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 ChoiceBeginner
Snowflake ID structure
system-designunique-idsnowflake - Multiple ChoiceBeginner
Service discovery
system-designservice-discovery - Multiple ChoiceBeginner
Liveness vs readiness probe
system-designhealth-checkskubernetes - Multiple ChoiceBeginner
Storage sizing with replication
system-designcapacity-estimationstorage - Multiple ChoiceBeginner
Bandwidth estimation
system-designcapacity-estimationbandwidth - Multiple ChoiceBeginner
SQS visibility timeout
system-designmessagingsqs - Multiple ChoiceBeginner
Cache warming
system-designcaching - Multiple ChoiceBeginner
Idempotent HTTP methods
system-designapi-designidempotency - Multiple ChoiceBeginner
Distributed tracing purpose
system-designobservabilitytracing - Multiple ChoiceBeginner
Failover active-passive vs active-active
system-designavailabilityfailover - Multiple ChoiceBeginner
Nines and downtime budget
system-designavailabilitynines - Multiple ChoiceBeginner
Back-of-envelope QPS from DAU
capacity-estimationqpsback-of-envelope - Multiple ChoiceBeginner
Why negative caching helps
cachingcache-penetrationnegative-caching - Multiple ChoiceBeginner
WAL durability on crash
waldurabilitycrash-recovery - Multiple ChoiceBeginner
Read-your-writes routing
replication-lagread-your-writesconsistency - Multiple ChoiceBeginner
Kafka partition ordering guarantee
kafkapartitioningordering - Multiple ChoiceBeginner
SQS visibility timeout semantics
sqsvisibility-timeoutat-least-once - Multiple ChoiceBeginner
Keyset pagination on a busy table
paginationcursorperformance - Multiple ChoiceBeginner
Snowflake ID clock rollback
snowflakeclock-skewunique-id - Multiple ChoiceBeginner
Circuit breaker half-open state
circuit-breakerresiliencefault-tolerance - Multiple ChoiceBeginner
Retry backoff with jitter
retrybackoffjitter - Multiple ChoiceBeginner
S3 strong read-after-write consistency
s3consistencyobject-storage - Multiple ChoiceBeginner
Multi-region active-active vs active-passive
multi-regionavailabilitytopology - Multiple ChoiceBeginner
Cross-region quorum latency cost
quorumlatencymulti-region - Multiple ChoiceBeginner
Hybrid logical clock purpose
hlcclockscausality - Multiple ChoiceBeginner
Bloom filter false negative property
bloom-filterprobabilisticmembership - Multiple ChoiceBeginner
HyperLogLog cardinality error
hyperloglogcardinalityapproximation - Multiple ChoiceBeginner
Skip list expected complexity
skip-listdata-structurescomplexity - Multiple ChoiceBeginner
Postgres autovacuum bloat cause
postgresvacuumbloat - Multiple ChoiceBeginner
Read replica routing lag hazard
read-replicasreplication-lagconsistency - Multiple ChoiceBeginner
Hash sharding hotspot risk
shardinghashinghotspots - Multiple ChoiceBeginner
Full jitter retry rationale
backoffjitterretries - Multiple ChoiceBeginner
Token bucket burst cap
token-bucketrate-limitingburst - 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 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 ChoiceBeginner
Feature Flag Kill Switch
feature-flagskill-switchrollout - 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
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