Glossary · Software Architecture
What is the strangler fig pattern?
Short answer
The strangler fig pattern is a way to replace a legacy system gradually: you put a routing layer in front of it, build new functionality piece by piece in a modern system, and redirect traffic for each piece as it is ready, until the old system can be switched off. It avoids the risk of a single big-bang rewrite.
Where the name comes from
A strangler fig grows around a host tree until it stands on its own and the host is gone. Martin Fowler used the image in 2004 for replacing software the same way.
How to apply it
- Intercept: route all requests through a proxy, API gateway or the web server, so you can decide per URL or feature where traffic goes.
- Pick a slice: start with a valuable, well-bounded feature, such as the product catalogue or invoicing.
- Rebuild and redirect: implement it in the new system, sync or migrate its data, and switch its routes over, ideally behind a feature flag so you can roll back.
- Repeat until little remains, then retire the old system.
Tips
- Agree on how data stays consistent while both systems run: one owner per entity, plus sync jobs or events.
- Measure parity: compare outputs of old and new for the same inputs before switching.
- Delete migrated code from the legacy side so the two don’t drift.
For PHP estates this often means moving from an old custom framework or PHP 5/7 codebase to Laravel or Symfony on a current PHP version, one module at a time.
