Distinguish reversible schema changes from irreversible data loss
The deployment template says every migration can be rolled back, even when original values are discarded.
- Focused work estimate
- 5h + prerequisites
- Priority in the scenario
- High
- Engineering practice
- Migration design · Recovery planning
Estimated field mix
- Database engineering60%
- Developer tooling20%
- System design20%
Field percentages are editorial estimates of the ticket's engineering focus. They total 100%; they are not measured time, proficiency scores, or ownership evidence.
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Project context
A fictional subscription service has accumulated migration scripts with unclear lock risks and no consistent rehearsal output.
Setup prerequisites
- Create a disposable local database with synthetic subscriptions and a separate scratch database; author all fixtures locally.
Preceding work
Complete these dependencies, or supply their agreed outputs before taking this ticket.
- BDBMIG-101 · Add a migration inventory with checksums and sequence gaps
- BDBMIG-103 · Prevent migration commands from targeting unapproved databases
- BDBMIG-105 · Test old and new application queries during expand-contract changes
- BDBMIG-102 · Flag table rewrites and long-held locks for review
- BDBMIG-104 · Capture lock waits during a synthetic concurrent workload
- BDBMIG-106 · Resume a chunked backfill after a process interruption
Acceptance criteria
- Classify schema reversal and data recovery separately.
- Identify the exact point original data becomes unavailable.
- Compare forward repair, backup restore, and application rollback for this migration.
Implementation constraints
- Do not invent recoverability beyond captured data.
Verification to include
- Rehearse the selected recovery with synthetic rows.
- Show the unrecoverable case when no original data was retained.
Deliverables
- Recovery decision record.
Rollout and recovery
Block contraction until recovery assumptions and retained-data lifetime are reviewed.
Value of the work
For the engineer: Practice migration tooling, lock reasoning, and clear developer feedback.
For the team: Provide reviewers with repeatable migration checks and explicit recovery limits.
Evidence boundaries
Outcome Evidence: Tests, patches, and runbooks are requested deliverables. They become Outcome Evidence only through a qualified Mission and immutable Evidence IDs.
Ownership Evidence: Independent adaptation must be observed under a declared verification policy and cite immutable Evidence IDs. Completing a planning ticket establishes no Ownership Evidence.