Expose cache freshness observations without claiming source correctness
The dashboard shows a high hit rate as proof the product data is correct, though hit count says nothing about revision freshness.
- Focused work estimate
- 1h 30m + prerequisites
- Priority in the scenario
- Medium
- Engineering practice
- Observability · Metric semantics
Estimated field mix
- Distributed systems50%
- Site reliability50%
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 wholesale portal caches product availability descriptions across several API instances. Delayed invalidations and slow refreshes bring back old content after edits.
Setup prerequisites
- Create two local cache clients and a synthetic authoritative product store.
- Use a fake clock and controllable read/write barriers.
Preceding work
Complete these dependencies, or supply their agreed outputs before taking this ticket.
- ACACHE-101 · Define cache keys from every product-view authority input
- ACACHE-102 · Specify what stale product data the portal may display
- ACACHE-103 · Record authoritative product revision with each cached projection
- ACACHE-104 · Commit product changes with a durable invalidation fact
- ACACHE-105 · Prevent a slow cache fill from overwriting a newer product revision
- ACACHE-106 · Handle delayed invalidation without evicting a newer cache revision unnecessarily
- ACACHE-107 · Bound duplicate refresh work across API instances
- ACACHE-108 · Fall back predictably when the shared cache is unavailable
- ACACHE-109 · Reconcile cache generations after a projection-schema release
Acceptance criteria
- Report hit/miss, source revision and observed freshness separately.
- Mark unknown source comparison explicitly.
- Avoid exposing product payloads in generic telemetry.
Implementation constraints
- Metrics describe cache behavior, not business-data correctness.
Verification to include
- Observe a fresh hit with a known source revision.
- Hide source revision availability and report unknown freshness despite a cache hit.
Deliverables
- Cache health projection and semantics notes
Rollout and recovery
Add freshness diagnostics before tuning hit-rate targets; retain unknown states during outages.
Value of the work
For the engineer: Practice cache consistency, version guards and recoverable invalidation.
For the team: Review fast derived reads without losing authoritative state or tenant isolation.
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.