OptimizeSeniorBackendAuto-graded
Stop concurrent cache misses from hammering the backend
Assesses senior judgement under load: noticing that many simultaneous requests duplicate one expensive job, and coordinating them safely.cache.py is a read-through cache wrapping an expensive loader — a database query or an upstream call — returning the cached value and calling the loader only on a miss. Single-threaded it is fine. Under concurrent load it has a stampede: when many requests want the same uncached key at once, they all see the miss and all call the loader before any of them stores a result, so one logical miss becomes a hundred calls. Make concurrent misses for one key collapse into a single load.40 min
python
You're done when
- Concurrent requests for the same missing key result in exactly one call to the loader.
- Every one of those requests still receives the correct value.
- Different keys are not blocked by each other — a miss on one key does not serialise the others.
- A key already in the cache is served from it without calling the loader.
- The cache remains correct once the in-flight load completes and later requests arrive.
What it measures
Performance optimizationRecognising that the cost here is duplicated work under concurrency, not the cache lookup itself.
Resilience engineeringProtecting a backend from a herd of simultaneous misses, which is how caches amplify an outage instead of absorbing it.
Concurrency designCoordinating per key so callers share one in-flight load without introducing a global bottleneck.
Your environment
Terminal + editorA full shell in the browser
Claude CodePre-authenticated, no key setup
Isolated microVMYours alone, wiped afterwards
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40 min
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