Expose replica freshness with a trustworthy unknown state
The dashboard marks a replica healthy when its consumer is running even though no source checkpoint has been observed.
- Focused work estimate
- 1h 30m + prerequisites
- Priority in the scenario
- Medium
- Engineering practice
- Observability · Data semantics
Estimated field mix
- Data engineering60%
- Site reliability40%
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 messaging service builds a read model from database changes. Restarts and schema changes can re-enable preferences that customers disabled.
Setup prerequisites
- Create synthetic preference records and a replayable local change log.
- Use adapters or fixtures without provisioning a production broker.
Preceding work
Complete these dependencies, or supply their agreed outputs before taking this ticket.
- ACDC-101 · Define the preference change envelope with source position
- ACDC-103 · Represent preference deletion as a durable tombstone
- ACDC-104 · Advance the feed checkpoint only with the projected transaction
- ACDC-105 · Reject a stale preference update after a partition retry
- ACDC-106 · Handoff a preference snapshot to live changes without a gap
- ACDC-107 · Evolve a preference value enum without silently coercing unknown values
- ACDC-108 · Compare source and replica preferences without exporting contacts
- ACDC-109 · Repair one preference partition while continuing unrelated reads
Acceptance criteria
- Show applied source position and receipt lag separately.
- Report unknown when no comparable source watermark exists.
- Define stale thresholds in the local operating contract.
Implementation constraints
- Never infer source completeness from process liveness.
Verification to include
- Advance source and replica positions and calculate the documented lag.
- Remove the source watermark and return unknown health.
Deliverables
- Freshness endpoint and unknown-state cases
Rollout and recovery
Add freshness as advisory status first; restore the prior view while preserving unknown semantics.
Value of the work
For the engineer: Practice change-feed ordering, snapshots and privacy-preserving deletion.
For the team: Review whether replicated customer preferences remain correct during 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.