Working with Legacy Systems
Working with Legacy Systems
A Practical Guide to the Systems we Inherit and Maintain
About the Book
There comes a point in everyones I.T. career when they become responsible for a legacy system. This is inevitable and I call it the Penelope Principle. Like the Peter Principle (where people are promoted to their level of incompetence) and the Dilbert Principle (incompetent workers are promoted to where they can do least damage - management) this should be accepted and worked with rather than being fought against. It is stated thus:
All I.T. workers will be promoted into a position where they become responsible for a Legacy System
However the I.T. industry is obsessed with new technologies and new projects. University courses, books, magazines and conferences focus on what is new and assume you always start with a clean slate. This isn't what actually occurs in the real world and I hope what follows fills some of this gap. This book is NOT intended to present a formal methodology but is aimed at all the Penelopes out there who need a guidebook to help them with their first legacy system.
About the Author
Bundles that include this book
Table of Contents
-
- Preface
-
Part 1 - Definition, Issues and Strategy
-
What is Legacy?
- An IT centric view of the world
- Systems Development in Context
- Systems Development Scaled with Time
- Examples of Real Legacy Systems
-
Common Issues
- No Documentation
- Lost Knowledge
- Hidden Knowledge
- Unused Functionality
- No Coherent Design/Inconsistent Implementation
- Fragility (vs. Stability)
- Tight Coupling
- Loose Coupling
- Zombie Technologies
- Licensing
- Regulation
- Politics
- Organisation Constraints and Change are reflected in the System
- External Processes have evolved to Fit around the System
- External Systems have evolved to Fit around the System
- Decaying data
- Now What?
-
Reasons to be Cheerful
- A legacy system is a successful system
- You have real users you can talk to
- You can learn a lot about the business
- You can have a large impact quickly
- It’s important, but not trendy, so you’ll get paid well!
-
Strategies
- Ignore it
- Investigation
- Maintenance
- Upgrade
- Migration
- Incremental Improvement
- Replacement
- A special note on Decommissioning
- Conclusion
-
What is Legacy?
-
Part 2 - Investigation and System Review
- Architectural Description and Review
-
Stakeholders
- Locate the stakeholders
- End Users
- Support Staff
- System’s Infrastructure Staff
- Audit and Compliance
- Budget Holders
- Third Party Suppliers
- Developers/Software Engineers/Database Analysts
- Document the Stakeholders
-
Architectural Sketches
- Why Sketch?
- Context Diagram
- Container Diagram
- Component Diagram
- C4 - Classes
-
Further Analysis
- What else can be done?
- Further Static Analysis
- Further Dynamic Analysis
-
Security Considerations
- Passwords
- Account Maintenance and Lifecycle
- Cryptographic Keys
- Certificates
- Third Party Passwords and Keys
- Lack of Patching
- Moving from Closed to Open Networks
- Modernising Data Formats
- New Development and Copied Data
- Server and Infrastructure Consolidation
- Improved Backup Processes
- Checklist of Questions and Actions
-
Part 3 - Making Changes
-
Safely Making Changes
- Virtualisation is your Friend
- 50,000 Foot view of the virtualisation process
- Actions to make a system safe to change
-
Preparation Issues
- External Dependencies Required for Testing
- Duplicate Test System Data Becomes Stale Quickly
- Internal Coupling Stops You Swapping Components
- Changes Cause Unexpected Errors
- Licensing and Regulation Issues Running Multiple Copies
- Structural Changes for Disaster Recovery/High Availability
-
Stabilisation
- Why?
- What have we Achieved Already?
- Bad Data
- Removing Unwanted Data
- Data Cleansing
- Removing Unneeded Components
- Removing Unneeded Tasks
- Process Cleansing
- Removing Unused User Options
- Don’t Underestimate a Resource Boost
- Re-Examine your Application Settings
- Optimise the User’s Actions
-
Leaving a Good Legacy
- Why Bother Leaving a Good Legacy?
- Documentation
- Tests
- Design Considerations
-
Safely Making Changes
-
Appendices
-
Appendix 1 - Example Legacy Scenario
- Very Brief Overview
- Why this example?
-
Appendix 2 - Legacy Project Questions
- Why a List?
- References and Bibliography
- Version History
- Acknowledgements and Thanks to:
-
Appendix 1 - Example Legacy Scenario
- Notes
The Leanpub 60-day 100% Happiness Guarantee
Within 60 days of purchase you can get a 100% refund on any Leanpub purchase, in two clicks.
See full terms
Do Well. Do Good.
Authors have earned$12,124,424writing, publishing and selling on Leanpub, earning 80% royalties while saving up to 25 million pounds of CO2 and up to 46,000 trees.
Learn more about writing on Leanpub
Free Updates. DRM Free.
If you buy a Leanpub book, you get free updates for as long as the author updates the book! Many authors use Leanpub to publish their books in-progress, while they are writing them. All readers get free updates, regardless of when they bought the book or how much they paid (including free).
Most Leanpub books are available in PDF (for computers) and EPUB (for phones, tablets and Kindle). The formats that a book includes are shown at the top right corner of this page.
Finally, Leanpub books don't have any DRM copy-protection nonsense, so you can easily read them on any supported device.
Learn more about Leanpub's ebook formats and where to read them
Top Books
- #1
Mastering STM32 - Second Edition
Carmine NovielloWith more than 1200 microcontrollers, STM32 is probably the most complete ARM Cortex-M platform on the market. This book aims to be the most complete guide around introducing the reader to this exciting MCU portfolio from ST Microelectronics and its official CubeHAL and STM32CubeIDE development environment.
- #2
Functional Programming Made Easier
Charles ScalfaniA Functional Programming book from beginner to advanced without skipping a single step along the way.
In my 40 years of programming, I've felt that programming books always let me down, especially Functional Programming books. So, I wrote the book I wish I had 5 years ago.
Functional Programming will never be easy, but it can be easier.
- #3
Jetpack Compose internals
Jorge CastilloJetpack Compose is the future of Android UI. Master how it works internally and become a more efficient developer with it. You'll also find it valuable if you are not an Android dev. This book provides all the details to understand how the Compose compiler & runtime work, and how to create a client library using them.
- #4
R Programming for Data Science
Roger D. PengThis book brings the fundamentals of R programming to you, using the same material developed as part of the industry-leading Johns Hopkins Data Science Specialization. The skills taught in this book will lay the foundation for you to begin your journey learning data science. Printed copies of this book are available through Lulu.
- #5
Windows 11 Field Guide
Rafael Rivera and Paul ThurrottThe Windows 11 Field Guide is a full-length e-book about the latest version of Microsoft Windows, aimed at those users who will upgrade from Windows 10 or acquire Windows 11 with a new PC.
- #6
node-opcua by example
Etienne RossignonGet the best out of node-opcua through a set of documented examples by the author himself that will allow you to create stunning OPCUA Servers or Clients.
- #7
C++20 - The Complete Guide
Nicolai M. JosuttisAll new language and library features of C++20 (for those who know previous C++ versions).
The book presents all new language and library features of C++20. Learn how this impacts day-to-day programming, to benefit in practice, to combine new features, and to avoid all new traps.
Buy early, pay less, free updates.
Other books:
- #8
Introducing EventStorming
Alberto BrandoliniThe deepest tutorial and explanation about EventStorming, straight from the inventor.
- #9
Node-OPCUA by example - edition 2022
Etienne RossignonGet the best out of node-opcua through a set of documented examples by the author himself that will allow you to create stunning OPCUA Servers or Clients.
- #10
Functional Event-Driven Architecture
Gabriel VolpeExplore the event-driven architecture (EDA) in a purely functional way. Learn to design and develop distributed systems that scale. Identify common design patterns in such systems.
Take your functional programming skills to the next level by joining me in developing a distributed system powered by Apache Pulsar and Fs2 streams, all in Scala 3!
Top Bundles
- #1
1500 QUIZ COMMENTATI (3 libri)
3 Books
Tre libri dei QUIZ MMG Commentati al prezzo di DUE! I QUIZ dei concorsi ufficiali di Medicina Generale relativi agli anni: 2000-2001-2003-2012-2013-2014-2015-2016-2017-2018-2019-2020-2021 +100 inediti Raccolti in unico bundle per aiutarvi nello studio e nella preparazione al concorso. All'interno di ogni libro i quiz sono stati suddivisi per... - #2
CCIE Service Provider Ultimate Study Bundle
2 Books
Piotr Jablonski, Lukasz Bromirski, and Nick Russo have joined forces to deliver the only CCIE Service Provider training resource you'll ever need. This bundle contains a detailed and challenging collection of workbook labs, plus an extensively detailed technical reference guide. All of us have earned the CCIE Service Provider certification... - #3
Pattern-Oriented Memory Forensics and Malware Detection
2 Books
This training bundle for security engineers and researchers, malware and memory forensics analysts includes two accelerated training courses for Windows memory dump analysis using WinDbg. It is also useful for technical support and escalation engineers who analyze memory dumps from complex software environments and need to check for possible... - #4
Software Architecture
2 Books
"Software Architecture for Developers" is a practical and pragmatic guide to modern, lightweight software architecture, specifically aimed at developers. You'll learn:The essence of software architecture.Why the software architecture role should include coding, coaching and collaboration.The things that you really need to think about before... - #6
The Python Craftsman
3 Books
The Python Craftsman series comprises The Python Apprentice, The Python Journeyman, and The Python Master. The first book is primarily suitable for programmers with some experience of programming in another language. If you don't have any experience with programming this book may be a bit daunting. You'll be learning not just a programming... - #10
Statistics - All Lee Baker's Books
10 Books
This bundle includes all of the statistics books that Lee Baker has published at LeanPub 13 Statistics Books*1 PlaceNo Hassle! All in both epub and mobi formats - perfect for all devices and screen sizes. --------------------------------------------------------- *The book listed as Getting Started With Statistics is a bundle of 4 books...
