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
- Multiple ChoiceBeginner
Token bucket burst cap
token-bucketrate-limitingburst - Multiple ChoiceIntermediate
Preventing seat oversell
concurrencybookingoversell - Multiple ChoiceHard
Saga compensation ordering
sagacompensationorchestration - Multiple ChoiceBeginner
Double-Entry Ledger Balance Invariant
ledgerdouble-entryinvariants - Multiple ChoiceBeginner
Purpose of Idempotency Keys in Payments
paymentsidempotencyretries - Multiple ChoiceBeginner
Append-Only Event Store for Finance
event-storefinanceaudit - Multiple ChoiceBeginner
Why Reconciliation Jobs Exist
reconciliationsettlementledger - Multiple ChoiceBeginner
Exactly-Once vs At-Least-Once Semantics
exactly-onceidempotencymessaging - Multiple ChoiceBeginner
Per-User Quiet Hours in Notifications
notificationsquiet-hourstimezone - Multiple ChoiceHard
Ledger Immutability and Corrections
ledgerimmutabilitycorrections - Multiple ChoiceIntermediate
Saga Choreography vs Orchestration Tradeoff
sagaorchestrationchoreography - Multiple ChoiceBeginner
Audit Trail Tamper Evidence
audittamper-evidenthash-chain - Multiple ChoiceBeginner
Push vs Pull Notification Delivery
notificationspushpull - Multiple ChoiceBeginner
CRDT vs OT for Collaboration
collaborationcrdtot - Multiple ChoiceHard
Adaptive Bitrate Ladder Design
videoabrhls - Multiple ChoiceHard
Sharded Timers for Job Scheduler
schedulershardingtimers - Multiple ChoiceIntermediate
Fan-Out on Write vs Read
news-feedfan-outcelebrity - Multiple ChoiceIntermediate
Stable Cursor Pagination
paginationfeedcursor - Multiple ChoiceIntermediate
Per-Conversation Ordering in Chat
chatorderingreceipts - Multiple ChoiceIntermediate
Oversell Prevention in Ticket Booking
bookinginventoryconcurrency - Multiple ChoiceIntermediate
Geohash vs S2 for Radius Search
proximitys2geohash - Multiple ChoiceBeginner
Feature Flag Kill Switch
feature-flagskill-switchrollout - Multiple ChoiceIntermediate
Consistent Rollout via Hashing
feature-flagsrollouthashing - Multiple ChoiceHard
Global and Per-Tenant Rate Limiting
rate-limitertenantdistributed - Multiple ChoiceIntermediate
Config Service Watch for Updates
configwatchpush - Multiple ChoiceHard
URL Shortener Hot Key Mitigation
url-shortenerhot-keycaching - Multiple ChoiceIntermediate
API Gateway Circuit Breaking
api-gatewaycircuit-breakerresilience - Multiple ChoiceBeginner
Multi-tenant data isolation strategy
multi-tenantisolationdatabase - Multiple ChoiceBeginner
Noisy neighbor mitigation in SaaS
multi-tenantnoisy-neighborquotas - Multiple ChoiceBeginner
MQTT topic strategy for IoT ingestion
iotmqttingestion - Multiple ChoiceBeginner
Time-series storage choice for IoT
iottime-seriesstorage - Multiple ChoiceBeginner
Log shipping durability trade-off
log-aggregationshippingdurability - Multiple ChoiceBeginner
Log retention tiering design
log-aggregationretentiontiering - Multiple ChoiceBeginner
Recommendation candidate generation stage
recommendationcandidate-generationranking - Multiple ChoiceBeginner
Feature store purpose in ML pipelines
recommendationfeature-storeml - Multiple ChoiceBeginner
A/B assignment via hashing
experimentationassignmenthashing - Multiple ChoiceBeginner
E-commerce oversell prevention approach
inventoryreservationconcurrency - Multiple ChoiceBeginner
CDN cache invalidation for atomic deploys
cdndeploycache-invalidation - Multiple ChoiceBeginner
Webhook retry with idempotency
webhooksidempotencyretries - Multiple ChoiceBeginner
Session vs JWT trade-off
identityjwtsessionrevocation - Multiple ChoiceBeginner
RBAC vs ABAC for fine-grained access
authorizationabacpolicy - Multiple ChoiceBeginner
Envelope encryption for secrets at rest
secretskmsenvelope-encryption - Multiple ChoiceBeginner
Content-addressed dedup in file storage
file-storagededupchunking - Multiple ChoiceBeginner
Sharded counters for real-time leaderboard
leaderboardshardingcounters - Multiple ChoiceBeginner
CDC to search index consistency
searchcdcconsistency - Multiple ChoiceBeginner
Mobile push token lifecycle handling
push-notificationstoken-managementapns-fcm - Multiple ChoiceIntermediate
Multi-region failover RTO vs RPO
multi-regionfailoverrto-rpo - Multiple ChoiceIntermediate
ETL vs ELT for schema drift
data-pipelineeltschema-drift - Multiple ChoiceIntermediate
Distributed tracing tail sampling rationale
tracingsamplingobservability - Multiple ChoiceIntermediate
Refresh token rotation security
oauth2refresh-tokenrotationsecurity - Multiple ChoiceIntermediate
Policy decision caching correctness
authorizationcachingpolicy-evaluation - Multiple ChoiceIntermediate
[Testing] Line Code execution measurement
qatestingtestingqa - Multiple ChoiceIntermediate
[Testing] Branch Code execution measurement
qatestingtestingqa - Multiple ChoiceIntermediate
[Testing] Mutation Code execution measurement
qatestingtestingqa - Multiple ChoiceIntermediate
[Testing] 80% myth Code execution measurement
qatestingtestingqa - Multiple ChoiceIntermediate
[Testing] Threshold CI Code execution measurement
qatestingtestingqa - 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
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