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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- Code AnalysisHard
Quorum read after failed write
system-designquorumconsistency - Code AnalysisBeginner
Unbounded result set in API
system-designapi-designpagination - Code AnalysisHard
Idempotency key stored after the effect
system-designidempotencyconcurrency - Code AnalysisBeginner
Fixed-window boundary burst
system-designrate-limiting - Code AnalysisHard
Fan-out read amplification
system-designnews-feedfan-out - Code AnalysisIntermediate
Queue with no DLQ and infinite retry
system-designmessagingdlq - Code AnalysisBeginner
Read-modify-write without atomicity
system-designconcurrencydistributed-counter - Code AnalysisIntermediate
Fire-and-forget event loss
system-designmessagingoutbox - Code AnalysisIntermediate
Thundering herd from synchronized cron
system-designcachingstampede - Code AnalysisBeginner
Writing to a replica
system-designdatabasesreplication - Code AnalysisHard
Offset commit before processing
system-designmessagingkafka - Code AnalysisIntermediate
Cache write without invalidation on update
system-designcachingcache-invalidation - Code AnalysisHard
Unbounded retry on non-transient error
system-designresilienceretry - Code AnalysisIntermediate
In-memory session per instance
system-designscalabilitystateless - Code AnalysisHard
CAS without version on concurrent edit
system-designconcurrencydatabases - Code AnalysisIntermediate
Sync call in an async event loop
system-designscalabilityconcurrency - Code AnalysisHard
Bloom filter with no eviction
system-designcachingbloom-filter - Code AnalysisIntermediate
Chatty microservice call
system-designmicroservicesperformance - Code AnalysisBeginner
Cache aside stampede on hot key expiry
cachingstampedehot-key - Code AnalysisIntermediate
TTL jitter to prevent avalanche
cachingavalanchettl-jitter - Code AnalysisHard
Commit before process loses messages
kafkaoffset-commitmessage-loss - Code AnalysisBeginner
Rebalance storm from long processing
kafkarebalanceconsumer-groups - Code AnalysisIntermediate
Connection pool exhaustion from leaked connections
connection-poolresource-leakreliability - Code AnalysisHard
Lost update from read-modify-write
transactionslost-updateconcurrency - Code AnalysisHard
Phantom read with range predicate
phantom-readisolation-levelsmvcc - Code AnalysisBeginner
N+1 query in ORM loop
ormn+1query-optimization - Code AnalysisBeginner
Index-killing function on column
indexingsargabilitysql - Code AnalysisIntermediate
Unbounded cardinality metric label
metricscardinalityobservability - Code AnalysisBeginner
Window boundary allows double rate
rate-limitingfixed-windowburst - Code AnalysisIntermediate
Non-idempotent retry double charge
paymentsidempotencyretry - Code AnalysisIntermediate
Unbounded queue without maxsize
backpressurequeuememory - Code AnalysisIntermediate
Quorum write to minority partition
quorumpartitioncap - Code AnalysisIntermediate
Write to a read replica
replicationread-replicarouting - Code AnalysisIntermediate
Outbox relay skipped without ordering
outboxcdcordering - Code AnalysisHard
LSM write amplification analyzer
lsmcompactionrocksdb - Code AnalysisIntermediate
Idempotent receiver dedupe check
idempotencyat-least-oncededupe - Code AnalysisIntermediate
Outbox pattern race condition
outboxexactly-oncerelay - Code AnalysisHard
Two-phase commit blocking analysis
2pcdistributed-transactionsblocking - Code AnalysisIntermediate
Kafka transaction + sink exactly-once
kafkaexactly-oncetransactions - Code AnalysisIntermediate
Redis distributed lock expiry race
distributed-lockredisfencing - Code AnalysisBeginner
GCRA smoothness tuning
gcrarate-limitingsmoothing - Code AnalysisBeginner
Ledger double-entry balance check
ledgerdouble-entryinvariants - Code AnalysisBeginner
Kafka per-key ordering pitfall
kafkaorderingpartitioning - Code AnalysisIntermediate
Circuit breaker half-open bug
circuit-breakerresiliencehalf-open - Code AnalysisIntermediate
Load shedding priority drop
load-sheddingadmission-controlpriority - Code AnalysisBeginner
Stale-while-revalidate cache bug
cachingswrcdn - Code AnalysisIntermediate
Cache key cardinality explosion
cachingcache-keyscardinality - Code AnalysisIntermediate
WebSocket reconnect storm
websocketreconnectthundering-herd - Code AnalysisBeginner
Count-min heavy hitters shape
count-minheavy-hitterssketch - Code AnalysisIntermediate
Cursor pagination stability bug
paginationcursorfeed - Code AnalysisBeginner
Transactional Outbox Pattern for Payments
outboxpaymentsevents - Code AnalysisBeginner
Saga Compensation for Money Transfer
sagatransfercompensation - Code AnalysisBeginner
Notification Fan-Out Deduplication
notificationsdedupretries - Code AnalysisBeginner
Geospatial Index for Ride Matching
ride-hailinggeospatialmatching - Code AnalysisBeginner
Trie Top-K for Search Autocomplete
autocompletetrietop-k - Code AnalysisHard
Time-Series Cardinality Explosion
metricscardinalitytime-series - Code AnalysisIntermediate
Non-Idempotent Payment Retry Handler
idempotencypaymentsretries - Code AnalysisBeginner
Dual Write Ledger and Event Bug
outboxdual-writeevents - Code AnalysisIntermediate
Non-Atomic Balance Check SQL
sqlconcurrencytoctou - Code AnalysisHard
Mutable Balance Ledger Correction
ledgerimmutabilitydouble-entry
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