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.
Filter by type
Filter by difficulty
Popular topics
150 challenges matching your filters
Clear topic filter- Multiple ChoiceBeginner
Stateless services and sticky sessions
system-designscalability - Multiple ChoiceBeginner
Vertical vs horizontal scaling limit
system-designscalability - Multiple ChoiceBeginner
L4 vs L7 load balancer
system-designload-balancing - Multiple ChoiceBeginner
Consistent hashing vs modulo hashing
system-designload-balancingconsistent-hashing - Multiple ChoiceIntermediate
Virtual nodes in consistent hashing
system-designconsistent-hashing - Multiple ChoiceBeginner
Cache-aside vs write-through
system-designcaching - Multiple ChoiceBeginner
Write-back cache risk
system-designcaching - Multiple ChoiceBeginner
LRU vs LFU eviction
system-designcachingeviction - Multiple ChoiceHard
Cache stampede / thundering herd
system-designcachingstampede - Multiple ChoiceHard
Probabilistic early expiration
system-designcachingstampede - Multiple ChoiceBeginner
Negative caching
system-designcaching - Multiple ChoiceIntermediate
TTL jitter
system-designcaching - Multiple ChoiceBeginner
CDN purpose
system-designcachingcdn - Multiple ChoiceIntermediate
CDN invalidation tradeoff
system-designcdncache-invalidation - Multiple ChoiceBeginner
Cache penetration
system-designcaching - Multiple ChoiceBeginner
Read replica lag
system-designdatabasesreplication - Multiple ChoiceIntermediate
Read-your-writes consistency
system-designconsistencyreplication - Multiple ChoiceBeginner
SQL vs NoSQL access pattern
system-designdatabasesnosql - Multiple ChoiceBeginner
Index write amplification
system-designdatabasesindexes - Multiple ChoiceHard
Sharding key choice
system-designdatabasessharding - Multiple ChoiceHard
Quorum read/write overlap
system-designconsistencyquorum - Multiple ChoiceBeginner
Leader election with consensus
system-designconsensusleader-election - Multiple ChoiceBeginner
CAP theorem statement
system-designcap-theorem - Multiple ChoiceIntermediate
PACELC extension
system-designpacelccap-theorem - Multiple ChoiceIntermediate
Serializable vs linearizable
system-designconsistency - Multiple ChoiceBeginner
Causal consistency
system-designconsistency - Multiple ChoiceBeginner
Eventual consistency tradeoff
system-designconsistency - Multiple ChoiceBeginner
At-least-once semantics
system-designmessagingidempotency - Multiple ChoiceIntermediate
Exactly-once reality
system-designmessagingkafka - Multiple ChoiceIntermediate
Idempotency key design
system-designapi-designidempotency - Multiple ChoiceIntermediate
Backpressure
system-designmessagingbackpressure - Multiple ChoiceBeginner
Dead letter queue
system-designmessagingdlq - Multiple ChoiceBeginner
Poison message and head-of-line blocking
system-designmessagingdlq - Multiple ChoiceBeginner
Token bucket burst behavior
system-designrate-limiting - Multiple ChoiceIntermediate
Distributed rate limiting accuracy
system-designrate-limitingdistributed - Multiple ChoiceIntermediate
Sliding window vs fixed window
system-designrate-limiting - Multiple ChoiceBeginner
Sliding window log memory cost
system-designrate-limiting - Multiple ChoiceIntermediate
gRPC vs REST
system-designapi-designgrpc - Multiple ChoiceBeginner
Cursor vs offset pagination
system-designapi-designpagination - Multiple ChoiceBeginner
API versioning rationale
system-designapi-designversioning - Multiple ChoiceHard
Two-phase commit blocking
system-designdistributed-transactions2pc - Multiple ChoiceIntermediate
Saga pattern compensation
system-designdistributed-transactionssaga - Multiple ChoiceIntermediate
Outbox pattern
system-designdistributed-transactionsoutbox - Multiple ChoiceBeginner
SLI vs SLO vs SLA
system-designobservabilityslo - Multiple ChoiceBeginner
p99 vs mean latency
system-designobservabilitylatency - Multiple ChoiceIntermediate
Availability math
system-designavailability - Multiple ChoiceIntermediate
Circuit breaker states
system-designavailabilitycircuit-breaker - Multiple ChoiceHard
Bulkhead isolation
system-designavailabilitybulkhead - Multiple ChoiceBeginner
Blob vs object storage
system-designstorages3 - Multiple ChoiceIntermediate
Pre-signed URLs
system-designstorages3 - Multiple ChoiceIntermediate
WebSocket vs long polling
system-designreal-timewebsockets - Multiple ChoiceHard
Fan-out on write vs read
system-designreal-timefan-outnews-feed - Multiple ChoiceBeginner
Inverted index
system-designsearchinverted-index - Multiple ChoiceIntermediate
Elasticsearch refresh vs commit
system-designsearchelasticsearch - Multiple ChoiceBeginner
URL shortener key generation
system-designurl-shortenerunique-id - Multiple ChoiceBeginner
Snowflake ID structure
system-designunique-idsnowflake - Multiple ChoiceIntermediate
Clock skew and Snowflake
system-designunique-idsnowflake - Multiple ChoiceIntermediate
Microservices vs monolith
system-designmicroservices - Multiple ChoiceBeginner
Service discovery
system-designservice-discovery - Multiple ChoiceBeginner
Liveness vs readiness probe
system-designhealth-checkskubernetes
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 — 3-minute matches, Elo ranking, and badge tiers on the line.