Challenge Library
Coding Challenges
5,893 curated questions across every arena — browse by topic, then jump into a live 2-minute battle to put them to the test.
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5,893 challenges total
- 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 - Code AnalysisIntermediate
At-Least-Once Consumer Duplicate Side Effect
exactly-onceconsumeridempotency - Code AnalysisHard
Reconciliation Missing Rows Bug
reconciliationsqlouter-join - Code AnalysisIntermediate
Notification Dedup Missing TTL
notificationsdeduprace - Code AnalysisHard
Quiet Hours Suppression Logic Bug
notificationsquiet-hourstimezone - Code AnalysisHard
Chat Ordering by Client Clock
chatorderingclocks - Code AnalysisIntermediate
Feature Flag Inconsistent Rollout Seed
feature-flagsrolloutconsistency - Code AnalysisIntermediate
Tenant filter missing in query
multi-tenantisolationsql - Code AnalysisIntermediate
Unbounded tenant metric key cardinality
iotcardinalitymetrics - Code AnalysisHard
Edge buffer overflow dropping IoT data
iotedge-bufferingmqtt - Code AnalysisIntermediate
Log dedup dropping distinct events
log-aggregationdedupidempotency - Code AnalysisHard
Log query scanning cold tier
log-aggregationquery-at-scaleindexing - Code AnalysisHard
Feature leakage across train/serve
recommendationfeature-storetrain-serve-skew - Code AnalysisIntermediate
A/B exposure logged after variant call
experimentationexposure-loggingbias - Code AnalysisHard
Sequential testing without peeking control
experimentationsequential-testingstatistics - Code AnalysisIntermediate
Exactly-once violated by non-idempotent sink
data-pipelineexactly-onceidempotency - Code AnalysisHard
Backfill double-counting aggregates
data-pipelinebackfillidempotency - Code AnalysisHard
Trace assembly with clock skew
tracingtrace-assemblyclock-skew - Code AnalysisIntermediate
JWT introspection missing signature check
identityjwtoidcsecurity - Code AnalysisHard
KMS secret rotation leaving stale ciphertext
secretsrotationkms - Code AnalysisIntermediate
List-objects ACL check per request
file-storageaclmetadata-db - Code AnalysisHard
Hot leaderboard partition on one key
leaderboardhot-partitionredis - Code AnalysisHard
Webhook deliveries without ordering guard
webhooksorderingdlqbackpressure - Multiple ChoiceIntermediate
Eventual consistency read path
architectureedaeventual-consistency - Multiple ChoiceHard
Compensating transaction design
architecturedistributedsagaidempotency - Multiple ChoiceIntermediate
Message ordering and idempotency
architectureedaidempotency - Multiple ChoiceHard
Outbox pattern
architectureedaoutbox - Multiple ChoiceBeginner
API versioning: additive change
architectureapiversioningbackward-compatibility - Multiple ChoiceIntermediate
Contract testing goal
architecturetestingcontract-testing - Multiple ChoiceBeginner
Test pyramid rationale
architecturetestingtest-pyramid - Multiple ChoiceBeginner
Stub vs mock vs fake
architecturetestingtest-doubles - Multiple ChoiceIntermediate
Seams and testability
architecturetestingseams - Multiple ChoiceBeginner
Dependency injection benefit
architecturetestingdi - Multiple ChoiceIntermediate
Branch by abstraction
architecturerefactoringbranch-by-abstraction - Multiple ChoiceHard
Technical debt: deliberate vs accidental
architecturetechnical-debtrefactoring - Multiple ChoiceIntermediate
Fitness functions
architectureevolutionary-architecturefitness-functions - Multiple ChoiceBeginner
ADRs purpose
architectureadrdocumentation - Multiple ChoiceIntermediate
Feature flags vs branches
architecturefeature-flagscicd - Multiple ChoiceHard
Feature flag debt
architecturefeature-flagstechnical-debt - Multiple ChoiceBeginner
Cyclic dependencies harm
architecturemodularitydependencies - Multiple ChoiceIntermediate
Stable Dependencies Principle
architecturemetricssdp - Multiple ChoiceBeginner
Package cohesion: CCP
architecturemodularitycohesion - Multiple ChoiceHard
Boundary leak via shared database
architecturemicroservicescoupling - Multiple ChoiceIntermediate
Idempotency key design
architectureresilienceidempotency - Multiple ChoiceBeginner
Facade reduces coupling
architecturedesign-patternsfacade - Multiple ChoiceIntermediate
Decorator ordering matters
architecturedesign-patternsdecorator - Multiple ChoiceIntermediate
Strategy vs State
architecturedesign-patternsstrategystate - Multiple ChoiceIntermediate
Bulkhead pattern
architectureresiliencebulkhead
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