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 · "Database"ready labs · catalogue order

#LabDifficultyTime

07The N+1 That Wasn'tDB load explodes, but the query count looks normal.3 attempts · 1 solved · 1 discussionsMed.26min 16The stale confirmationA successful update is followed by an old value.1 attempts · 1 solved · 0 discussionsMed.18min 27The Query That Loaded the WorldOrders list endpoint returns every tenant's rows and times out under load.0 attempts · 0 solved · 0 discussionsMed.15min 29The Config That Swallowed ProdProduction boots with the dev database URL and debug flags.3 attempts · 1 solved · 0 discussionsMed.15min 62Cache invalidation skipped on writeWrites update the database but the cache is never told, so reads keep returning the old value.0 attempts · 0 solved · 0 discussionsHard28min 68The Wallet That Let It SlideTwo concurrent transfers from the same account both succeed and the balance goes negative.0 attempts · 0 solved · 0 discussionsEasy14min 69Two For One EmailConcurrent registrations for the same email both succeed and create duplicate accounts.0 attempts · 0 solved · 0 discussionsMed.18min 72The Phantom ReadA read inside a transaction does not observe the same transaction's pending write.0 attempts · 0 solved · 0 discussionsMed.18min 106Inner join drops comments from soft-deleted authorsA LEFT JOIN needs an INNER JOIN — or vice versa — and rows silently disappear from the listing.0 attempts · 0 solved · 0 discussionsEasy15min 107Count that missed the where clauseA dashboard count helper ignores its filter and reports the wrong total.0 attempts · 0 solved · 0 discussionsEasy12min 108N+1 that silently drops rowsA per-row fetch helper filters on a property that does not exist, so every caller sees an empty result.0 attempts · 0 solved · 0 discussionsMed.20min 109Pagination skips rows on timestamp tiesCursor-based pagination drops every row whose updated_at equals the boundary timestamp on the previous page.0 attempts · 0 solved · 0 discussionsMed.22min 110Pseudo-unique constraint leaks duplicate accountsAn index assumed unique is queried by the wrong column, so duplicates slip through signup.0 attempts · 0 solved · 0 discussionsHard28min 138Idempotency key ignoredThe payment endpoint validates an idempotency key but drops it before the database call.3 attempts · 1 solved · 0 discussionsMed.20min

discussion · public Practice

What are developers debugging?

Questions, dead ends, and useful investigation clues from real public Practice threads.

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keep building the loop

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