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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All types Multiple Choice Code Analysis Code

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All levels Beginner Intermediate Hard

Popular topics

345 challenges matching your filters

Code AnalysisHard
Time-Series Cardinality Explosion
metricscardinalitytime-series
Code AnalysisHard
Mutable Balance Ledger Correction
ledgerimmutabilitydouble-entry
Code AnalysisHard
Reconciliation Missing Rows Bug
reconciliationsqlouter-join
Code AnalysisHard
Quiet Hours Suppression Logic Bug
notificationsquiet-hourstimezone
Code AnalysisHard
Chat Ordering by Client Clock
chatorderingclocks
Code AnalysisHard
Edge buffer overflow dropping IoT data
iotedge-bufferingmqtt
Code AnalysisHard
Log query scanning cold tier
log-aggregationquery-at-scaleindexing
Code AnalysisHard
Feature leakage across train/serve
recommendationfeature-storetrain-serve-skew
Code AnalysisHard
Sequential testing without peeking control
experimentationsequential-testingstatistics
Code AnalysisHard
Backfill double-counting aggregates
data-pipelinebackfillidempotency
Code AnalysisHard
Trace assembly with clock skew
tracingtrace-assemblyclock-skew
Code AnalysisHard
KMS secret rotation leaving stale ciphertext
secretsrotationkms
Code AnalysisHard
Hot leaderboard partition on one key
leaderboardhot-partitionredis
Code AnalysisHard
Webhook deliveries without ordering guard
webhooksorderingdlqbackpressure
Code AnalysisHard
Saga: non-idempotent compensation
architecturedistributedsagaidempotency
Code AnalysisHard
Dual-write inconsistency
architectureedaoutbox
Code AnalysisHard
Facade hides subsystem steps
architecturedesign-patternsfacade
Code AnalysisHard
Thundering herd on expiry
architectureresiliencecaching
Code AnalysisHard
Idempotency: naive dedupe key
architectureidempotencyconcurrency
Code AnalysisHard
Aggregate invariants vs eventual consistency
architecturedddconsistency
Code AnalysisHard
Anti-corruption layer absence
architecturedddacl
Code AnalysisHard
Feature flag interaction bug
architecturefeature-flagstesting
Code AnalysisHard
Gateway timeout as circuit breaker signal
architectureresiliencecircuit-breaker
Code AnalysisHard
Repository per aggregate only
architecturedddrepository
Code AnalysisHard
Event schema evolution break
architectureedaversioning
Code AnalysisHard
Saga pivot vs compensation ordering
architecturesagadistributed
Code AnalysisHard
Analyze: aggregate invariant leak
aggregateinvariantsddd
Code AnalysisHard
Violated Principle: Layering Leak
layeringdipboundaries
Code AnalysisHard
Projection Rebuild Requirement
event-sourcingprojection-rebuildupcasting
Code AnalysisHard
Saga vs Process Manager
sagaprocess-managerorchestration
Code AnalysisHard
Aggregate Design for Concurrency
aggregatesconcurrencyddd
Code AnalysisHard
Outbox vs CDC
outboxcdcdual-write
Code AnalysisHard
ACL Translation Layer
acldddintegration
Code AnalysisHard
Functional Core, Imperative Shell
functional-coreimperative-shelltestability
Code AnalysisHard
Conway's Law Implication
conways-laworg-designteam-topologies
Code AnalysisHard
Multi-Tenancy Isolation Choice
multi-tenancyisolationsilopoolbridge
Code AnalysisHard
Violated principle in subclass override
lspsolidcode-analysis
Code AnalysisHard
Wrong abstraction refactor
wrong-abstractionrefactoringcode-analysis
Code AnalysisHard
CRDT operation design
crdtcollaborative-editingcode-analysis
Code AnalysisHard
Rules engine vs hard-coded conditions
rules-enginedecision-tablescode-analysis
Code AnalysisHard
Which Failure Mode FMEA
fmeafailure-modestimeoutscode-analysis
Code AnalysisHard
Which Security Boundary Gap
zero-trustsecurity-architecturecode-analysis
Code AnalysisHard
Which Idempotency Violation
idempotencyat-least-oncecode-analysis
Code AnalysisHard
Which Blast Radius Flaw
shuffle-shardingblast-radiuscode-analysis
Code AnalysisHard
Which Admission Control Flaw
admission-controloverloadcode-analysis

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.

Coding Challenges

Time-Series Cardinality Explosion

Mutable Balance Ledger Correction

Reconciliation Missing Rows Bug

Quiet Hours Suppression Logic Bug

Chat Ordering by Client Clock

Edge buffer overflow dropping IoT data

Log query scanning cold tier

Feature leakage across train/serve

Sequential testing without peeking control

Backfill double-counting aggregates

Trace assembly with clock skew

KMS secret rotation leaving stale ciphertext

Hot leaderboard partition on one key

Webhook deliveries without ordering guard

Saga: non-idempotent compensation

Dual-write inconsistency

Facade hides subsystem steps

Thundering herd on expiry

Idempotency: naive dedupe key

Aggregate invariants vs eventual consistency

Anti-corruption layer absence

Feature flag interaction bug

Gateway timeout as circuit breaker signal

Repository per aggregate only

Event schema evolution break

Saga pivot vs compensation ordering

Analyze: aggregate invariant leak

Violated Principle: Layering Leak

Projection Rebuild Requirement

Saga vs Process Manager

Aggregate Design for Concurrency

Outbox vs CDC

ACL Translation Layer

Functional Core, Imperative Shell

Conway's Law Implication

Multi-Tenancy Isolation Choice

Violated principle in subclass override

Wrong abstraction refactor

CRDT operation design

Rules engine vs hard-coded conditions

Which Failure Mode FMEA

Which Security Boundary Gap

Which Idempotency Violation

Which Blast Radius Flaw

Which Admission Control Flaw

Practice now, dominate the arena