Battle-tested Strangler Fig migration workflow for transitioning JavaScript to TypeScript, legacy REST to tRPC, or monolithic endpoints to microservices.
Legacy Codebase Migration Workflow (Strangler Fig Pattern)
Goal
Incrementally modernize legacy codebases without risky, all-at-once rewrites.
Migration Steps
1. Establish Invariant Baselines
- Create end-to-end regression tests capturing current input/output behavior for the target module.
- Record sample real-world inputs and expected output snapshots.
2. Build the Modern Facade / Interface
- Define clean TypeScript interfaces for the module's public API.
- Implement an adapter layer routing requests between legacy and modern implementations.
3. Incremental File-by-File Modernization
- Convert files starting from the leaf dependencies (utility functions) up to top-level controllers.
- Replace
any with explicit types and runtime Zod validation.
4. Canary / Dual-Run Verification
- Log any discrepancies between legacy output and modern output.
- Validate performance and memory consumption under load.
5. Decommission Legacy Code
- Remove old helper functions and adapters once 100% traffic is verified on the modern path.