Measure saved origin work instead of celebrating the hit ratio
The cache reports a high hit ratio, but every hit still triggers a synchronous origin validation and costs almost as much as a miss.
- Focused work estimate
- 1h 30m + prerequisites
- Priority in the scenario
- Medium
- Engineering practice
- Cache measurement · Latency analysis
Estimated field mix
- Performance engineering100%
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 equipment-rental service caches availability summaries. A campaign sends repeated reads, while stock updates and shared expiry times create bursts against the origin. Build a local origin stub and cache-backed read API using synthetic depots and products.
Setup prerequisites
- Create a deterministic local availability origin and Redis-backed reader with synthetic tenant, depot and product data.
- Use a seeded hot-key distribution and controlled time; record cache capacity, TTLs, runtime and machine limits.
Preceding work
Complete these dependencies, or supply their agreed outputs before taking this ticket.
Acceptance criteria
- Record hits, misses, stale responses, origin calls, origin work duration and end-to-end request latency separately.
- Use bounded labels and report the same workload window and request counts for all rates.
- Define avoided origin calls against an uncached run of the identical seeded requests.
Implementation constraints
- Do not combine cache and origin error outcomes into successful hits; expose failures and retries explicitly.
Verification to include
- Replay a repeated-key fixture and reconcile request count with cache outcomes and actual stub calls.
- Enable synchronous validation deliberately and verify the report reveals that high hit rate did not remove origin work.
Deliverables
- Cache effectiveness report and aggregate counters
Rollout and recovery
Run counters beside the existing local behavior before policy changes; preserve raw attempt counts for comparisons.
Value of the work
For the engineer: Learn to evaluate caching through avoided work, bounded staleness, concurrency and recovery rather than hit rate alone.
For the team: Produce a reviewable cache policy and failure exercise for a read-heavy service with changing business data.
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.