Avoid rewriting a legacy system from scratch, by strangling it
- There comes a point when there is so much technical debt in your legacy project that you can no longer implement new features. Yet rewriting from scratch is risky and refactoring is expensive.
- ‘Strangle’ the codebase instead, progressively deleting the old codebase in favor of building a new one. It has less risk of breaking things and is less work overall.
- To strangle the codebase:
- Write new code that acts as a proxy for the old code: users come to the new system, which actually redirects to the old one behind the scenes.
- Build new modules to re-implement each of the legacy behaviors in the new codebase, such that there is no change from the user perspective.
- Progressively fade away the old code from use until users are entirely consuming the new modules.
- Delete the old, unused code.
- Use techniques such as wrap classes to add new behaviours to the system without changing existing code at first, which also separates new and old responsibilities while integrating them. Or you can use the domain-driven design (DDD), with the legacy system as a black box and building a bubble context where you apply DDD, interacting with the black box through an anti-corruption layer. Roll out the rewrites progressively.
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