Shadow-Verify-Migrate Pattern

Migrate between service implementations in three composed steps: shadow, verify, switch. Compose old and new behind a common interface. Delegate real traffic to the old, shadow a percentage to the new, and log mismatches to verify equivalence before anything real moves. Then replace the shadow client with a migration client that routes by configuration. Roll forward by adjusting config, roll back by reverting it. Finally, delete the old code.

The key insight: each step is a composition of the same two implementations behind the same interface; the caller never changes. This avoids the common anti-patterns of inheriting old into new (coupling lifecycles), if-else blocks inside the old implementation (degrading it during transition), or skipping verification entirely.