Challenge Library
Coding Challenges
5,893 curated questions across every arena — browse by topic, then jump into a live 3-minute battle to put them to the test.
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- Multiple ChoiceHard
[System Design] Duplicate detection Safe retries
qasys-designsys-designqa - Multiple ChoiceHard
[System Design] TTL on keys Safe retries
qasys-designsys-designqa - Multiple ChoiceHard
[System Design] Producer/consumer Async decoupling
qasys-designsys-designqa - Multiple ChoiceHard
[System Design] At-least-once Async decoupling
qasys-designsys-designqa - Multiple ChoiceHard
[System Design] DLQ Async decoupling
qasys-designsys-designqa - Multiple ChoiceHard
[System Design] Kafka vs RabbitMQ Async decoupling
qasys-designsys-designqa - Multiple ChoiceHard
[System Design] Backpressure Async decoupling
qasys-designsys-designqa - Multiple ChoiceHard
[System Design] Three pillars Monitor system health
qasys-designsys-designqa - Multiple ChoiceHard
[System Design] Prometheus Monitor system health
qasys-designsys-designqa - Multiple ChoiceHard
[System Design] Grafana Monitor system health
qasys-designsys-designqa - Multiple ChoiceHard
[System Design] Distributed tracing Monitor system health
qasys-designsys-designqa - Multiple ChoiceHard
[System Design] SLI/SLO/SLA Monitor system health
qasys-designsys-designqa - Multiple ChoiceHard
[System Design] Shard key Horizontal partition
qasys-designsys-designqa - Multiple ChoiceHard
[System Design] Range vs hash Horizontal partition
qasys-designsys-designqa - Multiple ChoiceHard
[System Design] Rebalance Horizontal partition
qasys-designsys-designqa - Multiple ChoiceHard
[System Design] Cross-shard joins Horizontal partition
qasys-designsys-designqa - Multiple ChoiceHard
[System Design] Consistent hashing Horizontal partition
qasys-designsys-designqa - Multiple ChoiceHard
Cache stampede / thundering herd
system-designcachingstampede - Multiple ChoiceHard
Probabilistic early expiration
system-designcachingstampede - Multiple ChoiceHard
Sharding key choice
system-designdatabasessharding - Multiple ChoiceHard
Quorum read/write overlap
system-designconsistencyquorum - Multiple ChoiceHard
Two-phase commit blocking
system-designdistributed-transactions2pc - Multiple ChoiceHard
Bulkhead isolation
system-designavailabilitybulkhead - Multiple ChoiceHard
Fan-out on write vs read
system-designreal-timefan-outnews-feed - Multiple ChoiceHard
Little's Law for concurrency
system-designcapacity-estimationlatency - Multiple ChoiceHard
Kafka consumer group rebalance
system-designmessagingkafka - Multiple ChoiceHard
Visibility timeout too short
system-designmessagingsqsidempotency - Multiple ChoiceHard
Lost update race
system-designconcurrencyconsistency - Multiple ChoiceHard
Optimistic vs pessimistic locking
system-designconcurrencydatabases - Multiple ChoiceHard
Tail-based sampling
system-designobservabilitytracing - Multiple ChoiceHard
Retry amplification
system-designavailabilityretry - Multiple ChoiceHard
Quorum without read repair
system-designquorumconsistency - Multiple ChoiceHard
Cache invalidation on write
system-designcachingcache-invalidation - Multiple ChoiceHard
LSM tree read amplification tradeoff
lsm-treestorage-engineread-amplification - Multiple ChoiceHard
Write skew under snapshot isolation
mvccwrite-skewisolation-levels - Multiple ChoiceHard
Raft safety vs Paxos roles
raftpaxosconsensus - Multiple ChoiceHard
Sliding window counter hybrid tradeoff
rate-limitingsliding-windowalgorithms - Multiple ChoiceHard
Distributed rate limiting consistency
rate-limitingdistributedconsistency - Multiple ChoiceHard
Fencing tokens for stale leaders
fencing-tokenssplit-brainleader-election - Multiple ChoiceHard
Redlock safety criticism
distributed-locksredlockclock-skew - Multiple ChoiceHard
OR-Set vs LWW register semantics
crdtor-setconflict-resolution - Multiple ChoiceHard
Cuckoo filter vs bloom filter trade-off
cuckoo-filterbloom-filterdeletion - Multiple ChoiceHard
t-digest for streaming percentiles
t-digestpercentilesstreaming - Multiple ChoiceHard
Tombstone accumulation in LSM
tombstoneslsmcompaction - Multiple ChoiceHard
HOT updates and indexed columns
postgreshot-updatesindexes - Multiple ChoiceHard
PgBouncer transaction vs session pooling
pgbouncerconnection-poolpostgres - Multiple ChoiceHard
Dual-write migration correctness
migrationdual-writeconsistency - Multiple ChoiceHard
Saga compensation ordering
sagacompensationorchestration - Multiple ChoiceHard
Ledger Immutability and Corrections
ledgerimmutabilitycorrections - Multiple ChoiceHard
Adaptive Bitrate Ladder Design
videoabrhls - Multiple ChoiceHard
Sharded Timers for Job Scheduler
schedulershardingtimers - Multiple ChoiceHard
Global and Per-Tenant Rate Limiting
rate-limitertenantdistributed - Multiple ChoiceHard
URL Shortener Hot Key Mitigation
url-shortenerhot-keycaching - Multiple ChoiceHard
[Testing] Playwright Test full user flows
qatestingtestingqa - Multiple ChoiceHard
[Testing] Page Object Model Test full user flows
qatestingtestingqa - Multiple ChoiceHard
[Testing] Visual testing Test full user flows
qatestingtestingqa - Multiple ChoiceHard
[Testing] Flaky tests Test full user flows
qatestingtestingqa - Multiple ChoiceHard
[Testing] CI integration Test full user flows
qatestingtestingqa - Multiple ChoiceHard
[TypeScript] ReturnType Extract types in conditionals
qatypescripttypescriptqa - Multiple ChoiceHard
[TypeScript] Promise unwrap Extract types in conditionals
qatypescripttypescriptqa
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