API Versioning and Backward Compatibility

1. Why This Is a Microservices Lesson at All

In a monolith, changing a function signature was safe work. The compiler found every caller, and one commit fixed them all together.

Between services there is no compiler. Your callers are other teams' running systems. They deploy on their own schedules, and deployments overlap by design, so old and new versions of everything are always running somewhere at once.

That makes every API a contract