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.
Now, this is technically risky for us, since you'll have the book or course files either way. But we're so confident in our products and services, and in our authors and readers, that we're happy to offer a full money back guarantee for everything we sell.
You can only find out how good something is by trying it, and because of our 100% money back guarantee there's literally no risk to do so!
So, there's no reason not to click the Add to Cart button, is there?
See full terms...
Earn $8 on a $10 Purchase, and $16 on a $20 Purchase
We pay 80% royalties on purchases of $7.99 or more, and 80% royalties minus a 50 cent flat fee on purchases between $0.99 and $7.98. You earn $8 on a $10 sale, and $16 on a $20 sale. So, if we sell 5000 non-refunded copies of your book for $20, you'll earn $80,000.
(Yes, some authors have already earned much more than that on Leanpub.)
In fact, authors have earnedover $14 millionwriting, publishing and selling on Leanpub.
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
