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 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 - Multiple ChoiceIntermediate
Capacity estimation: peak vs average
system-designcapacity-estimation - Multiple ChoiceBeginner
Storage sizing with replication
system-designcapacity-estimationstorage - Multiple ChoiceBeginner
Bandwidth estimation
system-designcapacity-estimationbandwidth - Multiple ChoiceIntermediate
Read-heavy vs write-heavy design
system-designcapacity-estimationdatabases - Multiple ChoiceHard
Little's Law for concurrency
system-designcapacity-estimationlatency - Multiple ChoiceIntermediate
Hot partition in Kafka
system-designmessagingkafkasharding - Multiple ChoiceHard
Kafka consumer group rebalance
system-designmessagingkafka - Multiple ChoiceIntermediate
RabbitMQ vs Kafka model
system-designmessagingkafkarabbitmq - Multiple ChoiceBeginner
SQS visibility timeout
system-designmessagingsqs - Multiple ChoiceHard
Visibility timeout too short
system-designmessagingsqsidempotency - Multiple ChoiceIntermediate
Thundering herd on cache key deletion
system-designcachingstampede - Multiple ChoiceBeginner
Cache warming
system-designcaching - Multiple ChoiceIntermediate
Bloom filter pre-check
system-designcachingbloom-filter - Multiple ChoiceBeginner
Idempotent HTTP methods
system-designapi-designidempotency - Multiple ChoiceHard
Lost update race
system-designconcurrencyconsistency
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.