Build it yourself.
More interviews now include an “implement this component” round: a rate limiter, an LRU cache, a consistent hashing ring. Each lab here is one of those, taken straight from a lesson. Write Python in the browser and run it against example tests and hidden edge cases.
0/8 cleared690 XP to earnPython 3.14 · runs on your device
- No install, no serverPython runs in your browser (Pyodide, CPython compiled to WebAssembly). The first run downloads it once, ~6.5 MB.
- Hidden testsEdge cases you only see after a run. They ship with the page, so they aren't secret: they're practice, not an exam.
- Works offlineSave a lab (or a whole world) and it runs with no internet. Your code autosaves in this browser.
World 01Data at Scale
- LAB · W01 · RATE LIMITINGNEWToken bucket rate limiterRefill by elapsed time, spend tokens per request, allow bursts up to capacity, and tell a rejected client when to retry.From W01 · D3 · Rate Limiting at ScaleEasy~25 min4 + 9 hidden tests+60 XP
- LAB · W01 · RATE LIMITINGNEWSliding window counter rate limiterTwo counters per key and a weighted estimate fix the fixed window's boundary burst, in O(1) memory.From W01 · D3 · Rate Limiting at ScaleMedium~30 min2 + 9 hidden tests+90 XP
- LAB · W01 · PARTITIONINGNEWConsistent hashing ring with virtual nodesHash nodes and keys onto a ring, walk clockwise with binary search, and prove that adding a node only moves the keys it takes over.From W01 · D1 · Partitioning Deep-DiveMedium~35 min3 + 9 hidden tests+90 XP
World 02Failure-First Design
- LAB · W02 · IDEMPOTENCYNEWIdempotency-key storeA retried payment returns the first response instead of charging twice, a reused key with a different body is refused, and failures can be retried.From W02 · D2 · Idempotency in PracticeMedium~35 min3 + 10 hidden tests+90 XP
- LAB · W02 · FAILURE HANDLINGNEWCircuit breaker state machineCLOSED, OPEN and HALF-OPEN with a sliding failure window, a recovery timer on a fake clock, and business errors that don't trip it.From W02 · D3 · Circuit BreakersMedium~35 min3 + 10 hidden tests+90 XP
- LAB · W02 · RETRIESNEWRetry with exponential backoff and full jitterRetry only what can succeed later, wait base × 2^attempt capped, pick a random point below it, and honour Retry-After.From W02 · Preview · Failure-First Design PreviewEasy~25 min2 + 10 hidden tests+60 XP