Fence a late worker after its rendering lease expires
Worker A pauses long enough to lose its lease. Worker B completes the same preview, then A resumes and overwrites B's artifact pointer with stale output.
- Focused work estimate
- 5h + prerequisites
- Priority in the scenario
- High
- Engineering practice
- Fencing tokens · Worker leases · Concurrency · Artifact consistency
Estimated field mix
- Distributed systems60%
- 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 internal document portal creates previews through a trusted mock renderer. Large batches crowd out small teams, failed documents retry forever, and operators lack a safe replay command. This exercise never executes uploaded code or real document macros.
Setup prerequisites
- Queue semantics
- Database transactions
- Operational metrics
Preceding work
Complete these dependencies, or supply their agreed outputs before taking this ticket.
- QUEUE-101 · Expose a document job's current phase and terminal reason
- QUEUE-102 · Reject oversized document batches before accepting work
- QUEUE-103 · Commit job acceptance and dispatch intent together
- QUEUE-104 · Stop retrying unsupported document formats
- QUEUE-105 · Keep one team's bulk import from occupying every worker
Acceptance criteria
- Associate each execution attempt with a monotonic fencing identity.
- Accept completion only from the current authorized lease holder.
- Keep rejected late completions observable and prevent them from changing terminal job state.
Implementation constraints
- Lease expiration alone does not stop a process; the persistence boundary must reject stale writes.
Verification to include
- Expire A, complete through B, then submit A's completion and inspect unchanged output.
- Replay B's completion and assert one terminal transition and artifact reference.
Deliverables
- Fenced completion operation and paused-worker reproduction
Rollout and recovery
Introduce fencing before extending concurrency; rollback dispatch while retaining fencing checks on completions.
Value of the work
For the engineer: Practice asynchronous lifecycle control, fair scheduling, bounded retries, and artifact recovery with a deterministic provider.
For the team: See how an engineer accounts for accepted work and reduces operational toil without granting operators broad data access.
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.