Apply duplicate and out-of-order payment callbacks idempotently
A delayed pending callback overwrites an already-confirmed payment.
- Focused work estimate
- 3h 30m + prerequisites
- Priority in the scenario
- High
- Engineering practice
- Transactions · Event ordering
Estimated field mix
- Integrations40%
- Distributed systems40%
- Database engineering20%
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 software vendor changes payment providers. Its application must tolerate unknown outcomes, signed callbacks, refunds, and inconsistent settlement reports.
Setup prerequisites
- Create a local payment-provider double and synthetic orders; use test-only signatures and execute no financial transactions.
Preceding work
Complete these dependencies, or supply their agreed outputs before taking this ticket.
Acceptance criteria
- Deduplicate provider event identity.
- Apply only allowed state transitions.
- Record ignored stale events for diagnosis.
Implementation constraints
- Commit state and processed-event record atomically.
Verification to include
- Replay a successful callback twice.
- Deliver pending after confirmed and preserve confirmation.
Deliverables
- Callback transition handler.
Rollout and recovery
Adopt per synthetic merchant; pause on unresolved transition conflicts.
Value of the work
For the engineer: Practice idempotent integrations, state reconciliation, and provider migration.
For the team: Provide a reviewable adapter design that protects order consistency and recovery.
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.