Maintaining Service During Migration

Zero-downtime migration requires careful architectural planning. Implement a facade pattern where the authentication service routes requests to appropriate backends based on user migration status. This allows both systems to operate simultaneously while users gradually migrate. The facade must handle edge cases like users attempting to authenticate mid-migration or race conditions between migration and authentication.

Load balancing becomes critical during migration as authentication patterns change. Users who haven't migrated still use fast (but insecure) legacy hashing, while migrated users use slower modern algorithms. This disparity can cause uneven load distribution. Implement intelligent routing that considers both migration status and current system load. Monitor response times carefully and adjust capacity as needed.