Practice catalogue updated Mon, Aug 10, 2026

Debug the signal.
Ship the fix.

Production-shaped debugging labs across APIs, config, concurrency, and infrastructure. Reproduce the failure, follow the evidence, and prove the smallest safe change.

Ready labs138playable now

Quick starts99under 20 min

Avg. time18minper lab

OSS labs4public history

01Reproducemake the failure visible

02Tracefollow the evidence

03Patchchange the smallest thing

04Verifyprove it holds

Featured labBuglyst · Caching & Performance

Cache stampede without coalescing

Concurrent cold-cache callers each kick off their own expensive compute, overwhelming the worker.

Hard25 minJavaScript#track#caching-performance#stampede

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14 of 138 labs · "Concurrency"ready labs · catalogue order

#LabDifficultyTime

28The Event That Fired TwiceWebhook side effects run twice after every reconnect.1 attempts · 0 solved · 0 discussionsMed.15min 34The Lock That Never Let GoAfter one failing job, every later attempt waits forever on the same lock.0 attempts · 0 solved · 0 discussionsMed.15min 36Dedupe window too shortWebhook dedupe cache drops entries long before providers stop retrying, so duplicates get processed twice.0 attempts · 0 solved · 0 discussionsEasy14min 38Event id mutated by middlewareThe dedupe fingerprint includes middleware-added fields, so the same webhook event is treated as a brand new event on every retry.0 attempts · 0 solved · 0 discussionsEasy14min 43Async dedupe raceTwo concurrent retries of the same webhook event both pass the dedupe check and both run the side effect because the set is mutated after an await.0 attempts · 0 solved · 0 discussionsHard26min 45Dedupe stuck on failureThe dedupe marker is written before the side effect runs and never rolled back on failure, so a transient failure permanently suppresses every retry of the same event.0 attempts · 0 solved · 0 discussionsHard24min 70Lock Around the Wrong AxisReservations for distinct slots owned by the same caller serialize unnecessarily.0 attempts · 0 solved · 0 discussionsMed.18min 74The Crossed WiresTwo concurrent moves between the same accounts deadlock because the lock order is inverted.0 attempts · 0 solved · 0 discussionsHard28min 116Await Missing in Fire-and-ForgetBatch runner swallows async failures because the per-item call is not awaited.0 attempts · 0 solved · 0 discussionsEasy12min 117forEach With an Async CallbackArray.prototype.forEach does not await async callbacks, so the batch reports success while per-item work is still running.0 attempts · 0 solved · 0 discussionsEasy12min 118Loop Mutates the Source ArrayprocessJobs splices skipped jobs out of the input array while iterating, so the iterator drifts past subsequent entries.0 attempts · 0 solved · 0 discussionsMed.18min 119Permit Released Twice on SuccesswithLock releases the permit in both the happy path and finally, so the semaphore silently over-credits.0 attempts · 0 solved · 0 discussionsMed.18min 120Rejection Escapes Several Call Hopsprepare() drops the await on its inner load() call, so the rejection escapes past try/catch and runJob reports success.0 attempts · 0 solved · 0 discussionsHard25min 133Duplicate webhook processingEach request registers another event listener, so later webhooks are processed multiple times.0 attempts · 0 solved · 0 discussionsMed.20min

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More ways to practice.

Move from a focused drill to open-source history or company-style patterns when you want a different kind of debugging pressure.

Focused exercises

Train code review, system tracing, CI diagnosis, incident response, and fast debugging instincts.

05 drills5 formats · short sessions

Open-source labs

Practice against safely recreated public bug-fix history from the libraries that actually broke.

04 playablereal bug-fix history

Company-style prep

Train backend and infrastructure failure patterns without affiliation claims.

30 patternsPattern-based · unofficial