Note: The book is 95% complete. The current version is missing Chapter 14 (User Interface). While there is no fixed release date, the first version of the book should be 100% complete mid-September 2020.

Abstract

This book explains and illustrates how to implement Domain-Driven Design, Command Query Responsibility Segregation and Event Sourcing in Node.js. The reader is taught to build software that is problem centered, well designed, event-based, reactive and scalable. Domain-Driven Design (DDD) is a methodology that focuses on the actual problem to solve and its involved knowledge areas. Command Query Responsibility Segregation (CQRS) promotes to separate a software into a write side and a read side. Event Sourcing is an architectural pattern where state is represented as a series of immutable events. All concepts are described in theory and put into practice with standalone examples and a fully functional sample application. This is done without third party technologies or frameworks. The code is written in JavaScript and uses Node.js as runtime. This book includes a source code bundle with all standalone examples and the sample application implementation. The bundle contains more than 5000 lines of JS code.

Target audience

The target reader is experienced with Node.js and wants to implement DDD, CQRS and Event Sourcing without deep-diving into theory. Naturally, this also implies advanced skills in JavaScript. The second potential reader has theoretical knowledge about the mentioned concepts, but struggles with translating them into actual code. For such a person, it might be required to build up the respective JavaScript and Node.js knowledge. Apart from the first four chapters, which cover the more conceptual parts, this book heavily focuses on code and implementation. Also, some of the covered topics are not explained to their full extent, but rather as far as needed. As a consequence, the book is not recommended to be used as a reference.

Chapter order and structure

The chapter order in this book is determined by what makes most sense in terms of building the sample application. As a consequence, the topics are laid out in a way so they can build upon each other. This also reduces the likelihood of referring to terms before they are explained. Every chapter has the same structure. First, the respective concepts are introduced and defined. Then, they are described in detail and illustrated either with drawings or code examples. At the end of each chapter, the discussed concepts are applied to the sample application. This is done by describing the working steps to take and by showing the according drawings or code. An exception is chapter 5, where there is no sample application section.

Programming paradigms

The content and the code examples in this book compromise an overall combination of imperative, declarative, object-oriented and functional programming. However, most parts apply the object-oriented paradigm and therefore partially contradict with a functional approach. There are different motivations for this. For one, the primary goal is to use the most prevalent style with regards to the target reader. Furthermore, at the time of writing, I am no expert in the functional paradigm and cannot apply and explain it adequately. Still, over the course of the book the domain-related implementations transition towards a more functional style. Also, individual infrastructural functionalities apply selected principles of it wherever useful. As these changes come automatically together with introducing specific concepts, there is no need for specialized upfront knowledge.

Code examples

This book includes a bundle that contains the implementations for all standalone examples and the sample application. Also, it provides the possibility to run the code when reading on a computer. For this purpose, the bundle includes a playground server that can execute Node.js code and display its output. Almost every implementation shown in the book is accompanied with a hyperlink labeled as "run code" or "run code usage". Clicking such a link opens a web page with an editable text area, an output window and control buttons.