Strangler application patterns and antipatterns

A few observations about what makes success more likely in application rewrite effort

1. DEALING WITH LEGACY SOFTWARE PATTERNS AND ANTIPATTERNS Matteo Vaccari @xpmatteo
2. Get a whiteboard, put on the colored stickies, and try not to be a pain in the arse. #agile #inanutshell
3. A company and its software
5. Legacy code == code that is difficult to change
6. The core domain of a profitable company is always complex
7. Refactor or rewrite? What to do with legacy code?
8. Why do we fear the big rewrite?
9. http://www.bonkersworld.net/building-software/
10. Things going against the big rewrite • Same people and same methods: they will get the same results • The old software contains knowledge and details that we’re not aware of • While we are busy rewriting the old features, competitors are busy implementing new ones • Why is there always time to do things over, but there is never time to do things right?
11. A tale of two companies • There once were two competing companies, with equal market share • One company rewrote their product… • … and many customers migrated to the other! • The company who kept the old codebase now has twice the market share
12. A digression on software design
13. A non-obvious fact
14. This is more complex…
15. …than these
16. Doing it wrong…
17. What are the factors that make the split work well?
18. Simplicity is achieved when… • Each box is understandable in isolation • Each box does only one thing • Each box represents a single concept • Each box is cohesive
19. How to connect A & B
20. Simplicity is achieved when… • Things are loosely coupled • Or, rather… • … they are coupled appropriately
21. Design is fractal!
22. And, similarly…
23. Or worse!
24. How to connect services?
25. Domain Events to the rescue!
26. A Domain Event
27. How to connect services? REST apis RSS-like event queue Messaging service
28. So how do we deal with legacy code?
30. Patterns for incremental improvement 1. Pay down your technical debt 2. Bubble context 3. Strangler application
31. Strangler application Database nuovo App Nuova Database legacy App Legacy Batch Sync Utenti Apache Perl filter Utenti migrati Utenti non migrati Router Old and new apps Update mechanism
32. A Strangler application is composed of: • A router • The old application • The new application(s) • An update mechanism
33. Another example
34. Strangler app antipatterns
35. The router is inside the legacy app
36. The new app is aware of the old one
37. For best results, the new app and the old should not be aware of each other! • A router • The old application • The new application(s) • An update mechanism
38. Conclusions?
40. twitter.com/xpmatteo thoughtworks.com THANK YOU WE ARE HIRING! YES, IN ITALY!