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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528 challenges matching your filters

Code AnalysisBeginner
Rebalance storm from long processing
kafkarebalanceconsumer-groups
Code AnalysisBeginner
N+1 query in ORM loop
ormn+1query-optimization
Code AnalysisBeginner
Index-killing function on column
indexingsargabilitysql
Code AnalysisBeginner
Window boundary allows double rate
rate-limitingfixed-windowburst
Code AnalysisBeginner
GCRA smoothness tuning
gcrarate-limitingsmoothing
Code AnalysisBeginner
Ledger double-entry balance check
ledgerdouble-entryinvariants
Code AnalysisBeginner
Kafka per-key ordering pitfall
kafkaorderingpartitioning
Code AnalysisBeginner
Stale-while-revalidate cache bug
cachingswrcdn
Code AnalysisBeginner
Count-min heavy hitters shape
count-minheavy-hitterssketch
Code AnalysisBeginner
Transactional Outbox Pattern for Payments
outboxpaymentsevents
Code AnalysisBeginner
Saga Compensation for Money Transfer
sagatransfercompensation
Code AnalysisBeginner
Notification Fan-Out Deduplication
notificationsdedupretries
Code AnalysisBeginner
Geospatial Index for Ride Matching
ride-hailinggeospatialmatching
Code AnalysisBeginner
Trie Top-K for Search Autocomplete
autocompletetrietop-k
Code AnalysisBeginner
Dual Write Ledger and Event Bug
outboxdual-writeevents
Code AnalysisBeginner
Anemic model allows invalid state
architecturedddanemic-model
Code AnalysisBeginner
SRP: god class
architecturesolidsrp
Code AnalysisBeginner
LSP: equals/hashCode mismatch
architecturesolidlsp
Code AnalysisBeginner
Coupling metric read
architecturemetricscoupling
Code AnalysisBeginner
Command with undo
architecturedesign-patternscommand
Code AnalysisBeginner
Feature flag stale branch
architecturefeature-flagstechnical-debt
Code AnalysisBeginner
Command/query separation violation
architecturecqrsprinciples
Code AnalysisBeginner
ISP in a module interface
architecturesolidisp
Code AnalysisBeginner
CQRS lag breaks read-your-writes
architecturecqrseventual-consistency
Code AnalysisBeginner
Anemic model with validation elsewhere
architecturedddanemic-model
Code AnalysisBeginner
Dependency inversion missing in service
architecturesoliddip
Code AnalysisBeginner
Choreography observability gap
architectureedasaga
Code AnalysisBeginner
Shared module creates cycle at package level
architecturemodularitydependencies
Code AnalysisBeginner
Bulkhead absence cascades
architectureresiliencebulkhead
Code AnalysisBeginner
Test double misuse
architecturetestingtest-doubles
Code AnalysisBeginner
Branch by abstraction setup
architecturerefactoringbranch-by-abstraction
Code AnalysisBeginner
Value object mutability bug
architecturedddvalue-object
Code AnalysisBeginner
Modular monolith boundary enforcement
architecturemodularitymonolith
Code AnalysisBeginner
Query side should not be reused for writes
architecturecqrsseparation
Code AnalysisBeginner
Anemic DTO vs domain object
architecturedddanemic-model
Code AnalysisBeginner
Strangler deletion debt
architecturemigrationstrangler-fig
Code AnalysisBeginner
Dependency injection container misuse
architecturedicoupling
Code AnalysisBeginner
Cyclic dependency via event handler
architecturemodularityeda
Code AnalysisBeginner
Analyze: feature envy smell
feature-envytell-dont-askrefactoring
Code AnalysisBeginner
Coupling Metrics: Instability
coupling-metricsmartin-metricsdependency-graph
Code AnalysisBeginner
Policy as Code
policy-as-codeopaauthorization
Code AnalysisBeginner
Abstraction leakage in repository
abstraction-leakagerepositorycode-analysis
Code AnalysisBeginner
Module boundary violation
module-boundariesbuild-enforcementcode-analysis
Code AnalysisBeginner
What Debt Quadrant Is This
debt-quadranttechnical-debtcode-analysis
Code AnalysisBeginner
Which YAGNI Violation
yagniover-engineeringcode-analysis
Code AnalysisBeginner
Which Delivery Strategy
progressive-deliverycanarycode-analysis
Code AnalysisBeginner
Which Observability Boundary Gap
tracingasynccorrelation-idcode-analysis
Code AnalysisBeginner
Which Documentation Approach
documentation-as-codestructurizradrcode-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

Rebalance storm from long processing

N+1 query in ORM loop

Index-killing function on column

Window boundary allows double rate

GCRA smoothness tuning

Ledger double-entry balance check

Kafka per-key ordering pitfall

Stale-while-revalidate cache bug

Count-min heavy hitters shape

Transactional Outbox Pattern for Payments

Saga Compensation for Money Transfer

Notification Fan-Out Deduplication

Geospatial Index for Ride Matching

Trie Top-K for Search Autocomplete

Dual Write Ledger and Event Bug

Anemic model allows invalid state

SRP: god class

LSP: equals/hashCode mismatch

Coupling metric read

Command with undo

Feature flag stale branch

Command/query separation violation

ISP in a module interface

CQRS lag breaks read-your-writes

Anemic model with validation elsewhere

Dependency inversion missing in service

Choreography observability gap

Shared module creates cycle at package level

Bulkhead absence cascades

Test double misuse

Branch by abstraction setup

Value object mutability bug

Modular monolith boundary enforcement

Query side should not be reused for writes

Anemic DTO vs domain object

Strangler deletion debt

Dependency injection container misuse

Cyclic dependency via event handler

Analyze: feature envy smell

Coupling Metrics: Instability

Policy as Code

Abstraction leakage in repository

Module boundary violation

What Debt Quadrant Is This

Which YAGNI Violation

Which Delivery Strategy

Which Observability Boundary Gap

Which Documentation Approach

Practice now, dominate the arena