Keep old API instances working during a column rename
A migration renames booking.start to booking.starts_at before old API instances finish draining. Half the requests fail until every instance updates.
- Focused work estimate
- 4h + prerequisites
- Priority in the scenario
- High
- Engineering practice
- Expand-contract migrations · Compatibility · Release sequencing
Estimated field mix
- Database engineering60%
- Platform engineering40%
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 scheduling service has an API, a worker, and PostgreSQL. Releases use immutable images on two application instances. The team needs compatibility checks, staged traffic, and a rehearsed rollback without introducing a new orchestration platform.
Setup prerequisites
- HTTP health checks
- CI pipelines
- Database migrations
Preceding work
Complete these dependencies, or supply their agreed outputs before taking this ticket.
Acceptance criteria
- Define expand, compatible application rollout, backfill, and contract stages.
- Prove old and new application versions work during the supported overlap.
- Block destructive cleanup while the old reader version remains active.
Implementation constraints
- Include rollback compatibility explicitly; a reverse rename alone is not a release strategy.
Verification to include
- Run mixed-version synthetic requests through the expanded schema.
- Attempt cleanup with an old instance present and assert the gate blocks.
Deliverables
- Versioned migration plan and mixed-version compatibility checks
Rollout and recovery
Apply only expansion first; rollback application traffic before the contract stage if errors appear.
Value of the work
For the engineer: Practice release contracts, compatibility windows, measurable canaries, and incident decisions on a modest application topology.
For the team: Review whether an engineer can make deployments diagnosable and reversible while identifying when rollback is unsafe.
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.