CQRS Read-Model Rebuilds with Versioned Cutover

Read models drift. Rebuilds are inevitable. Downtime is optional.

Versioned Strategy

1. Build V2 projection in parallel.
2. Compare V1/V2 on sampled traffic.
3. Shift cohorts gradually.
4. Keep rollback path warm.

if cohort(userId) == "canary":
  query(V2)
else:
  query(V1)

Confidence comes from diff quality, not migration speed.