Strangler fig pattern -- incremental legacy migration

Rewriting a legacy system from scratch is risky and takes too long. The big bang rewrite often fails because you lose institutional knowledge, miss edge cases, and cannot ship value during the rewrite period.

Incrementally replace parts of the legacy system: (1) Put a proxy/facade in front of the legacy system. (2) Implement new features in the new system. (3) Route specific endpoints/features to the new system. (4) Gradually migrate existing features one by one. (5) Decommission the legacy system when empty. This lets you ship the new system incrementally and roll back individual features.

Named after strangler fig trees that grow around host trees. The new system grows alongside the old one until it fully replaces it. This eliminates the big-bang risk.