artodeto's blog about coding, politics and the world

By Benjamin Eberlei and Kore Nordmann.

What Is Legacy Code

• hard or impossible to fix errors or implement new features
• hard or impossible to implement automatic testing

What Is The Talk About

• common legacy code issues
• how to verify we do not breack everything
• refactoring legacy code

Issues With Legacy Code

• code issues
• inversion of control
• data structures
• application or business logic
• shared state

Code Issues

• code size
• high efferent coupling
• code duplication
• ...
• Use phpmd to locate size or complexity errors.

Efferent Coupling

• many outgoing coupling for one class in other classes
• can be located with pdepend

Inversion Of Control

• Missing abstractions
• Inline new
• Static calls
• Inline life cycle management (only value objects and exceptions should be newed inline)

Data Structure

• Missing abstractions
• Inline new
• Static calls
• Inline life cycle management (only value objects and exceptions should be newed inline)

Data Structure

• with arrays, you have a bad documentation
• use simple value objects (also known as dto) and they are not immutable
• split logic and try to avoid big models (seperation of concerns (roles/contextes) on model logic
• branch by abstraction
  • introduce facade / proxy for old code
  • call old code from facade / proxy implementation
  • write new shiny facade / proxy implementation
  • use new implementation
  • delete old code

Application Or Business Logic

• mixed business logic (application logic) in large controllers is the most common and pressing issue with legacy code (fat controllers)
• try to use solutions like business requirement
• extract business logic like service layer pattern
• introduce (domain) events (in the same request)
• introduce message queues (not in the same request)

Shared State

• we know that global state is bad
• sessions are (often) cross-request global state in bash maps
• method execution pathes depending on current object state are horrible to track down

Testing Changes

• if your legacy code is hard to test, try to test it from the http protocoll point of view
• use behat as behaviour driven development (BDD)
• use mink with "goute", "sahi" or "zombie.js", "selenium", "selenium 2"

Extra

• do small checks when refactoring (good indicator are many, many commits)
• test your primary use cases (put the chaos in the cage)
• be bold (enough) and remove old code