling out half a data structure
4 // before discovering a JSON syntax error.
5 var d decodeState
6 err := checkValid(data, &d.scan)
7 if err != nil {
8 return err
9 }
10
11 d.init(data)
12 return d.unmarshal(v)
13 }
You might be thinking, “why use such a short and useless variable name like d? It doesn’t tell me anything about what the variable is holding.” It’s a fair point, especially considering that for years we’ve been told that having descriptive variable names is very important. But the authors of Go had something else in mind, and many people have come to embrace short variable names. From a page containing advice on reviewing Go code:
“Variable names in Go should be short rather than long. This is especially true for local variables with limited scope. Prefer c to lineCount. Prefer i to sliceIndex.”
Shorter variable names make control flow easier to follow, and allow the reader of the code to focus on the important logic, such as function calls. A general rule of thumb is, if a variable spans less than 10 lines, use a single character. If it spans more, use a descriptive name. At the same time, try to minimize variable span, and functions shorter than 15 lines. Most of the time, this produces readable, idiomatic Go code.
Golint
golint is a linter for Go, and it differs from gofmt in that it prints style mistakes, whereas gofmt reformats your code. To install golint, run:
go get -u github.com/golang/lint/golint
As with gofmt, you can’t tell golint to ignore certain errors. However, golint is not meant to be used as a standard, and will sometimes have false positives and false negatives. On Go Report Card we’ve noticed a lot of repositories with golint errors like the following:
Line 29: warning: exported type Entry should have comment or be unexported (golint)
This is just suggesting that an exported type should have a comment, otherwise it should be unexported. This is nice for godoc, which displays the type’s comment right below it. You might also see a warning like this:
Line 5: warning: if block ends with a return statement, so drop this else and outdent its block (golint)
Here’s a piece of code where that warning would show up when running golint:
1 func truncate(s, suf string, l int) string {
2 if len(s) < l {
3 return s
4 } else {
5 return s[:l] + suf
6 }
7 }
What golint is saying here is that because we return on line 3, there’s no need for the else on the following line. Thus our code can become:
1 func truncate(s, suf string, l int) string {
2 if len(s) < l {
3 return s
4 }
5 return s[:l] + suf
6 }
Which is a bit smaller and easier to read.
That’s all we’re going to cover on golint - we do suggest using it because it can show you ways to make your code simpler as well as more suitable for godoc. There’s no need to fix all of its warnings though, if you think it’s too noisy.
Error Handling
Error handling may take some getting used to when learning Go. In Go, your functions will normally return whatever values you want to return, as well as an optional error value. To give a simple example:
1 // lineCount returns the number of lines in a given file
2 func lineCount(filepath string) (int, error) {
3 out, err := exec.Command("wc", "-l", filepath).Output()
4 if err != nil {
5 return 0, err
6 }
7 // wc output is like: 999 filename.go
8 count, err := strconv.Atoi(strings.Fields(string(out))[0])
9 if err != nil {
10 return 0, err
11 }
12
13 return count, nil
14 }
Note the multiple if err != nil checks. These are very common in idiomatic Go code, and sometimes people who are new to Go have trouble adjusting to having to write them all the time. You may see it as unnecessary code duplication. Why not have try/except like other languages?
We had similar thoughts when first starting out with Go, but warmed up to the error checking. When you’re that strict about returning and checking errors, it’s hard to miss where and why an error is happening.
We could even go ahead and make those errors more specific:
1 // lineCount returns the number of lines in a given file
2 func lineCount(filepath string) (int, error) {
3 out, err := exec.Command("wc", "-l", filepath).Output()
4 if err != nil {
5 return 0, fmt.Errorf("could not run wc -l: %s", err)
6 }
7 // wc output is like: 999 filename.go
8 count, err := strconv.Atoi(strings.Fields(string(out))[0])
9 if err != nil {
10 return 0, fmt.Errorf("could not convert wc -l output to integer: %s", err)
11 }
12
13 return count, nil
14 }
Just make sure not to capitalize the error string unless beginning with proper nouns or acronyms, because the error will be logged in the caller with something like this:
1 filepath := "/home/gopher/somefile.txt"
2 lines, err := lineCount(filepath)
3 if err != nil {
4 log.Printf("ERROR: lineCount(%q): %s", filepath, err)
5 }
and the error line will flow better without a capital letter appearing in the middle of the log line:
2017/09/21 03:57:55 ERROR: lineCount("/home/gopher/somefile.txt"): Could not run wc -l
vs.
2017/09/21 03:57:55 ERROR: lineCount("/home/gopher/somefile.txt"): could not run wc -l
Wrapping Up
Compared to most languages, Go is very opinionated about proper style. It can take getting used to, but the advantage is that Go projects all follow the same style. This reduces mental overhead and lets you focus on the logic of the program. For more examples of idiomatic Go code, we recommend reading the Go source code itself. One way to do this is to browse Go standard library’s godoc documentation. Clicking on a function name on godoc.org will take you to a page which displays the source containing that function. Don’t be afraid to read the source - it is very approachable and, partly due to it being run through gofmt, very readable.
Strings
In Go, string literals are defined using double quotes, similar to other popular programming languages in the C family:
1 package main
2
3 import "fmt"
4
5 func ExampleString() {
6 s := "I am a string - 你好"
7 fmt.Println(s)
8 // Output: I am a string - 你好
9 }
As the example shows, Go string literals and code may also contain non-English characters, like the Chinese 你好 3.
Appending to Strings
Strings can be appended to with the addition (+) operator:
1 package main
2
3 import "fmt"
4
5 func ExampleAppend() {
6 greeting := "Hello, my name is "
7 greeting += "Inigo Montoya"
8 greeting += "."
9 fmt.Println(greeting)
10 // Output: Hello, my name is Inigo Montoya.
11 }
This method of string concatenation is easy to read, and great for simple cases. But while Go does allow us to concatenate strings with the + (or +=) operator, it is not the most efficient method. It is best used only when very few strings are being added, and not in a hot code path. For a discussion on the most efficient way to do string concatenation, see the later chapter on optimization.
In most cases, the built-in fmt.Sprintf function available in the standard library is a better choice for building a string. We can rewrite the previous example like this:
fmt.Sprintf to build a string 1 package main
2
3 import "fmt"
4
5 func ExampleFmtString() {
6 name := "Inigo Montoya"
7 sentence := fmt.Sprintf("Hello, my name is %s.", name)
8 fmt.Println(sentence)
9 // Output: Hello, my name is Inigo Montoya.
10 }
The %s sequence is a special placeholder that tells the Sprintf function to insert a string in that position. There are also other sequences for things that are not strings, like %d for integers, %f for floating point numbers, or %v to leave it to Go to figure out the type. These sequences allow us to add numbers and other types to a string without casting, something the + operator would not allow due to type conflicts. For example:
fmt.Printf to combine different types of variables in a string 1 package main
2
3 import "fmt"
4
5 func ExampleFmtComplexString() {
6 name := "Inigo Montoya"
7 age := 32
8 weight := 76.598
9 t := "Hello, my name is %s, age %d, weight %.2fkg"
10 fmt.Printf(t, name, age, weight)
11 // Output: Hello, my name is Inigo Montoya, age 32, weight 76.60kg
12 }
Note that here we used fmt.Printf to print the new string directly. In previous examples, we used fmt.Sprintf to first create a string variable, then fmt.Println to print it to the screen (notice the S in Sprintf, short for string). In the above example, %d is a placeholder for an integer, %.2f a for a floating point number that should be rounded to the second decimal, and %s a placeholder for a string, as before. These codes are analogous to ones in the printf and scanf functions in C, and old-style string formatting in Python. If you are not familiar with this syntax, have a look at the documentation for the fmt package. It is both expressive and efficient, and used liberally in Go code.
What would happen if we tried to append an integer to a string using the plus operator?
1 package main
2
3 func main() {
4 s := "I am" + 32 + "years old"
5 }
Running this with go run, Go returns an error message during the build phase:
1 $ go run bad_append.go
2 # command-line-arguments
3 ./bad_append.go:4: cannot convert "I am" to type int
4 ./bad_append.go:4: invalid operation: "I am" + 32 (mismatched types string an\
5 d int)
As expected, Go’s type system catches our transgression, and complains that it cannot append an integer to a string. We should rather use fmt.Sprintf for building strings that mix different types.
Next we will have a look at a very useful standard library package that allows us to perform many common string manipulation tasks: the built-in strings package.
Splitting strings
The strings package is imported by simply adding import "strings", and provides us with many string manipulation functions. One of these is a function that split a string by separators, and obtain a slice of strings:
1 package main
2
3 import "fmt"
4 import "strings"
5
6 func ExampleSplit() {
7 l := strings.Split("a,b,c", ",")
8 fmt.Printf("%q", l)
9 // Output: ["a" "b" "c"]
10 }
The strings.Split function takes a string and a separator as arguments. In this case, we passed in "a,b,c" and the separator “,” and received a string slice containing the separate letters a, b, and c as strings.
Counting and finding substrings
Using the strings package, we can also count the number of non-overlapping instances of a substring in a string with the aptly-named strings.Count. The following example uses strings.Count to count occurrences of both the single letter a, and the substring ana. In both cases we pass in a string4. Notice that we get only one occurrence of ana, even though one may have expected it to count ana both at positions 1 and 3. This is because strings.Count returns the count of non-overlapping occurrences.
1 package main
2
3 import (
4 "fmt"
5 "strings"
6 )
7
8 func ExampleCount() {
9 s := "banana"
10 c1 := strings.Count(s, "a")
11 c2 := strings.Count(s, "ana")
12 fmt.Println(c1, c2)
13 // Output: 3 1
14 }
If we want to know whether a string contains, starts with, or ends with some substring, we can use the strings.Contains, strings.HasPrefix, and strings.HasSuffix functions, respectively. All of these functions return a boolean:
1 package main
2
3 import (
4 "fmt"
5 "strings"
6 )
7
8 func ExampleContains() {
9 str := "two gophers on honeymoon"
10 if strings.Contains(str, "moon") {
11 fmt.Println("Contains moon")
12 }
13 if strings.HasPrefix(str, "moon") {
14 fmt.Println("Starts with moon")
15 }
16 if strings.HasSuffix(str, "moon") {
17 fmt.Println("Ends with moon")
18 }
19 // Output: Contains moon
20 // Ends with moon
21 }
For finding the index of a substring in a string, we can use strings.Index. Index returns the index of the first instance of substr in s, or -1 if substr is not present in s:
1 package main
2
3 import "fmt"
4 import "strings"
5
6 func ExampleIndex() {
7 an := strings.Index("banana", "an")
8 am := strings.Index("banana", "am")
9 fmt.Println(an, am)
10 // Output: 1 -1
11 }
The strings package also contains a corresponding LastIndex function, which returns the index of the last (ie. right-most) instance of a matching substring, or -1 if it is not found.
The strings package contains many more useful functions. To name a few: ToLower, ToUpper, Trim, Equals and Join, all performing actions that match their names. For more information on these and other functions, refer to the strings package docs. As a final example, let’s see how we might combine some of the functions in the strings package in a real program, and discover some of its more surprising functions.
Advanced string functions
The program below repeatedly takes input from the user, and declares whether the typed sentence is palindromic. For a sentence to be palindromic, we mean that the words should be the same when read forwards and backwards. We wish to ignore punctuation, and assume the sentence is in English, so there are spaces between words. Take a look and notice how we use two new functions from the strings package, FieldsFunc and EqualFold, to keep the code clear and concise.
1 package main
2
3 import (
4 "bufio"
5 "fmt"
6 "os"
7 "strings"
8 "unicode"
9 )
10
11 // getInput prompts the user for some text, and then
12 // reads a line of input from standard input. This line
13 // of text is then returned.
14 func getInput() string {
15 fmt.Print("Enter a sentence: ")
16 scanner := bufio.NewScanner(os.Stdin)
17 scanner.Scan()
18 return scanner.Text()
19 }
20
21 func isNotLetter(c rune) bool {
22 return !unicode.IsLetter(c)
23 }
24
25 // isPalindromicSentence returns whether or not the given sentence
26 // is palindromic. To calculate this, it splits the string into words,
27 // then creates a reversed copy of the word slice. It then checks
28 // whether the reverse is equal (ignoring case) to the original.
29 // It also ignores any non-alphabetic characters.
30 func isPalindromicSentence(s string) bool {
31 // split into words and remove non-alphabetic characters
32 // in one operation by using FieldsFunc and passing in
33 // isNotLetter as the function to split on.
34 w := strings.FieldsFunc(s, isNotLetter)
35
36 // iterate over the words from front and back
37 // simultaneously. If we find a word that is not the same
38 // as the word at its matching from the back, the sentence
39 // is not palindromic.
40 l := len(w)
41 for i := 0; i < l/2; i++ {
42 fw := w[i] // front word
43 bw := w[l-i-1] // back word
44 if !strings.EqualFold(fw, bw) {
45 return false
46 }
47 }
48
49 // all the words matched, so the sentence must be
50 // palindromic.
51 return true
52 }
53
54 func main() {
55 // Go doesn't have while loops, but we can use for loop
56 // syntax to read into a new variable, check that it's not
57 // empty, and read new lines on subsequent iterations.
58 for l := getInput(); l != ""; l = getInput() {
59 if isPalindromicSentence(l) {
60 fmt.Println("... is palindromic!")
61 } else {
62 fmt.Println("... is not palindromic.")
63 }
64 }
65 }
Save this code to palindromes.go, and we can then run it with go run palindromes.go.
1 $ go run palindromes.go
2 Enter a sentence: This is magnificent!
3 ... is not palindromic.
4 Enter a sentence: This is magnificent, is this!
5 ... is palindromic!
6 Enter a sentence:
As expected, when we enter a sentence that reads the same backwards and forwards, ignoring punctuation and case, we get the output ... is palindromic!. Now, let’s break down what this code is doing.
The getInput function uses a bufio.Scanner from the bufio package to read one line from standard input. scanner.Scan() scans until the end of the line, and scanner.Text() returns a string containing the input line.
The meat of this program is in the isPalindromicSentence function. This function takes a string as input, and returns a boolean indicating whether the sentence is palindromic, word-for-word. We also want to ignore punctuation and case in the comparison. First, on line 34, we use strings.FieldsFunc to split the string at each Unicode code point for which the isNotLetter function returns true. In Go, you can pass around functions like any other value. A function’s type signature describes the types of its arguments and return values. Our isNotLetter function satisfies the function signature specified by FieldsFunc, which is to take a rune as input, and return a boolean. Runes are a special character type in the Go language - for now, just think of them as more or less equivalent to a single character, like char in Java.
In isNotLetter, we return false if the passed in rune is a letter as defined by the Unicode standard, and true otherwise. We can achieve this in a single line by using unicode.IsLetter, another built-in function provided by the standard unicode library.
Putting it all together, strings.FieldsFunc(s, isNotLetter) will return a slice of strings, split by sequences of non-letters. In other words, it will return a slice of words.
Next, on line 40, we iterate over the slice of words. We keep an index i, which we use to create both fw, the word at index i, and bw, the matching word at index l - i - 1. If we can walk all the way through the slice without finding two words that are not equal, we have a palindromic sentence. And we can stop halfway through, because then we have already done all the necessary comparisons. The next table shows how this process works for an example sentence as i increases. As we walk through the slice, words match, and so we continue walking until we reach the middle. If we were to find a non-matching pair, we can immediately return false, because the sentence is not palindromic.
| “Fall” | “leaves” | “as” | “soon” | “as” | “leaves” | “fall” | EqualFold | |
|---|---|---|---|---|---|---|---|---|
| i=0 | fw | bw | true | |||||
| i=1 | fw | bw | true | |||||
| i=2 | fw | bw | true |
The equality check of strings is performed on line 44 using strings.EqualFold - this function compares two strings for equality, ignoring case.
Finally, on line 58, we make use of the semantics of the Go for loop definition. The basic for loop has three components separated by semicolons:
- the init statement: executed before the first iteration
- the condition expression: evaluated before every iteration
- the post statement: executed at the end of every iteration
We use these definition to instantiate a variable l and read into it from standard input, conditionally break from the loop if it is empty, and set up reading for each subsequent iteration in the post statement.
Ranging over a string
When the functions in the strings package don’t suffice, it is also possible to range over each character in a string:
1 package main
2
3 import "fmt"
4
5 func ExampleIteration() {
6 s := "ABC你好"
7 for i, r := range s {
8 fmt.Printf("%q(%d) ", r, i)
9 }
10 // Output: 'A'(0) 'B'(1) 'C'(2) '你'(3) '好'(6)
11 }
You might be wondering about something peculiar about the output above. The printed indexes start from 0, 1, 2, 3 and then jump to 6. Why is that? This is the topic in the next chapter, Supporting Unicode.
Supporting Unicode
Part of preparing production-ready code is making sure that it behaves as expected for supported languages and inputs. In the previous chapter, we showed how easy Go makes many common string manipulation tasks. Many of these examples were implicitly English-centric, with only some hints that there may be more brewing below the surface when it comes to handling international character sets. In this chapter we will examine strings in greater depth, and learn how to write bug-free, production-ready code that handles strings in any language supported by the Unicode standard.
We will start by taking a detour through the history of string encodings. This will then inform the rest of our discussion on handling different character sets in Go.
A very brief history of string encodings
What are string encodings, and why do we need them? You can skip this section if you already know the difference between Unicode and UTF-8, and between a character and a Unicode code point.
Consider how a computer might represent a string of text. Because computers operate on binary, human-readable text needs to be represented as binary numbers in some way. Early computer pioneers came up with one such scheme, which they called ASCII (pronounced ASS-kee). ASCII is one way of mapping characters to numbers. For example, A is 65 (binary 0100 0001, or hexadecimal 0x41), B is 66, C is 67, and so on. We could represent the ASCII-encoded string “ABC” in hexadecimal notation, like so:
ASCII defines a mapping for 127 different characters, using exactly 7 bits. For the old 8-bit systems, this was perfect. The only problem is ASCII only covers unaccented English letters.
As computers became more widespread, other countries also needed to represent their text in binary format, and unaccented English letters were not enough. So a plethora of new encodings were invented. Now when code encountered a string, it also needed to know which encoding the string is using in order to map the bytes to the correct human-readable characters.
Identifying this as a problem, a group called the Unicode consortium undertook the herculean task of assigning a number to every letter used in any language. Such a magic number is called a Unicode code point, and is represented by a U+ followed by a hexadecimal number. For example, the string “ABC” corresponds to these three Unicode code points:
Notice how for the string “ABC”, the hexadecimal numbers are the same as for ASCII.
So Unicode assigns each character with a number, but it does not specify how this number should be represented in binary. This is left to the encoding. The most popular encoding of the Unicode standard is called UTF-8. UTF-8 is popular because it has some nice properties.
One nice property of UTF-8 is that every code point between 0-127 is stored in a single byte. Only code points 128 and above are stored using 2, 3, or up to 6 bytes. Because the first 128 Unicode code points were chosen to match ASCII, this has the side effect that English text looks exactly the same in UTF-8 as it did in ASCII. (Notice how the hexadecimally-encoded ASCII of “ABC” from earlier is the same as the Unicode code points for the same letters.)
In this chapter we will keep things simple by focusing on only these two encodings: ASCII and UTF-8. UTF-8 has become the universal standard, and supports every language your application might need, from Chinese to Klingon. But the same principles apply for any encoding, and should your application need to handle the conversion from other encodings, most common encodings are available in the golang.org/x/text/encoding package.
For a more complete history of string encoding, we recommend Joel Spolsky’s excellent blog post from 2003, titled The Absolute Minimum Every Software Developer Absolutely, Positively Must Know About Unicode and Character Sets (No Excuses!).
With an understanding of what string encodings are and why they exist, let’s now turn to how they are handled in Go.
Strings are byte slices
In Go, strings are read-only (or immutable) byte slices. The byte slice representing the string is not required to hold Unicode text, UTF-8 text, or any other specific encoding. In other words, strings can hold arbitrary bytes. For example, we can take a slice of bytes, and convert it to a string:
1 package main
2
3 import (
4 "fmt"
5 )
6
7 func main() {
8 b := []byte{65, 66, 67}
9 s := string(b)
10 fmt.Println(s)
11 // Output: ABC
12 }
In this byte slice to string conversion, Go makes no assumptions about the encoding. To Go, this is just another byte slice, now with the type of string. When the string gets printed by fmt.Println, again, Go is just sending some bytes to standard output. The terminal that outputs the bytes to the screen needs to use the appropriate character encoding for the bytes to render correctly as human-readable text. In this case, either ASCII or UTF-8 encodings would do.
Printing strings
For debugging strings it is often useful to see the raw bytes in different forms. Strings starting with a % symbol, like %s or %q, are placeholders for parameters when passed into certain functions in the fmt package, like fmt.Printf, fmt.Sprintf, which returns a formatted string, and fmt.Scanf, which reads a variable from input. Package fmt implements formatted I/O with functions similar to C’s printf and scanf. The format ‘verbs’ are derived from C’s but are simpler.
The next program showcases some different ways to print strings in Go using the fmt package.
1 package main
2
3 import (
4 "fmt"
5 )
6
7 func showVerb(v, s string) {
8 n := fmt.Sprintf(v, s)
9 fmt.Println(v, "\t", n)
10 }
11
12 func main() {
13 s := "ABC 你好"
14 showVerb("%s", s)
15 showVerb("%q", s)
16 showVerb("%+q", s)
17 showVerb("%x", s)
18 showVerb("% x", s)
19 showVerb("%# x", s)
20 }
Running this produces the following output:
1 %s ABC 你好
2 %q "ABC 你好"
3 %+q "ABC \u4f60\u597d"
4 %x 41424320e4bda0e5a5bd
5 % x 41 42 43 20 e4 bd a0 e5 a5 bd
6 %# x 0x41 0x42 0x43 0x20 0xe4 0xbd 0xa0 0xe5 0xa5 0xbd
The showVerb function is a single line that prints the given verb v, plus the string s formatted using that verb. We use the fmt.Sprintf function to create a new string formatted with the passed in verb v. On the next line we print v and the new string n, to see what it looks like.
The verb following the % character specifies how Go should format the given parameter. There are many verbs to choose from, including some verbs for specific data types. The following verbs can be used for any data type 5:
1 %v the value in a default format
2 when printing structs, the plus flag (%+v) adds field names
3 %#v a Go-syntax representation of the value
4 %T a Go-syntax representation of the type of the value
5 %% a literal percent sign; consumes no value
and these verbs are only for strings and slices of bytes:
1 %s the uninterpreted bytes of the string or slice
2 %q a double-quoted string safely escaped with Go syntax
3 %x base 16, lower-case, two characters per byte
4 %X base 16, upper-case, two characters per byte
Other common verbs include those for integers (e.g. %d for numbers in base 10) and booleans (%b). You can refer to the fmt package documentation for the full list.
As our example demonstrated, some verbs allow special flags between the % symbol and the verb. From the fmt package documentation:
1 + always print a sign for numeric values;
2 guarantee ASCII-only output for %q (%+q)
3 - pad with spaces on the right rather than the left (left-justify the field)
4 # alternate format: add leading 0 for octal (%#o), 0x for hex (%#x);
5 0X for hex (%#X); suppress 0x for %p (%#p);
6 for %q, print a raw (backquoted) string if strconv.CanBackquote
7 returns true;
8 always print a decimal point for %e, %E, %f, %F, %g and %G;
9 do not remove trailing zeros for %g and %G;
10 write e.g. U+0078 'x' if the character is printable for %U (%#U).
11 ' ' (space) leave a space for elided sign in numbers (% d);
12 put spaces between bytes printing strings or slices in hex (% x, % X)
13 0 pad with leading zeros rather than spaces;
14 for numbers, this moves the padding after the sign
Flags are ignored by verbs that do not expect them.
There are plenty more options for formatting with the fmt package, and we recommend reading the documentation for a full breakdown of all the available options. One aspect not covered so far is that it is possible to specify a width by an optional decimal number immediately preceding the verb. By default the width is whatever is necessary to represent the value, but when provided, width will pad with spaces until there are least that number of runes. This is not the same as in C, which uses bytes to count the width. So what exactly are runes?
Runes and safely ranging over strings
Previously we stated that Go makes no assumptions about the string encoding when it converts bytes to the string type. This is true. The string type carries with it no information about its encoding. But in certain instances, Go does need to make assumptions about the underlying encoding. One such case is when we range over a string.
Let’s return to the last example from the previous chapter, and range over a string that contains some non-ASCII characters:
1 package main
2
3 import "fmt"
4
5 func ExampleIteration() {
6 s := "ABC你好"
7 for i, r := range s {
8 fmt.Printf("%q(%d) ", r, i)
9 }
10 // Output: 'A'(0) 'B'(1) 'C'(2) '你'(3) '好'(6)
11 }
Running this, we get the following output:
1 'A'(0) 'B'(1) 'C'(2) '你'(3) '好'(6)
The range keyword returns two values on each iteration: an integer indicating the current position in the string, and the current rune. rune is a built-in type in Go, and it is meant to contain a single Unicode character. As such, it is an alias of int32, and contains 4 bytes. So on every iteration, we get the current position in the string, in this case called i, and a rune called r. We use the %q and %d verbs to print out the current rune and position in the string.
By now it might be clear why the position variable i jumps from 3 to 6 between 你 and 好. Under the hood, instead of going byte by byte, the range keyword is fetching the next rune in the string.
Text Normalization
In the Unicode standard, there are often several ways to represent the same string. For example, the acute e in the word café can be represented in a string as a single rune ("\u00e9") or as an e followed by an acute accent ("e\u0301"). According to the standard, these two should be treated as equivalent. Is this what happens in Go?
1 package main
2
3 import (
4 "fmt"
5
6 "golang.org/x/text/unicode/norm"
7 )
8
9 func main() {
10 a := string([]rune{0x00E9})
11 b := string([]rune{0x0065, 0x0301})
12 fmt.Println(a, b)
13 fmt.Println("same:", a == b)
14
15 normA := norm.NFC.String(a)
16 normB := norm.NFC.String(b)
17 fmt.Println(normA, normB)
18 fmt.Println("same:", normA == normB)
19
20 // Output: é é
21 //same: false
22 //é é
23 //same: true
24 }
As the example demonstrates, the answer is “no”. String comparison, by default, does not consider the two versions of é equivalent. However, the example also shows how we can use the golang.org/x/text/unicode/norm package to write the strings into NFC (“Normalization Form C”) before doing the comparison, in which case they do come back as equivalent. Unicode provides four normalization forms: NFD, NFC, NFKD and NFKC. These are summarized in the table below, taken from the Unicode standard documentation:
| Form | Description |
|---|---|
| Normalization Form D (NFD) | Canonical Decomposition |
| Normalization Form C (NFC) | Canonical Decomposition, followed by Canonical Composition |
| Normalization Form KD (NFKD) | Compatibility Decomposition |
| Normalization Form KC (NFKC) | Compatibility Decomposition, followed by Canonical Composition |
Here “decomposition” means writing the “e” and accent separately, as in "e\u0301", and “composition” means writing them as a single character, i.e. "\u00e9". The details of these are far beyond the scope of this book, but the important thing for our purposes is that there are several instances where normalizing text can be a good idea. We will discuss a few of these next.
Handling look-alikes
Some Unicode characters look exactly alike, yet use different underlying codes. This can pose a security threat. For example, can you tell the difference between users Kevin and Kevin? One Kevin is using K (unicode \u004B) and the other is using K (Kelvin sign, unicode \u212A). The second Kevin is likely doing this to circumvent detection mechanisms that check whether usernames are taken, and might intend on impersonating the first Kevin.
Fortunately, the golang.org/x/text/unicode/norm package comes to the rescue. We can use this to mitigate (but not completely eliminate) the risk. In the following example, we again norm.NFC.String to convert the strings to canonical form before doing comparisons:
1 package main
2
3 import (
4 "fmt"
5
6 "golang.org/x/text/unicode/norm"
7 )
8
9 func equalNorm(a, b string) bool {
10 return norm.NFC.String(a) == norm.NFC.String(b)
11 }
12
13 func main() {
14 user1 := `Kevin` // uses normal K "\u004B"
15 user2 := `Kevin` // uses Kelvin K "\u212A"
16 fmt.Println("naive equal:", user1 == user2)
17 fmt.Println("norm equal:", equalNorm(user1, user2))
18
19 // Output: naive equal: false
20 //norm equal: true
21 }
This trick can help us detect similar usernames that may be an attempt to impersonate someone else and treat visually similar names as equivalent. It’s not perfect, though. Some characters that look the same are still treated as unequal. For example, the Latin o, Greek ο, and Cyrillic о are still different characters because they come from different alphabets, even though they look the same.
Saving bytes on the wire
Generally speaking, the NFC form will not save all that much in terms of bytes stored in your database or sent over the wire. But in some languages, like Korean, the savings can be significant. You should do benchmarking for your own application, but it is well worth considering converting text to NFC form before storing it in a database or sending it to other applications.
Correctly modifying Unicode text
The norm package can also be of use when replacing text. Consider an example, adapted from the official Go blog post, where we wish to replace the word cafe in some text. Without taking multi-rune character boundaries into account, an instance of cafe followed by an accented "e" might become cafeś (with the accent now incorrectly on the s) after replacement. We can handle this if we transform the text to canonical form first before doing replacement.
1 package main
2
3 import (
4 "fmt"
5 "strings"
6
7 "golang.org/x/text/unicode/norm"
8 )
9
10 func replaceNormalized(text string, old, new []string) string {
11 for i := 0; i < len(old) && i < len(new); i++ {
12 text = strings.ReplaceAll(norm.NFC.String(text), old[i], new[i])
13 }
14 return text
15 }
16
17 func main() {
18 text := "We went to eat at multiple cafe" + string(rune(0x0301)) // accented\
19 e in decomposed form
20 incorrect := strings.ReplaceAll(text, "cafe", "cafes")
21 fmt.Println("Incorrect:", incorrect)
22
23 correct := replaceNormalized(text, []string{"cafe", "café"}, []string{"cafes\
24 ", "cafés"})
25 fmt.Println("Correct:",correct)
26
27 // Output: Incorrect: We went to eat at multiple cafeś
28 //Correct: We went to eat at multiple cafés
29 }
Summary
In this chapter we covered:
- how Go treats unicode characters in strings differently from other languages. Be sure you test with unicode characters when ranging over strings or counting characters, unless your input guarantees to have only characters from the ASCII subset.
- Text normalization and handling look-alike characters, as well as some other edge cases related to Unicode to look out for when writing production-ready Go code.
If you would like to learn more about this topic, also see the Go blog posts Strings, bytes, runes and characters in Go and Text Normalization in Go.
Concurrency
Go’s concurrency model is based on what are called “goroutines” - essentially lightweight threads. To invoke a goroutine, we use the go keyword:
If we were to add the above to a main() function and run it, we would probably see no output. This is because the main exited before the goroutine had time to finish. To see this in action, let’s add a sleep to give the goroutine some time to run:
1 package main
2
3 import (
4 "fmt"
5 "time"
6 )
7
8 func main() {
9 go func() {
10 fmt.Println("hello, world")
11 }()
12
13 time.Sleep(1 * time.Second)
14 }
$ go run main.go
hello, world
What would happen if we had multiple goroutines? Let’s try:
1 package main
2
3 import (
4 "fmt"
5 "time"
6 )
7
8 func main() {
9 go func() {
10 fmt.Println("hello, world 1")
11 }()
12
13 go func() {
14 fmt.Println("hello, world 2")
15 }()
16
17 go func() {
18 fmt.Println("hello, world 3")
19 }()
20
21 time.Sleep(1 * time.Second)
22 }
$ go run main.go
hello, world 2
hello, world 1
hello, world 3
We notice the goroutines all ran, and not in the order in which they were written. This is the expected behavior.
But we can’t use time.Sleep everywhere in our code to wait for our goroutines to finish, right? In the next sections we’ll discuss how to organize our goroutines.
sync.WaitGroup
With sync.WaitGroup we can avoid using time.Sleep to wait for our goroutines to finish. Instead, we create a sync.WaitGroup and add 1 to its counter for every goroutine we expect to launch. Then, inside each goroutine we decrement the counter. Finally we call the Wait() method on the WaitGroup to wait for all of our goroutines to finish. Let’s modify our previous example to use a sync.WaitGroup:
1 package main
2
3 import (
4 "fmt"
5 "sync"
6 )
7
8 func main() {
9 var wg sync.WaitGroup
10 wg.Add(1)
11 go func() {
12 defer wg.Done()
13 fmt.Println("hello, world 1")
14 }()
15
16 wg.Add(1)
17 go func() {
18 defer wg.Done()
19 fmt.Println("hello, world 2")
20 }()
21
22 wg.Add(1)
23 go func() {
24 defer wg.Done()
25 fmt.Println("hello, world 3")
26 }()
27
28 wg.Wait()
29 }
When we run this code, we get the same or similar output to when we were using time.Sleep:
$ go run main.go
hello, world 3
hello, world 1
hello, world 2
All of our goroutines finished, and we didn’t need to use a time.Sleep to wait for them.
errgroup
errgroup is similar to sync.WaitGroup but provides convenient error handling functionality.
1 package main
2
3 import (
4 "fmt"
5 "log"
6 "os/exec"
7 "strconv"
8 "strings"
9 "sync/atomic"
10
11 "golang.org/x/sync/errgroup"
12 )
13
14 // lineCount returns the number of lines in a given file
15 func lineCount(filepath string) (int, error) {
16 out, err := exec.Command("wc", "-l", filepath).Output()
17 if err != nil {
18 return 0, fmt.Errorf("could not run wc -l: %s", err)
19 }
20
21 // wc output is like: 999 filename.go
22 count, err := strconv.Atoi(strings.Fields(string(out))[0])
23 if err != nil {
24 return 0, fmt.Errorf("could not convert wc -l output to integer: %s", err)
25 }
26
27 return count, nil
28 }
29
30 func main() {
31 var totalLineCount int64
32 var g errgroup.Group
33
34 files := []string{
35 "foo.txt",
36 "bar.txt",
37 "baz.txt",
38 }
39
40 for _, file := range files {
41 file := file
42 g.Go(func() error {
43 lc, err := lineCount(file)
44 if err != nil {
45 return err
46 }
47
48 atomic.AddInt64(&totalLineCount, int64(lc))
49
50 return nil
51 })
52 }
53
54 if err := g.Wait(); err != nil {
55 log.Fatal(err)
56 }
57
58 log.Printf("total line count: %d", totalLineCount)
59 }
In this example, we have a list of files for which we want to get the word count concurrently. We loop over each filename and launch a goroutine with the errgroup’s Go function. The lineCount function calls out to the wc command. Note that due to how closures work with goroutines, we must set file := file before launching the goroutine (https://golang.org/doc/faq#closures_and_goroutines).
Inside the goroutine, we get the line count and error return values from lineCount. If we encounter an error, we return it. After launching the goroutines, we call g.Wait() to wait for them to finish. We also get an error back if any errors were returned in the process:
$ go run main.go
2020/05/21 13:51:59 total line count: 17
To force an error, let’s remove one of the files we’re checking:
$ rm foo.txt
$ go run main.go
2020/05/21 13:52:35 could not run wc -l: exit status 1
exit status 1
Channels
Channels can be used to send and receive values. They’re often used in goroutines; a goroutine will do some work and then send the result to the channel that was passed in as an argument:
1 package main
2
3 import "fmt"
4
5 func main() {
6 ch := make(chan string)
7
8 go func(ch chan string) {
9 ch <- "hello, world 1"
10 }(ch)
11
12 go func(ch chan string) {
13 ch <- "hello, world 2"
14 }(ch)
15
16 go func(ch chan string) {
17 ch <- "hello, world 3"
18 }(ch)
19
20 a, b, c := <-ch, <-ch, <-ch
21
22 fmt.Println(a)
23 fmt.Println(b)
24 fmt.Println(c)
25 }
This code is functionally equivalent to our sync.WaitGroup example in the previous section. Note that we must declare the channel before it’s used, as we see with ch := make(chan string). We create the channel, then each goroutine sends a string onto the channel. Finally we select 3 values from the channel and print them out at the end of the program.
What would happen if we only sent 2 strings but tried to select 3?
1 package main
2
3 import "fmt"
4
5 func main() {
6 ch := make(chan string)
7
8 go func(ch chan string) {
9 ch <- "hello, world 1"
10 }(ch)
11
12 go func(ch chan string) {
13 ch <- "hello, world 2"
14 }(ch)
15
16 // go func(ch chan string) {
17 // ch <- "hello, world 3"
18 // }(ch)
19
20 a, b, c := <-ch, <-ch, <-ch
21
22 fmt.Println(a)
23 fmt.Println(b)
24 fmt.Println(c)
25 }
$ go run main.go
fatal error: all goroutines are asleep - deadlock!
goroutine 1 [chan receive]:
main.main()
/Users/gopher/foo.go:20 +0xf2
exit status 2
Goroutines in web handlers
We mentioned that goroutines run until the main function exits. This means that a web handler can create goroutines to do background processing and can return before the goroutines finish. Let’s see an example:
1 package main
2
3 import (
4 "log"
5 "net/http"
6 "time"
7 )
8
9 func main() {
10 http.HandleFunc("/", func(w http.ResponseWriter, req *http.Request) {
11 go func() {
12 time.Sleep(3 * time.Second)
13 log.Println("hello, world")
14 }()
15
16 return
17 })
18
19 log.Println("Running on :8080...")
20 log.Fatal(http.ListenAndServe("127.0.0.1:8080", nil))
21 }
If we run this code in one terminal, we should see:
$ go run main.go
2018/11/02 13:21:23 Running on :8080...
Now in a second terminal, let’s do a curl:
$ curl localhost:8080
$
The curl returns almost immediately. Now if we wait a couple of seconds, we should see our print statement in the first terminal:
$ go run main.go
2018/11/02 13:21:23 Running on :8080...
2018/11/02 13:21:26 hello, world
Meaning the goroutine continued running in the background after our HTTP handler returned.
Pollers
Goroutines are useful when writing pollers as well. Let’s say we have a Go program that functions as a web server, but we also want to poll for some data in the background. We’ll keep the web server functionality small for the sake of simplicity in our example. Let’s poll for advisory information from BART, the public transportation system serving the San Francisco Bay Area:
1 package main
2
3 import (
4 "encoding/json"
5 "io/ioutil"
6 "log"
7 "net/http"
8 "time"
9 )
10
11 const bsaEndpoint = "http://api.bart.gov/api/bsa.aspx?cmd=bsa&key=MW9S-E7SL-2\
12 6DU-VV8V&json=y"
13
14 type bsaResponse struct {
15 Root struct {
16 Advisories []BSA `json:"bsa"`
17 }
18 }
19
20 // BSA is a BART service advisory
21 type BSA struct {
22 Station string
23 Description struct {
24 Text string `json:"#cdata-section"`
25 }
26 }
27
28 func poll() {
29 ticker := time.NewTicker(5 * time.Second)
30 defer ticker.Stop()
31 for {
32 select {
33 case <-ticker.C:
34 resp, err := http.Get(bsaEndpoint)
35 if err != nil {
36 log.Println("ERROR: could not GET bsaEndpoint:", err)
37 }
38 defer resp.Body.Close()
39
40 b, err := ioutil.ReadAll(resp.Body)
41 if err != nil {
42 log.Println("ERROR: could not parse response body:", err)
43 }
44
45 var br bsaResponse
46 err = json.Unmarshal(b, &br)
47 if err != nil {
48 log.Println("ERROR: json.Unmarshal:", err)
49 }
50
51 if len(br.Root.Advisories) > 0 {
52 for _, adv := range br.Root.Advisories {
53 log.Println(adv.Station, adv.Description.Text)
54 }
55 }
56 }
57 }
58 }
59
60 func main() {
61 http.HandleFunc("/", func(w http.ResponseWriter, req *http.Request) {
62 go func() {
63 log.Println("hello, world 1")
64 }()
65
66 return
67 })
68
69 go poll()
70 log.Println("Running on :8080...")
71 log.Fatal(http.ListenAndServe("127.0.0.1:8080", nil))
72 }
Let’s go over what this code does. We have an HTTP handler like our previous example but without the time.Sleep. If we do curl localhost:8080, we should not only see an immediate response, but also a hello, world log statement in the console running our program. After we set up our handler, we run our poll function in a goroutine.
The poll function is making a GET request every 5 seconds to a public BART API endpoint which contains advisory information, such as delays at certain stations. After making the request and unmarshalling it, we print the results.
Why does poll need to be run in a goroutine? Since it contains an infinite loop (for{ ... }), if we were to run it on its own we would never get to the code following it.
Race conditions
When writing concurrent code, we must be careful not to create any race conditions. A race condition occurs in Go when two goroutines access the same variable and at least one access is a write.6
We discuss race conditions and the race detector in detail in the [Tooling]{#racedetector} section. Here we’ll go over a simple example and fix it, but please see the Tooling section for advice on preventing data races in your build.
Consider the following example of two goroutines updating a map concurrently:
1 package main
2
3 import (
4 "log"
5 "time"
6 )
7
8 func main() {
9 m := map[string]string{}
10
11 go func() {
12 m["test"] = "hello, world 1"
13 }()
14
15 go func() {
16 m["test"] = "hello, world 2"
17 }()
18
19 time.Sleep(time.Second)
20 log.Println(m["test"])
21 }
Running this code gives us the following output:
$ go run main.go
2018/11/02 17:29:27 map[test:hello, world 2]
Since these two goroutines are updating the same map concurrently, however, there is a data race. We can prevent this by using sync.Map:
1 package main
2
3 import (
4 "log"
5 "sync"
6 "time"
7 )
8
9 func main() {
10 var m = &sync.Map{}
11
12 go func() {
13 m.Store("test", "hello, world 1")
14 }()
15
16 go func() {
17 m.Store("test", "hello, world 2")
18 }()
19
20 time.Sleep(time.Second)
21
22 val, _ := m.Load("test")
23 log.Println(val)
24 }
This works fine, but the documentation warns us that sync.Map should only be used in the following situations:
The Map type is optimized for two common use cases: (1) when the entry for a given key is only ever written once but read many times, as in caches that only grow, or (2) when multiple goroutines read, write, and overwrite entries for disjoint sets of keys. In these two cases, use of a Map may significantly reduce lock contention compared to a Go map paired with a separate Mutex or RWMutex.7
So let’s take their advice and use map paired with a Mutex:
1 package main
2
3 import (
4 "log"
5 "sync"
6 "time"
7 )
8
9 type safeMap struct {
10 sync.Mutex
11 m map[string]string
12 }
13
14 func main() {
15 var sm = safeMap{
16 m: make(map[string]string),
17 }
18
19 go func() {
20 sm.Lock()
21 defer sm.Unlock()
22 sm.m["test"] = "hello, world 1"
23 }()
24
25 go func() {
26 sm.Lock()
27 defer sm.Unlock()
28 sm.m["test"] = "hello, world 2"
29 }()
30
31 time.Sleep(time.Second)
32
33 sm.Lock()
34 log.Println(sm.m["test"])
35 sm.Unlock()
36 }
We can add Store, Load, Delete etc. methods to our safeMap type as well:
1 package main
2
3 import (
4 "log"
5 "sync"
6 "time"
7 )
8
9 type safeMap struct {
10 sync.Mutex
11 m map[string]string
12 }
13
14 func (sm *safeMap) Store(key, val string) {
15 sm.Lock()
16 defer sm.Unlock()
17 sm.m[key] = val
18 }
19
20 func (sm *safeMap) Load(key string) (val string, exists bool) {
21 sm.Lock()
22 defer sm.Unlock()
23 val, ok := sm.m[key]
24 return val, ok
25 }
26
27 func (sm *safeMap) Delete(key, val string) {
28 sm.Lock()
29 defer sm.Unlock()
30 delete(sm.m, key)
31 }
32
33 func main() {
34 var sm = safeMap{
35 m: make(map[string]string),
36 }
37
38 go func() {
39 sm.Store("test", "hello, world 1")
40 }()
41
42 go func() {
43 sm.Store("test", "hello, world 2")
44 }()
45
46 time.Sleep(time.Second)
47
48 val, _ := sm.Load("test")
49 log.Println(val)
50 }
$ go run main.go
2018/11/02 18:45:57 hello, world 2
It also runs with the race detector enabled, no data races found:
$ go run -race main.go
2018/11/02 18:46:22 hello, world 2
Testing
A critical part of any production-ready system is a complete test suite. If you have not written tests before, and wonder why they are important, this introduction is for you. If you already understand the importance of proper testing, you can skip to the particulars of writing tests in Go, in writing tests.
Why do we need tests?
A line of reasoning we sometimes hear, is that “my code clearly works, why do I need to write tests for it?” This is a natural enough question, but make no mistake, a modern production-ready system absolutely must have automated tests. Let’s use an analogy from the business world to understand why this is so: double entry bookkeeping.
Double entry is the idea that every financial transaction has equal and opposite effects in at least two different accounts. For example, if you spend $10 on groceries, your bank account goes down by $10, and your groceries account goes up by $10. This trick allows you to see, at a glance, simultaneously how much money is in your bank account, and how much you spent on groceries. It also allows you to spot mistakes. Suppose a smudge in your books made the $10 entry look like $18. The total balance of your assets would no longer match your liabilities plus equity - there would be an $8 difference. We can compare entries in the bank account with entries in the groceries account to discover which amount is incorrect. Before double-entry bookkeeping, it was much harder to prove mistakes, and impossible to see different account balances at a glance. The idea revolutionized bookkeeping, and underpins accounting to this day.
Back to tests. For every piece of functionality we write, we also write a test. The test should prove that the code works in all reasonable scenarios. Like double-entry bookkeeping, tests are our way to ensure our system is correct, and remains correct. Your system might work now - you might even prove it to yourself by trying out some cases manually. But systems, especially production systems, require changes over time. Requirements change, environments change, bugs emerge, new features become needed, inefficiencies are discovered. All these things will require changes to be made to the code. After making these changes, will you still be sure that the system is correct? Will you run through manual test cases after every change? What if someone else is maintaining the code? Will they know how to test changes? How much time will it take you to manually perform these test cases?
Automated tests cost up-front investment, but they uncover bugs early, improve maintainability, and save time in the long run. Tests are the checks and balances to your production system.
Many books and blog posts have been written about good testing practice. There are even movements that promote writing tests first, before writing the code. We don’t think it’s necessary to be quite that extreme, but if it helps you write good tests, then more power to you. No production system is complete without a test suite that makes sensible assertions on the code to prove it correct.
Now that we have discussed the importance of testing in general, let’s see how tests are written in Go. As we’ll see, testing was designed with simplicity in mind.
Writing Tests
Test files in Go are located in the same package as the code being tested, and end with the suffix _test.go. Usually, this means having one _test.go to match each code file in the package. Below is the layout of a simple package for testing prime numbers.
- prime
- prime.go
- prime_test.go
- sieve.go
- sieve_test.go
This is a very typical Go package layout. Go packages contain all files in the same directory, including the tests. Now, let’s look at what the test code might look like in prime_test.go.
prime_test.go that tests a function called IsPrime 1 package main
2
3 import "testing"
4
5 // TestIsPrime tests that the IsPrime function
6 // returns true when the input is prime, and false
7 // otherwise.
8 func TestIsPrime(t *testing.T) {
9 // check a prime number
10 got := IsPrime(19)
11 if got != true {
12 t.Errorf("IsPrime(%d) = %t, want %t", 19, got, true)
13 }
14
15 // check a non-prime number
16 got = IsPrime(21)
17 if got != false {
18 t.Errorf("IsPrime(%d) = %t, want %t", 21, got, false)
19 }
20 }
We start by importing the testing package. Then, on line 8, we define a test as a normal Go function taking a single argument: t *testing.T. All tests must start with the word Test and have a single argument, a pointer to testing.T. In the function body, we call the function under test, IsPrime. First we pass in the integer 19, which we expect should return true, because 19 is prime. We check this assertion with a simple if statement on line 11, if got != true. If the statement evaluates to false, t.Errorf is called. Errorf formats its arguments in a way analogous to Printf, and records the text in the error log. We repeat a similar check for the number 21, this time asserting that the IsPrime function returns true, because 21 is not prime.
We can run the tests in this package using go test. Let’s see what happens:
It passed! But did it actually run our TestIsPrime function? Let’s check by adding the -v (verbose) flag to the command:
Our test is indeed being executed. The -v flag is a useful trick to remember, and we recommend running tests with it turned on most of the time.
All tests in Go follow essentially the same format as the TestIsPrime. The Go authors made a conscious decision not to add specific assertion functions, advising instead to use the existing control flow tools that come with the language. The result is that tests look very similar to normal Go code, and the learning curve is minimal.
Table-driven tests
Our initial TestIsPrime test is a good start, but it only tests two numbers. The code is also repetitive. We can do better by using what is called a table-driven test. The idea is to define all the inputs and expected outputs first, and then loop through each case with a for loop.
prime_test.go that tests a function called IsPrime 1 package main
2
3 import "testing"
4
5 // TestIsPrime tests that the IsPrime function
6 // returns true when the input is prime, and false
7 // otherwise.
8 func TestIsPrimeTD(t *testing.T) {
9 cases := []struct {
10 give int
11 want bool
12 }{
13 {19, true},
14 {21, false},
15 {10007, true},
16 {1, false},
17 {0, false},
18 {-1, false},
19 }
20
21 for _, c := range cases {
22 got := IsPrime(c.give)
23 if got != c.want {
24 t.Errorf("IsPrime(%d) = %t, want %t", c.give, got, c.want)
25 }
26 }
27 }
In the refactored test, we use a slice of an anomymous struct to define all the inputs we want to test. We then loop over each test case, and check that the output matches what we want. This is much cleaner than before, and it only took a few keystrokes to add more test cases into the mix. We now also check some edge cases: inputs of 0, 1, 10007, and negative inputs. Let’s run the test again and check that it still passes:
It looks like the IsPrime function works as advertised! To be sure, let’s add a test case that we expect to fail:
...
{-1, false},
// 17 is prime, so this test should fail:
{17, false},
}
...
We run go test -v again to see the results:
This time go test reports that the test failed, and we see the error message we provided to t.Errorf.
Writing error messages
In the tests above, we had the following code:
if got != c.want {
t.Errorf("IsPrime(%d) = %t, want %t", c.give, got, c.want)
}
The ordering of the if statement is not accidental: by convention, it should be actual != expected, and the error message uses that order too. This is the recommended way to format test failure messages in Go 8. In the error message, first state the function called and the parameters it was called with, then the actual result, and finally, the result that was expected. We saw before that this results in a message like
prime_test.go:25: IsPrime(17) = true, want false
This makes it clear to the reader of the error message what function was called, what happened, and what should have happened. The onus is on you, the test author, to leave a helpful message for the person debugging the code in the future. It is a good idea to assume that the person debugging your failing test is not you, and is not your team. Make both the name of the test and the error message relevant.
Testing HTTP Handlers
Let’s look at an example of testing that comes up often when developing web applications: testing an HTTP handler. First, let’s define a comically simple HTTP handler that writes a friendly response:
1 package main
2
3 import (
4 "fmt"
5 "net/http"
6 )
7
8 // helloHandler writes a friendly "Hello, friend :)" response.
9 func helloHandler(w http.ResponseWriter, r *http.Request) {
10 fmt.Fprintln(w, "Hello, friend :)")
11 }
12
13 func main() {
14 http.HandleFunc("/hello", helloHandler)
15 http.ListenAndServe(":8080", nil)
16 }
The httptest package provides us with the tools we need to test this handler as if it were running in a real web server. The TestHTTPHandler function in the following example illustrates how to use httptest.NewRecorder() to send a real request to our friendly helloHandler, and read the resulting response.
1 package main
2
3 import (
4 "net/http"
5 "net/http/httptest"
6 "testing"
7 )
8
9 func TestHTTPHandler(t *testing.T) {
10 // Create a request to pass to our handler.
11 req, err := http.NewRequest("GET", "/hello", nil)
12 if err != nil {
13 t.Fatal(err)
14 }
15
16 // We create a ResponseRecorder, which satisfies
17 // the http.ResponseWriter interface, to record
18 // the response.
19 r := httptest.NewRecorder()
20 handler := http.HandlerFunc(helloHandler)
21
22 // Our handler satisfies http.Handler, so we can call
23 // the ServeHTTP method directly and pass in our
24 // Request and ResponseRecorder.
25 handler.ServeHTTP(r, req)
26
27 // Check that the status code is what we expect.
28 if r.Code != http.StatusOK {
29 t.Errorf("helloHandler returned status code %v, want %v",
30 r.Code, http.StatusOK)
31 }
32
33 // Check that the response body is what we expect.
34 want := "Hello, friend :)\n"
35 got := r.Body.String()
36 if got != want {
37 t.Errorf("helloHandler returned body %q want %q",
38 got, want)
39 }
40 }
In this example we see the t.Fatal method used for the first time. This method is similar to t.Error, but unlike t.Error, if t.Fatal is called, the test will not execute any further. This is useful when a condition happens that will cause the rest of the test to be unnecessary. In our case, if our call to create a request on line 11 were to fail for some reason, the call to t.Fatal ensures that we log the error and abandon execution immediately. Anagolous to t.Errorf, there is also a t.Fatalf method, which takes arguments the same way as fmt.Printf.
On line 19 we create a new httptest.Recorder with which to record the response. We also create handler, which is helloHandler, but now of type http.HandlerFunc. We can do this, because helloHandler uses the appropriate signature defined by http.HandlerFunc:
type HandlerFunc func(ResponseWriter, *Request)
http.HandlerFunc is an adapter to allow the use of ordinary functions as HTTP handlers. As the final step of the setup, we pass the recorder and the request we created earlier in to handler.ServeHTTP(r, req). Now we can use the fields provided by httptest.Recorder, like Code and Body, to make assertions against our HTTP handler, as shown in the final lines of the test function.
Mocking
Imagine you need to test code that uses a third party library. Perhaps this library is a client library to an external API, or perhaps it performs database operations. In your unit tests, it is best to assume that the library does its job, and only test your functions and their interactions. This allows your test case failures to accurately reflect where the problem is, rather than leave the question of whether it’s your function, or the library, that’s at fault. There is a place for tests that include third party libraries, and that place is in integration tests, not unit tests.
Interfaces
How do we go about testing our functions, but not the libraries they use? The answer: interfaces. Interfaces are an incredibly powerful tool in Go.
In Java, interfaces need to be explicitly implemented. You rely on your third party vendor to provide an interface that you can use to stub methods for tests. In Go, we don’t need to rely on the third party author; we can define our own interface. As long as our interface defines a subset of the methods implemented by the library, the library will automatically implement our interface.
The next example illustrates one particular case where mocking is very useful: testing code that relies on random number generation.
1 package eightball
2
3 import (
4 "math/rand"
5 "time"
6 )
7
8 // randIntGenerator is an interface that includes Intn, a
9 // method in the built-in math/rand package. This allows us
10 // to mock out the math/rand package in the tests.
11 type randIntGenerator interface {
12 Intn(int) int
13 }
14
15 // EightBall simulates a very simple magic 8-ball,
16 // a magical object that predicts the future by
17 // answering yes/no questions.
18 type EightBall struct {
19 rand randIntGenerator
20 }
21
22 // NewEightBall returns a new EightBall.
23 func New() *EightBall {
24 return &EightBall{
25 rand: rand.New(rand.NewSource(time.Now().UnixNano())),
26 }
27 }
28
29 // Answer returns a magic eightball answer
30 // based on a random result provided by
31 // randomGenerator. It supports only four
32 // possible answers.
33 func (e EightBall) Answer(s string) string {
34 n := e.rand.Intn(3)
35 switch n {
36 case 0:
37 return "Definitely not"
38 case 1:
39 return "Maybe"
40 case 2:
41 return "Yes"
42 default:
43 return "Absolutely"
44 }
45 }
We define a simple eightball package that implements a simple Magic 8-Ball. We ask it a yes/no question, and it will return its prediction of the future. As you might expect, it completely ignores the question, and just makes use of a random number generator. But random numbers are hard to test, because they change all the time. One option would be to set the random seed in our code, or in our tests. This is indeed an option, but it doesn’t allow us to specifically test the different outcomes without some trial and error. Instead, we create an randIntGenerator interface, which has only one method, Intn(int) int. This method signature is the same as the Intn method implemented by Go’s built-in math/rand package. Instead of using the math/rand package directly in Answer, we decouple our code by referencing the Intn method on the EightBall’s rand interface. Since EightBall.rand is not exported, users of this package will not be aware of this interface at all. To create the struct, they will need to call the New method, which assigns the built-in struct from math/rand struct to satisfy our interface. So to package users the code looks the same, but under the hood, we can now mock out the call to Intn in our tests:
1 package eightball
2
3 import (
4 "testing"
5 )
6
7 type fixedRandIntGenerator struct {
8 // the number that should be "randomly" generated
9 randomNum int
10
11 // record the paramater that Intn gets called with
12 calledWithN int
13 }
14
15 func (g *fixedRandIntGenerator) Intn(n int) int {
16 g.calledWithN = n
17 return g.randomNum
18 }
19
20 func TestEightBall(t *testing.T) {
21 cases := []struct {
22 randomNum int
23 want string
24 }{
25 {0, "Definitely not"},
26 {1, "Maybe"},
27 {2, "Yes"},
28 {3, "Absolutely"},
29 {-1, "Absolutely"}, // default case
30 }
31
32 for _, tt := range cases {
33 g := &fixedRandIntGenerator{randomNum: tt.randomNum}
34 eb := EightBall{
35 rand: g,
36 }
37
38 got := eb.Answer("Does this really work?")
39 if got != tt.want {
40 t.Errorf("EightBall.Answer() is %q for num %d, want %q",
41 got, tt.randomNum, tt.want)
42 }
43
44 if g.calledWithN != 3 {
45 t.Errorf("EightBall.Answer() did not call Intn(3) as expected")
46 }
47 }
48 }
Sometimes, when the existing code uses a specific library implementation, it takes refactoring to use interfaces to mock out impementation details. However, the resulting code is more decoupled. The tests run faster (e.g. when mocking out external network calls) and are more reliable. Don’t be afraid to make liberal use of interfaces. This makes for more decoupled code and more focused tests.
GoMock
Another way to generate mocks for tests is GoMock. WIP
Generating Coverage Reports
To generate test coverage percentages for your code, run the go test -cover command. Let’s make a quick example and a test to go with it.
We’re going to write a simple username validation function. We want our usernames to only contain letters, numbers, and the special characters “-“, “_”, and “.”. Usernames also cannot be empty, and they must be less than 30 characters long. Here’s our username validation function:
1 package validate
2
3 import (
4 "fmt"
5 "regexp"
6 )
7
8 // Username validates a username. We only allow
9 // usernames to contain letters, numbers,
10 // and special chars "_", "-", and "."
11 func Username(u string) (bool, error) {
12 if len(u) == 0 {
13 return false, fmt.Errorf("username must be > 0 chars")
14 }
15 if len(u) > 30 {
16 return false, fmt.Errorf("username too long (must be < 30 chars)")
17 }
18 validChars := regexp.MustCompile(`^[a-zA-Z1-9-_.]+$`)
19 if !validChars.MatchString(u) {
20 return false, fmt.Errorf("username contains invalid character")
21 }
22
23 return true, nil
24 }
Now let’s write a test for it:
1 package validate
2
3 import "testing"
4
5 var usernameTests = []struct {
6 in string
7 wantValid bool
8 }{
9 {"gopher", true},
10 }
11
12 func TestUsername(t *testing.T) {
13 for _, tt := range usernameTests {
14 valid, err := Username(tt.in)
15 if err != nil && tt.wantValid {
16 t.Fatal(err)
17 }
18
19 if valid != tt.wantValid {
20 t.Errorf("Username(%q) = %t, want %t", tt.in, valid, tt.wantValid)
21 }
22 }
23 }
As you can see, we’re not covering very many cases. Let’s see what exactly our test coverage is for this function:
62.5% is a bit too low. This function is simple enough that we can get close to 100% coverage. We’d like to know exactly what parts of the function are not being covered. This is where the coverage profile and HTML report come in.
To generate a test coverage profile, we run go test -coverprofile=coverage.out:
We can now get a breakdown of coverage percentages per function, although we only have one function so it’s not very interesting:
What we really want to see is a line-by-line breakdown. We can get this with the HTML report, which we’ll cover in the next section.
HTML Coverage Reports
We can generate an HTML coverage report using the same coverage.out file from before, by running the following command:
This should open up a browser and show us an HTML page like the following:
Now we can see exactly where we need to improve our coverage. We need to cover the cases where the username length is either 0 or > 30, as well as the case where the username contains an invalid character. Let’s update our test for those cases:
1 package validate
2
3 import "testing"
4
5 var usernameTests = []struct {
6 in string
7 wantValid bool
8 }{
9 {"", false},
10 {"gopher", true},
11 {"gopher$", false},
12 {"abcdefghijklmnopqrstuvwxyzabcde", false},
13 }
14
15 func TestUsername(t *testing.T) {
16 for _, tt := range usernameTests {
17 valid, err := Username(tt.in)
18 if err != nil && tt.wantValid {
19 t.Fatal(err)
20 }
21
22 if valid != tt.wantValid {
23 t.Errorf("Username(%q) = %t, want %t", tt.in, valid, tt.wantValid)
24 }
25 }
26 }
Now if we re-run go test -coverprofile=coverage.out to get a new coverage profile, and then go tool cover -html=coverage.out to view the HTML report again, we should see all green:
Writing Examples
We can also write example code and the go test tool will run our examples and verify the output. godoc renders examples underneath the function’s documentation.
Let’s write an example for our username validation function:
1 package validate
2
3 import (
4 "fmt"
5 "log"
6 )
7
8 func ExampleUsername() {
9 usernames := []struct {
10 in string
11 valid bool
12 }{
13 {"", false},
14 {"gopher", true},
15 {"gopher$", false},
16 {"abcdefghijklmnopqrstuvwxyzabcde", false},
17 }
18 for _, tt := range usernames {
19 valid, err := Username(tt.in)
20 if err != nil && tt.valid {
21 log.Fatal(err)
22 }
23
24 fmt.Printf("%q: %t\n", tt.in, valid)
25 }
26 // Output:
27 // "": false
28 // "gopher": true
29 // "gopher$": false
30 // "abcdefghijklmnopqrstuvwxyzabcde": false
31 }
Note the Output: at the bottom. That’s a special construct that tells go test what the standard output of our example test should be. go test is actually going to validate that output when it runs the tests.
If we run a local godoc server with godoc -http:6060, and navigate to our validate package, we can also see that godoc renders the example, as expected:
If we click “Example” we’ll see our example code:
Another note about examples is that they have a specific naming convention. We named our example above ExampleUsername because we wrote an example for the Username function. But what if we want to write an example for a method on a type? Let’s say we had a type User with a method ValidateName:
1 type User struct {
2 Name string
3 }
4
5 func (u *User) ValidateName() (bool, error) {
6 ...
7 }
Then our example code would look like this:
1 func ExampleUser_ValidateName() {
2 ...
3 }
where the convention for writing examples for methods on types is ExampleT_M().
If we need multiple examples for a single function, we append an underscore and a lowercase letter. For example with our Validate function, we could have ExampleValidate, ExampleValidate_second, ExampleValidate_third, and so on.
In the next chapter, we will discuss one last important use of the Go testing package: benchmarking.
Benchmarks
The Go testing package contains a benchmarking tool for examining the performance of our Go code. In this chapter, we will use the benchmark utility to progressively improve the performance of a piece of code. We will then discuss advanced benchmarking techniques to ensure that we are measuring the right thing.
A simple benchmark
Let’s suppose we have a simple function that computes the nth Fibonacci number. The sequence \(F_{n}\) of Fibonacci numbers is defined by the recurrence relation, \(F_{n} = F_{n-1} + F_{n-2}\), with \(F_{0} = 0, F_{1} = 1\). That is, every number after the first two is the sum of the two preceding ones:
Because the sequence is recursively defined, a function that calculates the nth Fibonacci number is often used to illustrate programming language recursion in computer science text books. Below is such a function that uses the definition to recursively calculate the nth Fibonacci number.
1 package fibonacci
2
3 // F returns the nth Fibonnaci number.
4 func F(n int) int {
5 if n <= 0 {
6 return 0
7 } else if n == 1 {
8 return 1
9 }
10 return F(n-1) + F(n-2)
11 }
Let’s make sure it works by writing a quick test, as we saw in the chapter on Testing.
1 // fibonacci_test.go
2 package fibonacci
3
4 import "testing"
5
6 func TestF(t *testing.T) {
7 cases := []struct {
8 n int
9 want int
10 }{
11 {-1, 0},
12 {0, 0},
13 {1, 1},
14 {2, 1},
15 {3, 2},
16 {8, 21},
17 }
18
19 for _, tt := range cases {
20 got := FastF(tt.n)
21 if got != tt.want {
22 t.Errorf("F(%d) = %d, want %d", tt.n, got, tt.want)
23 }
24 }
25 }
Running the test, we see that indeed, our function works as promised:
Now, this recursive Fibonacci function works, but we can do better. How much better? Before we rewrite this function, let’s establish a baseline to which we can compare our future efficiency improvements. Go provides a benchmark tool as part of the testing package. Anagalous to TestX(t *testing.T), we create benchmarks with BenchmarkX(b *testing.B):
1 // fibonacci_bench_test.go
2 package fibonacci
3
4 import "testing"
5
6 var numbers = []int{
7 0, 10, 20, 30,
8 }
9
10 func BenchmarkF(b *testing.B) {
11 // run F(n) b.N times
12 m := len(numbers)
13 for n := 0; n < b.N; n++ {
14 F(numbers[n%m])
15 }
16 }
The BenchmarkF function can be saved in any file ending with _test.go to be included by the testing package. The only real surprise in the code is the for loop defined on line 13,
13 for n := 0; n < b.N; n++ {
The benchmark function must run the target code b.N times. During benchmark execution, b.N is adjusted until the benchmark function lasts long enough to be timed reliably.
To run the benchmark, we need to instruct go test to run benchmarks using the -bench flag. Similar to the -run command-line argument, -bench also accepts a regular expression to match the benchmark functions we want to run. To run all the benchmark functions, we provide -bench=.. go test will first run all the tests (or those matched by -run, if provided), and then run the benchmarks. The output for our benchmark above looks is as follows:
The output tells us that the benchmarks ran on a Linux x86-64 environment. Furthermore, the testing package executed our one benchmark, BenchmarkF. It ran the b.N loop 1000 times, and each iteration (i.e. each call to F) lasted 1,255,534ns (~1.2ms) on average.
1.2ms per call seems a bit slow! Especially considering that the numbers we provided to the Fibonacci function were quite small. Let’s improve our original function by not using recursion.
1 package fibonacci
2
3 // FastF returns the nth Fibonnaci number,
4 // but does not use recursion.
5 func FastF(n int) int {
6 var a, b int = 0, 1
7 for i := 0; i < n; i++ {
8 a, b = b, a+b
9 }
10 return a
11 }
This new function FastF, is equivalent to the original, but uses only two variables and no recursion to calculate the final answer. Neat! Let’s check whether it’s actually any faster. We can do this by adding a new benchmark function for FastF:
func BenchmarkFastF(b *testing.B) {
// run FastF(n) b.N times
m := len(numbers)
for n := 0; n < b.N; n++ {
FastF(numbers[n%m])
}
}
Again we run go test -bench=.. This time we will see the output of both benchmarks:
The output is telling us that F still took around 1245008ns per execution, but FastF took only 20.3ns! The benchmark proves that our non-recursive FastF is indeed orders of magnitude faster than the textbook recursive version, at least for the provided inputs.
Comparing benchmarks
The benchcmp tool parses the output of two go test -bench runs and compares the results.
To install, run:
Let’s output the benchmark for the original F function from earlier to a file, using BenchmarkF:
The file will look as follows:
Now instead of implementing FastF, we copy the FastF logic into our original F function:
1 package fibbonaci
2
3 // F returns the nth Fibonnaci number.
4 func F(n int) int {
5 var a, b int = 0, 1
6 for i := 0; i < n; i++ {
7 a, b = b, a+b
8 }
9 return a
10 }
and re-run the benchmark, outputting to a file called new.txt:
new.txt should look like this:
Now let’s run benchcmp on the results:
We can see the old performance, new performance, and a delta. In this case, the new version of F performs so well that it reduced the runtime of the original by 99.9987%. Thus rounded to two decimals, we get a delta of -100.00%.
Resetting benchmark timers
We can reset the benchmark timer if we don’t want the overall benchmark timing to include the execution time of our setup code.
A benchmark from the crypto/aes package in the Go source code provides an example of this:
1 package aes
2
3 import "testing"
4
5 func BenchmarkEncrypt(b *testing.B) {
6 tt := encryptTests[0]
7 c, err := NewCipher(tt.key)
8 if err != nil {
9 b.Fatal("NewCipher:", err)
10 }
11 out := make([]byte, len(tt.in))
12 b.SetBytes(int64(len(out)))
13 b.ResetTimer()
14 for i := 0; i < b.N; i++ {
15 c.Encrypt(out, tt.in)
16 }
17 }
As we can see, there is some setup done in the benchmark, then a call to b.ResetTimer() to reset the benchmark time and memory allocation counters.
Benchmarking memory allocations
The Go benchmarking tools also allow us to output the number memory allocations by the benchmark, alongside the time taken by each iteration. We do this by adding the -benchmem flag. Let’s see what happens if we do this on our Fibonnaci benchmarks from before.
We now have two new columns on the right: the number of bytes per operation, and the number of heap allocations per operation. For our Fibonnaci functions, both of these are zero. Why is this? Let’s add the -gcflags=-m option to see the details. The output below is truncated to the first 10 lines:
The Go compiler performs escape analysis. If an allocation does not escape the function, it can be stored on the stack. Variables placed on the stack avoid the costs involved with a heap allocation and the garbage collector. The omission of the fibonacci.go file from the output above implies that no variables from our F and FastF functions escaped to the heap. Let’s take another look at the FastF function to see why this is:
func FastF(n int) int {
var a, b int = 0, 1
for i := 0; i < n; i++ {
a, b = b, a+b
}
return a
}
In this function, the a, b, and i variables are declared locally and do not need to be put onto the heap, because they are not used again when the function exits. Consider what would happen if, instead of storing only the last two values, we naively stored all values calculated up to n:
1 package fibonacci
2
3 // FastHighMemF returns the nth Fibonacci number, but
4 // stores the full slice of intermediate
5 // results, consuming more memory than necessary.
6 func FastHighMemF(n int) int {
7 if n <= 0 {
8 return 0
9 }
10
11 r := make([]int, n+1)
12 r[0] = 0
13 r[1] = 1
14 for i := 2; i <= n; i++ {
15 r[i] = r[i-1] + r[i-2]
16 }
17 return r[n]
18 }
Running the same test and benchmark from before on this high-memory version of F, we get:
This time our function used 132 bytes per operation, due to our use of a slice in the function. If you are wondering why the number is 132 specifically: the exact number of bytes is sensitive to the numbers we use in the benchmark. The higher the input n, the more memory the function will allocate. The average of the values used in the benchmark (0, 10, 20, 30) is 15. Because this was compiled for a 64-bit machine, each int will use 8 bytes (8x8=64 bits). The slice headers also use some bytes. We still have zero heap allocations per operation, due to all variables being contained within the function. We will discuss advanced memory profiling and optimization techniques in Optimization.
Modulo vs Bitwise-and
In our Fibonacci benchmarks so far, we have made use of a list of four integer test cases:
6 var nums = []int{
7 0, 10, 20, 30,
8 }
which we then loop over in the BenchmarkF function:
13 for n := 0; n < b.N; n++ {
14 FastF(nums[n%m])
15 }
But when it comes down to the nanoseconds, modulo is a relatively slow computation to do on every iteration. It can actually have an impact on the accuracy of our results! Let’s peek at the Go assembler code. Go allows us to do this with the go tool compile -S command, which outputs a pseudo-assembly language called ASM. In the command below, we filter the instructions for the line we are interested in with grep:
The details of this output are not as important as it is to notice how many instructions there are. Now, let’s rewrite the code to use bitwise-and (&) instead of modulo %:
12 m := len(nums)-1
13 for n := 0; n < b.N; n++ {
14 FastF(nums[n&m])
15 }
Now, the ASM code becomes:
This is considerably shorter than before. In other words, the Go runtime will need to perform fewer operations, but the results will be the same. We can use modulo instead of ampersand because we have exactly four items in our nums slice. In general, \(n \% m == n \& (m - 1)\) if \(m\) is a power of two. For example,
If you are not yet convinced, expand the binary version of the bitwise-and operations to show that this is true:
0 & 3 = 00b & 11b = 0
1 & 3 = 01b & 11b = 1
2 & 3 = 10b & 11b = 2
3 & 3 = 11b & 11b = 3
...
To evaluate the impact of changing from modulo to ampersand on the benchmark results, let us create two benchmarks for FastF, one with modulo and the other with bitwise-and:
$ go test -bench=BenchmarkFastF
goos: linux
goarch: amd64
BenchmarkFastFModulo-4 100000000 16.7 ns/op
BenchmarkFastFBitwiseAnd-4 200000000 7.40 ns/op
PASS
ok _/Users/productiongo/benchmarks/bench_bitwise_and 4.058s
The version using bitwise-and runs twice as fast. Our original benchmark was spending half the time recalculating modulo operations! This is unlikely to have a big impact on benchmarks of bigger functions, but when benchmarking small pieces of code, using bitwise-and instead of modulo will make the benchmark results more accurate.
Tooling
In this chapter we will discuss some tooling you may find useful for writing and running production Go applications.
Godoc
Godoc uses the comments in your code to generate documentation. You can use it via the command line, or as an HTTP server where it will generate HTML pages.
To install, run:
go get golang.org/x/tools/cmd/godoc
You can run godoc on the standard library, or packages in your own GOPATH. To see the documentation for encoding/json for example, run:
$ godoc encoding/json
To see the documentation for a specific function, such as Marshal:
$ godoc encoding/json Marshal
To run the HTTP server locally:
$ godoc -http:6060
This will run a godoc HTTP server locally, where you can see the generated HTML documentation for packages in your GOPATH as well as the standard library.
There is also a hosted version of godoc at https://godoc.org. If you host your code on GitHub for example, godoc.org can generate the documentation for it. It’s a good idea to keep your comments clean and up to date in case others are checking your package’s page on godoc.org. Golint is useful for surfacing parts of your code that need comments.
More can be found at the official Go blog post “Godoc: documenting Go code”, but to summarize the main points:
“The convention is simple: to document a type, variable, constant, function, or even a package, write a regular comment directly preceding its declaration, with no intervening blank line.”
Package-level comments take the form of a comment directly above the package declaration, starting with “Package [name] …”, like so:
1 // Package sort provides primitives for sorting slices and user-defined
2 // collections.
3 package sort
but if your package comment is long, you can split it out into a separate doc.go file, which only contains the comment and a package clause. See net/http/doc.go for an example.
Go Guru
Go Guru is “a tool for answering questions about Go source code.”9 It ships with a command-line tool, but you can also integrate it into an editor. You can install it with:
go get -u golang.org/x/tools/cmd/guru
You can find a list of supported editor integrations on the Using Go Guru document linked in the godoc for guru. In this section we’ll show screenshots of guru in vim.
We’ll assume you have vim-go installed; if not please see the “Editor Integration” section of the “Installing Go” chapter in the beginning of this book.
Let’s take a look at one of the questions that guru answers for us: “what concrete types implement this interface?”
Navigate to the line that contains an interface, and type :GoImplements:
Then hit Enter. Your vim window should split and your cursor should be in the quickfix list on the bottom half, with a list of files containing concrete structs that implement the interface:
Hit Enter on any one of those and you’ll jump to the struct definition in the listed file. To get back to the list, do <Ctrl-W> j. Then you can scroll through it as before, or quit out of it as usual with :q.
Go ahead and try some of the other guru commands like :GoReferrers, :GoCallees, and :GoCallers. You can find more help on guru for vim-go at the vim-go-tutorial, and a list of guru queries and other help output with guru -help.
Race Detector
Go comes with a builtin mechanism for detecting race conditions. There are multiple ways to invoke it:
In this section we’ll write some code that contains a data race, and catch it with a test with the race detector enabled. A data race occurs when two goroutines try to access the same object in memory concurrently, one of which is trying to write to the object, and there is no lock in place to control access to the object.
1 package cat
2
3 import (
4 "log"
5 )
6
7 // Cat is a small, carnivorous mammal with excellent night vision
8 type Cat struct {
9 noise string
10 }
11
12 // SetNoise sets the noise that our cat makes
13 func (c *Cat) SetNoise(n string) {
14 c.noise = n
15 }
16
17 // Noise makes the cat make a noise
18 func (c *Cat) Noise() string {
19 return c.noise
20 }
21
22 func updateCat(c *Cat) {
23 go c.SetNoise("にゃん")
24
25 log.Println(c.Noise())
26 }
In the above code, we execute a goroutine that sets the Noise attribute of the Cat argument to "にゃん". That goroutine goes off and runs in the background and the flow of execution continues to where we attempt to log c.Noise. This causes a race condition as we might write the value in the goroutine at the same time as reading it in the log.Println call.
Without considering the race condition and reading the code from top to bottom, we expect updateCat to set the noise attribute for the passed-in "cat" to "にゃん". So let’s write a test that makes that assertion for us:
1 package cat
2
3 import (
4 "testing"
5 )
6
7 func TestUpdateCat(t *testing.T) {
8 c := Cat{noise: "meow"}
9 c = updateCat(c)
10 if got := c.Noise(); got != "にゃん" {
11 t.Fatalf("c.Noise() = %q, want %q", got, "にゃん")
12 }
13 }
When running this test normally with go test, we might get lucky and it will pass:
When we run the test with the race detector enabled, however, we’ll see something different:
That’s a lot of output, but let’s take a look at the first two blocks of text:
This tells us exactly where our data race is. Our package’s filename is cat.go, so if we narrow it down to the lines containing our file, we can see the write occurred here:
and the read here:
And indeed if we check our code, those are the lines where we attempt to set c.noise, as well as read c.noise.
So how do we fix this? We’re going to need a lock around our data structure. We’ll use a sync.Mutex to lock our Cat structure whenever we read or write to it:
1 package cat
2
3 import (
4 "log"
5 "sync"
6 )
7
8 // Cat is a small, carnivorous mammal with excellent night vision
9 type Cat struct {
10 mu sync.Mutex
11 noise string
12 }
13
14 // SetNoise sets the noise that our cat makes
15 func (c *Cat) SetNoise(n string) {
16 c.mu.Lock()
17 defer c.mu.Unlock()
18 c.noise = n
19 }
20
21 // Noise makes the cat make a noise
22 func (c *Cat) Noise() string {
23 c.mu.Lock()
24 defer c.mu.Unlock()
25
26 return c.noise
27 }
28
29 func updateCat(c *Cat) *Cat {
30 go c.SetNoise("にゃん")
31
32 log.Println(c.Noise())
33
34 return c
35 }
Now we run the test again:
and the race condition is fixed.
You can also build your application with the race detector enabled, and see potential data races while running the application, or during an integration test. As an example, let’s make an API that accepts user-contributed entries about the countries the user has visited, and a description of their trip:
1 package main
2
3 import (
4 "encoding/json"
5 "flag"
6 "fmt"
7 "log"
8 "net/http"
9 "strings"
10 "unicode"
11 )
12
13 var (
14 addr = flag.String("http", "127.0.0.1:8000", "HTTP listen address")
15 countries = map[string]string{
16 "england": "England",
17 "japan": "Japan",
18 "southafrica": "South Africa",
19 "unitedstates": "United States",
20 }
21 )
22
23 // Entry is a user-submitted entry summarizing their trip
24 // to a given country
25 type Entry struct {
26 Country string
27 Text string
28 }
29
30 // removeSpaces is a function for strings.Map that removes
31 // all spaces from a string
32 func removeSpaces(r rune) rune {
33 if unicode.IsSpace(r) {
34 return -1
35 }
36 return r
37 }
38
39 // normalizeCountry takes an Entry and normalizes its country name
40 func normalizeCountry(e *Entry) error {
41 co := strings.Map(removeSpaces, strings.ToLower(e.Country))
42 if _, ok := countries[co]; !ok {
43 return fmt.Errorf("invalid country %q", e.Country)
44 }
45
46 e.Country = countries[co]
47
48 return nil
49 }
50
51 // EntryPostHandler is the POST endpoint for entries
52 func EntryPostHandler(w http.ResponseWriter, req *http.Request) {
53 decoder := json.NewDecoder(req.Body)
54 defer req.Body.Close()
55 var entry Entry
56 err := decoder.Decode(&entry)
57 if err != nil {
58 log.Printf("ERROR: could not decode Entry: %s", err)
59 w.WriteHeader(http.StatusBadRequest)
60 w.Write([]byte(fmt.Sprintf("Could not decode Entry: %s\n", err)))
61 return
62 }
63
64 go func(e *Entry) {
65 normalizeCountry(e)
66 }(&entry)
67
68 log.Printf("INFO: received Entry %v", entry)
69 }
70
71 func main() {
72 http.HandleFunc("/entries", EntryPostHandler)
73
74 log.Printf("Running on %s ...", *addr)
75 log.Fatal(http.ListenAndServe(*addr, nil))
76 }
You can probably spot the race condition already - we’re trying to normalize the country name in a goroutine in the background, then immediately trying to log the entry. Running the server with go run normally won’t give us any errors:
and we can even POST an Entry:
we then see this on the server:
which is wrong - we’re supposed to be normalizing the country name to “England”. Let’s see what happens when we run the server with -race enabled:
That looks fine, but now let’s try to POST an Entry again:
and there is our data race. We could fix this race in a similar manner to the way we fixed the cat race earlier, but instead let’s try using a sync.WaitGroup:
1 package main
2
3 import (
4 "encoding/json"
5 "flag"
6 "fmt"
7 "log"
8 "net/http"
9 "strings"
10 "sync"
11 "unicode"
12 )
13
14 var (
15 addr = flag.String("http", "127.0.0.1:8000", "HTTP listen address")
16 countries = map[string]string{
17 "england": "England",
18 "japan": "Japan",
19 "southafrica": "South Africa",
20 "unitedstates": "United States",
21 }
22 )
23
24 // Entry is a user-submitted entry summarizing their trip
25 // to a given country
26 type Entry struct {
27 Country string
28 Text string
29 }
30
31 // removeSpaces is a function for strings.Map that removes
32 // all spaces from a string
33 func removeSpaces(r rune) rune {
34 if unicode.IsSpace(r) {
35 return -1
36 }
37 return r
38 }
39
40 // normalizeCountry takes an Entry and normalizes its country name
41 func normalizeCountry(e *Entry) error {
42 co := strings.Map(removeSpaces, strings.ToLower(e.Country))
43 if _, ok := countries[co]; !ok {
44 return fmt.Errorf("invalid country %q", e.Country)
45 }
46
47 e.Country = countries[co]
48
49 return nil
50 }
51
52 // EntryPostHandler is the POST endpoint for entries
53 func EntryPostHandler(w http.ResponseWriter, req *http.Request) {
54 decoder := json.NewDecoder(req.Body)
55 defer req.Body.Close()
56 var entry Entry
57 err := decoder.Decode(&entry)
58 if err != nil {
59 log.Printf("ERROR: could not decode Entry: %s", err)
60 w.WriteHeader(http.StatusBadRequest)
61 w.Write([]byte(fmt.Sprintf("Could not decode Entry: %s\n", err)))
62 return
63 }
64
65 var wg sync.WaitGroup
66 wg.Add(1)
67 go func(e *Entry) {
68 defer wg.Done()
69 normalizeCountry(e)
70 }(&entry)
71 wg.Wait()
72
73 log.Printf("INFO: received Entry %v", entry)
74 }
75
76 func main() {
77 http.HandleFunc("/entries", EntryPostHandler)
78
79 log.Printf("Running on %s ...", *addr)
80 log.Fatal(http.ListenAndServe(*addr, nil))
81 }
You can see that we now have a sync.WaitGroup, onto which we add a delta of 1 to the counter. Inside the goroutine we decrement the counter with defer wg.Done(), then we block until the counter is zero with wg.Wait(). Since we’re blocking until the goroutine finishes, there is no longer a data race:
and we see that our country is normalized now to “England”.
Go Report Card
Full disclosure: we are the authors of this free and open source tool.
Go Report Card is a web application that gives packages a grade based on how well they pass various linters and tools. It is a popular application in the open source community, with thousands of projects using the badge to indicate the code quality. You can try it on goreportcard.com if your source code is open source, or run the server locally to use the tool on your internal network or private repositories.
Staticcheck
Staticcheck is a static analysis tool for Go programs. It helps for simplifying code and catches some issues such as unused error values. It also shows some style suggestions such as suggesting that the first character of an error string should not be capitalized.10 Here is an example of running staticcheck on the Go Report Card source:
And here is another example of staticcheck being run on code that doesn’t set an error value properly:
1 package main
2
3 import (
4 "errors"
5 "fmt"
6 )
7
8 func badFunc() error {
9 return errors.New("an error has occurred")
10 }
11
12 func foo() error {
13 return nil
14 }
15
16 func main() {
17 var err error
18
19 if err := foo(); err == nil {
20 err = badFunc()
21 }
22
23 fmt.Println(err)
24 }
We might think that the err = badFunc() line would set the outside var err error to errors.New("an error has occurred"), but actually when we print the err in the main scope it prints <nil>:
Staticcheck will alert us that the err value inside the if statement is never used:
Go-fuzz
Go-fuzz is a randomized testing tool for Go packages. Fuzzing is a method of testing code by providing various types of input to a program and monitoring that program for crashes or memory leaks. Having an automated fuzzing process is a great way to get ahead of potential input bugs and allows us to write more robust programs.
To download go-fuzz, run:
go get -u github.com/dvyukov/go-fuzz/go-fuzz github.com/dvyukov/go-fuzz/go-fuzz-build
Now we must add a Fuzz function in the package we want to fuzz. Let’s use Nihongo.io, a Japanese dictionary written in Go, as an example:
1 package dictionary
2
3 import (
4 "compress/gzip"
5 "log"
6 "os"
7 )
8
9 func Fuzz(data []byte) int {
10 file, err := os.Open("../../data/edict2.json.gz")
11 if err != nil {
12 log.Fatal("Could not load edict2.json.gz: ", err)
13 }
14 defer file.Close()
15
16 reader, err := gzip.NewReader(file)
17 if err != nil {
18 log.Fatal("Could not create reader: ", err)
19 }
20
21 dict, err := Load(reader)
22 if err != nil {
23 log.Fatal("Could not load dictionary: ", err)
24 }
25
26 dict.Search(string(data), 10)
27
28 return 1
29 }
This file exists in the github.com/gojp/nihongo/lib/dictionary package. We then run go-fuzz-build and then go-fuzz (in our case with a high `-timeout flag since it takes some time to load the dictionary) to start the fuzzing:
and we see that fuzzer is running successfully.
According to the go-fuzz README:
The function must return 1 if the fuzzer should increase priority of the given input during subsequent fuzzing (for example, the input is lexically correct and was parsed successfully); -1 if the input must not be added to corpus even if gives new coverage; and 0 otherwise; other values are reserved for future use.
govalidate
govalidate is a useful command for validating your Go environment. It runs such checks as whether you have the latest Go version installed, whether your $PATH environment variable contains $GOPATH/bin, and others.
Pinning tools with Go modules
It’s good to ensure that new contributors can clone a repo and run all the necessary tools. If a project enforces in the build that staticcheck must pass, contributors should be able to run it locally. One way to do this is with a tools.go file.
Let’s assume we want contributors and our build to run staticcheck. We create a tools/ directory with a tools.go file:
1 // +build tools
2
3 package tools
4
5 import (
6 _ "honnef.co/go/tools/cmd/staticcheck"
7 )
Our Makefile can have:
and when we run make staticcheck, if we don’t already have staticcheck installed, go will install it based on the generated entry in go.mod.
Sometimes we want our projects to use specific versions of these tools. To do this, modify the tool’s version in go.mod.
Security
In this chapter, we’ll discuss a few ways to improve the security of our Go server. Most are quick and easy to implement. In reality a large production environment will require a robust security setup with a Web Application Firewall (WAF) and other tools, but here we will only discuss improvements for our Go code.
Keep Go Up to Date
One of the most important things we can do to ensure the security of our Go environment is to keep our Go version up to date.
The golang-announce mailing list will announce (in a message prefixed with [security]) when a minor revision will be released with security fixes. For example, Go version 1.11.3 was released in December 2018 with a few security fixes, including a fix in go get -u for a remote execution bug.
See more about Go’s security policy here.
CSRF
Cross-Site Request Forgery, or CSRF, is “an attack that forces an end user to execute unwanted actions on a web application in which they’re currently authenticated.”11
Imagine a user is logged into a bank’s website and receives a link from someone with malicious intent. The HTML of the malicious page redirects the user to the bank’s website and the money transfer form is pre-filled with the malicious user’s account and routing information. The malicious HTML might look like this:
1 <!DOCTYPE html>
2 <html>
3 <head>
4 <meta charset="UTF-8" />
5 <title>CSRF</title>
6 </head>
7 <body onload="document.forms[0].submit()">
8 <form action="http://insecurebank.com/transfer" onsubmit="" method="POST">
9 <input type="hidden" name="account" value="5555555555" />
10 <input type="hidden" name="routing" value="555555555" />
11 <input type="hidden" name="action" value="save" />
12 </form>
13 </body>
14 </html>
This attack might work if the bank is not protected against CSRF attacks.
To prevent CSRF attacks in our application, we must generate a unique per-user CSRF token and embed it in our HTML forms as a hidden field. The token is generated per-session, and must be sufficiently unique such that an attacker cannot guess it. If our web server doesn’t receive a CSRF token in a POST, or the token received doesn’t match the token from the user’s session, the request should be denied.
Luckily there are packages such as nosurf and gorilla/csrf which make it easy to protect our web application against CSRF attacks.
HTTP Strict Transport Security (HSTS)
HTTP Strict Transport Security helps protect websites against protocol downgrade attacks and cookie hijacking.12
Setting the Strict-Transport-Security on our web server will allow us to tell clients (web browsers) to only access our site over HTTPS, and never HTTP. A max-age expiration in seconds must be set with the header. In an example Go web server it could look like this, with a wrapper handler:
1 package main
2
3 import (
4 "flag"
5 "fmt"
6 "log"
7 "net/http"
8 )
9
10 var (
11 addr = flag.String("http", ":8000", "HTTP listen address")
12 )
13
14 func headerWrap(h http.HandlerFunc) http.HandlerFunc {
15 return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
16 w.Header().Add("Strict-Transport-Security", "max-age=31536000; includeSubDo\
17 mains")
18 h.ServeHTTP(w, r)
19 })
20 }
21
22 func homeHandler(w http.ResponseWriter, r *http.Request) {
23 fmt.Fprint(w, "Hello, world")
24 }
25
26 func main() {
27 flag.Parse()
28 http.HandleFunc("/", headerWrap(homeHandler))
29
30 log.Printf("Running on %s ...", *addr)
31 log.Fatal(http.ListenAndServe(*addr, nil))
32 }
If we start the server and then run curl -i localhost:8000 in another terminal, we can see that the header is set:
We would wrap all of our handlers with headerWrap to ensure they all have the proper headers set.
Another option would be to set the header in the web server configuration, such as in /etc/nginx/sites-available/[config].
Content Security Policy (CSP)
A Content-Security-Policy header helps prevent attacks such as cross-site scripting (XSS). The CSP header value allows us to specify the origins of our content.
For example, to ensure that all content comes from the site’s own origin, the following header can be set in the response:
1 Content-Security-Policy: default-src 'self'
SQL Injection
SQL injection is a common website vulnerability in which an attacker enters a query into a text input box in order to either retrieve more data than is meant to be returned, or to run arbitrary commands on the database. Any backend code which creates SQL strings directly from HTML forms is vulnerable. For example, if we have code in our backend for a form on our website that looks like this:
1 username := r.FormValue("username")
2 query := "SELECT * from users WHERE username='" + username + "';"
then an attacker could put the following malicious query into the HTML form that passes username to the backend:
1 ' OR 1=1; '
and we would end up with a query that looks like this:
1 SELECT * from users WHERE username='' OR 1=1; '';
This is problematic because now the query is selecting everything from the users table rather than scoping the query to a single username. This is a simplified example; there are many ways a query could be vulnerable, and many SQL injection queries besides our ' OR 1=1; ' example. And attackers will often use tools which try lots of different known SQL injections on webpages to see if the results they get back are anomalous and showing them that a page is vulnerable.
The way we can protect against SQL injection is by using what’s called “prepared statements.” Prepared statements are precompiled SQL statements which then insert values into the correct positions in the query. Go’s database/sql package does this for us automatically:
1 query := "SELECT * from users WHERE username=?"
2 username := r.FormValue("username")
3 rows, err := db.Query(query, username)
4 if err != nil {
5 log.Fatal(err)
6 }
7 defer rows.Close()
8 ...
See Also
- https://github.com/microcosm-cc/bluemonday
- https://github.com/unrolled/secure
- https://infosec.mozilla.org/guidelines/web_security
- https://developer.mozilla.org/en-US/docs/Web/HTTP/CSP
- https://en.wikipedia.org/wiki/SQL_injection
- https://www.owasp.org/index.php/SQL_Injection
- https://golang.org/pkg/database/sql/#DB.Prepare
- https://golang.org/pkg/database/sql/#DB.Query
Going into Production
In the preceding chapters we covered a lot of ground, from Go basics to advanced corner cases that production systems often need to concern themselves with. By now, your application should be running locally, well-tested, documented and ready for its first users. Good job! It’s time to show it to the world. This is an exciting phase, but it comes with a whole new set of challenges. Your code is about to move from development to production, changing things forever. In this chapter, we present a high-level checklist of what it takes to get a modern Go application production-ready and running on the Internet, hopefully serving many satisfied users.
Continuous Integration & Continuous Deployment (CI/CD)
Continuous integration is a general term that describes the process of running tests and builds automatically whenever a team member adds new code to the codebase. Continuous integration in a production-ready Go system often also includes running miscellaneous checks and linters.
It is possible to write your own continuous integration system, but the effort is likely not worth it. There are many existing continuous integration systems out there. Some, like Travis CI, are hosted for you and are free for open source projects. Others, like Drone, are open source and free for self-hosting, but also comes with enterprise versions. We tabulated some popular options that currently support Go in the following table:
| CI Service | Open Source | Configuration | Free version available |
|---|---|---|---|
| Travis CI | No | .travis.yml | Yes, for open source projects |
| Drone CI | Yes | .drone.yml that describes | Yes, as well as enterprise |
| Docker build steps | options. Always self-hosted | ||
| Jenkins | Yes | Jenkinsfile, either | Yes, if self-hosted. Paid hosted |
| declarative or scripted | versions also exist. | ||
| CircleCI | No | .circleci/config.yml | Yes, for open source projects |
| hosted on GitHub or Bitbucket | |||
| GitHub Actions | No | .github/workflows/actions.yml | Yes, for open source projects |
| hosted on GitHub |
In larger organizations, the decision of which system to use will likely already have been made for you. But if you’re doing this on your own or in a team without an existing CI/CD solution, you should bias towards simplicity so that your energy can be focused on improving the application itself. Github Actions and Travis CI are two good, fully managed solutions for cost-conscious small projects. Jenkins and Drone CI, in contrast, are open source options that can be run on your own infrastructure for free. These are good solutions if you are willing to put in more time to save a few dollars. The best choice will be project-dependent, and we encourage you to do your own research.
Continuous Deployment (CD) is similar to Continuous Integration in that it is an automated process that kicks off as soon as code is pushed to the central repository. It is usually, but not always, run on the same system as CI. After CI steps complete, a CD process will automatically deploy the latest version to Staging and/or Production environments, or make images available in a central repository for later deployment, ideally with a click of a button. Different engineers can have differing preferences when it comes to how often, and in how automated of a fashion, code should be deployed. This can even be a contentious topic, if not handled with care. You (and your team) will need to decide where you would like to fall on the scale between manual ad-hoc deployments over SSH and fully automated deployments as soon as code is pushed. That said, there is strong evidence that high-performing teams are consistently pushing themselves to get closer to the automated side of this scale 13. Intuitively, fewer manual in a deployment lead to fewer mistakes, less time wasted and more frequent feedback from users.
Deployment
This section is a work in progress.
Logging
This section is a work in progress.
Monitoring
The key difference between running code in development and running code in production is that in production, you are no longer an active part of the loop. During development, you might notice an error or a bug and fix it. But in production, unless you have good observability over your system as it runs and others use it, you will not know when the system encounters problems. You may not be watching the logs, or you may be asleep! Besides, who has the time to stare at server logs all day long? Instead, you want to be sure that if something goes wrong, you will be notified and able to diagnose the issue quickly. This is a critical part of a production-ready web application, and is the focus of this chapter.
Metrics and Dashboards
For reasons of redundancy, latency or scale, a production web application is typically deployed to multiple servers. As soon as this happens, it becomes important to have high-level visibilty over what is happening on each of the servers, without needing to log into them first. Centralized logging, discussed in Logging, is one way to send logs from all your servers to a centralized location for ease of use. This can allow you to discover errors and even get the information you need to debug them. However, logs alone don’t always tell the full story. If there are no error logs, does that mean users are not experiencing any issues? Or can it be that the server stopped running? Or stopped sending logs? How much disk and memory pressure do we have on each server, is there likely to be an outage soon? Are users experiencing acceptable latency?
A production application should be coupled with a dashboard that can answer these questions (and many others) from a high level. Dashboards like these can be powered by logs, but typically are not. Instead, engineers turn to specialized timeseries databases to collect high-level metrics across all servers. These are then displayed on a web interface where a developer acquainted with the system can tell at a glance whether anything is out of place.
Alerting
This section is a work in progress.
Further Reading
The Go Programming Language Specification
Effective Go is a good overview document for how to write idiomatic Go code.
Golang Weekly is a weekly newsletter about Go.
The Go Blog is the official blog for Go.
Gophers Slack is a Slack community of Go developers. There is a #golang-newbies channel, #performance, #golang-jobs, and many more.
Acknowledgements
The Go gopher was designed by Renée French.
Licenses
The Go source code license
github.com/gorilla/mux
Notes
1https://golang.org/doc/faq#no_pointer_arithmetic↩
2https://godoc.org/golang.org/x/tools/cmd/goimports↩
3你好, pronounced nĭ hăo, is Hello in Chinese. ↩
4Remember, Go does not support function overloading, so a single character should be passed as a string if the function expects a string, like most functions in the strings standard library.↩
5https://golang.org/pkg/fmt/#pkg-overview↩
6https://golang.org/doc/articles/race_detector.html↩
7https://golang.org/pkg/sync/#Map↩
8https://github.com/golang/go/wiki/CodeReviewComments#useful-test-failures↩
9https://godoc.org/golang.org/x/tools/cmd/guru↩
10https://github.com/golang/go/wiki/CodeReviewComments#error-strings↩
11https://www.owasp.org/index.php/Cross-Site_Request_Forgery_(CSRF)↩
12https://en.wikipedia.org/wiki/HTTP_Strict_Transport_Security↩
13Accelerate: The Science of Lean Software and DevOps: Building and Scaling High Performing Technology Organizations↩