Cache stampede without coalescing Debugging Lab

Cache stampede without coalescingCaching & PerformanceHardPreparing

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Incident Dossier

Cache stampede without coalescing

When a popular cache key expires, every concurrent caller triggers its own expensive recomputation. The downstream service saturates and reports start taking 30 seconds instead of 200 ms. The cache helper lacks in-flight coalescing.

Initial failing signalCompute runs once per concurrent cold-cache callerExpected compute() runs exactly once for 10 concurrent callers · received compute() runs 10 times — one per caller — stampeding the worker

Progress

1Reproduce the bugRun checks

2Edit the fixOpen

3Pass checksOpen

4Submit fixOpen

Reproduction Steps

Run npm test.
Notice that the compute counter equals the number of concurrent callers.
Inspect getOrCompute in src/report.js.
Identify that every cold-cache call kicks off its own compute — there is no in-flight dedup.
Add an in-flight promise map keyed by cache key so concurrent callers share the running compute.

Focus Mode

Start

src/report.js

Likely area

src/report.js

Challenge Coach

Hints reveal broad direction first, then narrow toward the suspect path without showing hidden tests or exact solution code.

TopicCaching & Performance

DifficultyHard

Est. time25 min

Workspacepreparing

Explorer

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Evidence Map

Failing signal loaded from challenge API
Workspace files loaded from workspace API
Results rendered only after API run/submit

Autosaved

workspace

Outputread-onlyready$ prepare workspace

Preparing workspace