Migration Pattern — Failures

Pattern: Migration
Component: failures.md
Version: 1.1 | Updated: 2026-07-16

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Failure 1: Contract first

Symptom: Column dropped with cutover; rollback needs backup restore.
Cause: Treated migration as single deploy.
Recovery: Restore; redesign expand → dual → cutover → contract.

Failure 2: Unknown consumers

Symptom: Nightly job or partner feed breaks post-cutover.
Cause: Incomplete inventory.
Recovery: Halt contract; restore dual if needed; finish inventory; re-soak.

Failure 3: No reconciliation

Symptom: API healthy; finance reports disagree.
Cause: No count/semantic gates.
Recovery: Run recon; repair data; delay contract until green.

Failure 4: Dual-write forever

Symptom: Two writers still required months later.
Cause: No contract exit criteria.
Recovery: Define mismatch/error thresholds and a calendar date; execute contract.

Failure 5: Fake rollback

Symptom: Plan says “redeploy previous” after destructive rewrite.
Cause: Rollback ignored data plane.
Recovery: Rewrite rollback for data; rehearse; mark true PONR.