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.
Filter by type
Filter by difficulty
Popular topics
5,893 challenges total
- Multiple ChoiceHard
Optimistic vs pessimistic locking
system-designconcurrencydatabases - Multiple ChoiceIntermediate
Split-brain
system-designconsensusavailability - Multiple ChoiceIntermediate
Fencing tokens
system-designconsensusleader-election - Multiple ChoiceIntermediate
Outbox relay ordering
system-designoutboxmessaging - Multiple ChoiceBeginner
Distributed tracing purpose
system-designobservabilitytracing - Multiple ChoiceHard
Tail-based sampling
system-designobservabilitytracing - Multiple ChoiceIntermediate
Metrics cardinality explosion
system-designobservabilitymetrics - Multiple ChoiceIntermediate
Exponential backoff with jitter
system-designavailabilityretry - Multiple ChoiceHard
Retry amplification
system-designavailabilityretry - Multiple ChoiceBeginner
Failover active-passive vs active-active
system-designavailabilityfailover - Multiple ChoiceBeginner
Nines and downtime budget
system-designavailabilitynines - Multiple ChoiceIntermediate
Idempotency store TTL
system-designidempotencyapi-design - Multiple ChoiceHard
Quorum without read repair
system-designquorumconsistency - Multiple ChoiceIntermediate
Read-through cache
system-designcaching - Multiple ChoiceHard
Cache invalidation on write
system-designcachingcache-invalidation - Multiple ChoiceBeginner
Back-of-envelope QPS from DAU
capacity-estimationqpsback-of-envelope - Multiple ChoiceIntermediate
Consistent hashing node removal impact
consistent-hashingrebalancingpartitioning - Multiple ChoiceBeginner
Why negative caching helps
cachingcache-penetrationnegative-caching - Multiple ChoiceHard
LSM tree read amplification tradeoff
lsm-treestorage-engineread-amplification - Multiple ChoiceBeginner
WAL durability on crash
waldurabilitycrash-recovery - Multiple ChoiceHard
Write skew under snapshot isolation
mvccwrite-skewisolation-levels - Multiple ChoiceIntermediate
Quorum read-write overlap formula
quorumreplicationconsistency - Multiple ChoiceBeginner
Read-your-writes routing
replication-lagread-your-writesconsistency - Multiple ChoiceHard
Raft safety vs Paxos roles
raftpaxosconsensus - Multiple ChoiceBeginner
Kafka partition ordering guarantee
kafkapartitioningordering - Multiple ChoiceIntermediate
Exactly-once via idempotent producer
kafkaexactly-oncetransactions - Multiple ChoiceBeginner
SQS visibility timeout semantics
sqsvisibility-timeoutat-least-once - Multiple ChoiceHard
Sliding window counter hybrid tradeoff
rate-limitingsliding-windowalgorithms - Multiple ChoiceHard
Distributed rate limiting consistency
rate-limitingdistributedconsistency - Multiple ChoiceIntermediate
GraphQL resolver N+1
graphqln+1batching - Multiple ChoiceBeginner
Keyset pagination on a busy table
paginationcursorperformance - Multiple ChoiceHard
Fencing tokens for stale leaders
fencing-tokenssplit-brainleader-election - Multiple ChoiceBeginner
Snowflake ID clock rollback
snowflakeclock-skewunique-id - Multiple ChoiceIntermediate
Error budget burn policy
sloerror-budgetreliability - Multiple ChoiceBeginner
Circuit breaker half-open state
circuit-breakerresiliencefault-tolerance - Multiple ChoiceBeginner
Retry backoff with jitter
retrybackoffjitter - Multiple ChoiceIntermediate
Tail sampling versus head sampling
tracingtail-samplingobservability - Multiple ChoiceBeginner
S3 strong read-after-write consistency
s3consistencyobject-storage - Multiple ChoiceHard
Redlock safety criticism
distributed-locksredlockclock-skew - Multiple ChoiceBeginner
Multi-region active-active vs active-passive
multi-regionavailabilitytopology - Multiple ChoiceIntermediate
Data residency constraint enforcement
gdprdata-residencycompliance - Multiple ChoiceBeginner
Cross-region quorum latency cost
quorumlatencymulti-region - Multiple ChoiceHard
OR-Set vs LWW register semantics
crdtor-setconflict-resolution - Multiple ChoiceBeginner
Hybrid logical clock purpose
hlcclockscausality - Multiple ChoiceBeginner
Bloom filter false negative property
bloom-filterprobabilisticmembership - Multiple ChoiceHard
Cuckoo filter vs bloom filter trade-off
cuckoo-filterbloom-filterdeletion - Multiple ChoiceBeginner
HyperLogLog cardinality error
hyperloglogcardinalityapproximation - Multiple ChoiceHard
t-digest for streaming percentiles
t-digestpercentilesstreaming - Multiple ChoiceBeginner
Skip list expected complexity
skip-listdata-structurescomplexity - Multiple ChoiceIntermediate
Size-tiered vs leveled compaction
lsmcompactionwrite-amplification - Multiple ChoiceHard
Tombstone accumulation in LSM
tombstoneslsmcompaction - Multiple ChoiceIntermediate
B+tree vs LSM read/write trade-off
btreelsmtrade-offs - Multiple ChoiceBeginner
Postgres autovacuum bloat cause
postgresvacuumbloat - Multiple ChoiceHard
HOT updates and indexed columns
postgreshot-updatesindexes - Multiple ChoiceIntermediate
Connection pool sizing math
connection-poolsizingcapacity - Multiple ChoiceHard
PgBouncer transaction vs session pooling
pgbouncerconnection-poolpostgres - Multiple ChoiceBeginner
Read replica routing lag hazard
read-replicasreplication-lagconsistency - Multiple ChoiceBeginner
Hash sharding hotspot risk
shardinghashinghotspots - Multiple ChoiceHard
Dual-write migration correctness
migrationdual-writeconsistency - Multiple ChoiceBeginner
Full jitter retry rationale
backoffjitterretries
Practice now, dominate the arena
Every challenge here is drawn from the live Code Royale pool. Review a question, then head to the arena to answer under pressure against a real opponent — 2-minute matches, Elo ranking, and badge tiers on the line.