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Aporeon

Technical interview practice, from one developer to another.

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Backend System Design Foundation

Reliability

Failure handling, rate limiting, retries, timeouts, dependency behavior, and operational resilience.

32 drillsPractice one at a timeOrdered pathConcept questionComparison questionArchitecture scenarioDebugging scenario

Recommended start

Middleware Ordering

Explains how middleware order affects authentication, authorization, error handling, routing, CORS, and response behavior.

Medium~7 minStep 1 of 32

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32 of 32 shown

1Middleware OrderingRecommended startExplains how middleware order affects authentication, authorization, error handling, routing, CORS, and response behavior.Concept questionMedium~7 min.NETASP.NET CoreC#Practice 2CancellationToken in APIsPropagates request cancellation through async dependencies while keeping committed side effects and timeout policy explicit.Concept questionMedium~7 min.NETASP.NET CoreC#Practice 3AuthN vs AuthZSeparates identity establishment from permission decisions and applies ASP.NET Core schemes, policies, tenant boundaries, ownership checks, and correct challenge or forbid behavior.Comparison questionEasy~5 min.NETASP.NET CoreC#Practice 4Idempotent POSTMakes POST retries return one durable business outcome by combining scoped keys, request identity, atomic ownership, response replay, and reconciliation.Architecture scenarioHard~12 min.NETASP.NET CoreC#Practice 5Correlation IDsUse correlation IDs, structured logs, and logging scopes to make API failures traceable in production.Architecture scenarioMedium~10 min.NETASP.NET CoreC#Practice 6Dependency TimeoutsBuilds dependency-specific timeout budgets that contain slow SQL, HTTP, and cache calls without creating unsafe retries or ambiguous writes.Architecture scenarioMedium~10 min.NETASP.NET CoreC#Practice 7Slow API DiagnosisInvestigate a slow API by checking database queries, blocking async, dependency latency, serialization, logging, and runtime pressure.Debugging scenarioHard~12 min.NETASP.NET CoreC#Practice 8401 vs 403 DebuggingDebugs ASP.NET Core authentication challenges and authorization forbids across schemes, token validation, policies, claims, tenants, and resource checks.Debugging scenarioMedium~10 min.NETASP.NET CoreC#Practice 9No Correlation IDHandle a production support case where logs exist but related request events cannot be connected.Debugging scenarioMedium~10 min.NETASP.NET CoreC#Practice 10Dependency Timeout IncidentInvestigate and mitigate API failures caused by a slow or failing downstream dependency.Debugging scenarioHard~12 min.NETASP.NET CoreC#Practice 11Rate LimitingDesign and debug rate limiting while protecting the API without blocking legitimate users and integrations.Architecture scenarioHard~12 min.NETASP.NET CoreC#Practice 12Duplicate Create RetryInvestigate and fix an API where client retries create duplicate orders, payments, or records.Debugging scenarioHard~12 min.NETASP.NET CoreC#Practice 13Thread Pool StarvationDiagnose an ASP.NET Core API that becomes slow because request threads are blocked or exhausted.Debugging scenarioHard~12 min.NETASP.NET CoreC#Practice 14Transaction BoundariesDraws a short atomic database boundary around one invariant while moving network side effects into explicit, recoverable workflow states.Architecture scenarioHard~12 min.NETC#EF CorePractice 15API ObservabilityDefine logs, metrics, traces, alerts, and dashboards needed to operate an ASP.NET Core API in production.Architecture scenarioHard~12 min.NETASP.NET CoreC#Practice 16SQL DeadlocksUses deadlock evidence to correct EF Core transaction scope, query plans, and resource order, with bounded retries only for repeat-safe victims.Debugging scenarioHard~12 min.NETC#EF CorePractice 17Isolation LevelsExplains transaction isolation levels with practical .NET data-access examples and production trade-offs.Concept questionHard~9 min.NETC#EF CorePractice 18EF Core RetriesExplains EF Core connection resiliency and retries with practical .NET data-access examples and production trade-offs.Concept questionHard~9 min.NETC#EF CorePractice 19EF Query DiagnosticsExplains EF Core query performance diagnostics with practical .NET data-access examples and production trade-offs.Debugging scenarioHard~12 min.NETC#EF CorePractice 20Outbox PatternUses a transactional outbox to make a local state change durably publishable while accepting at-least-once delivery and operating the backlog safely.System designHard~20 min.NETC#EF CorePractice 21High DB CPUExplains high database CPU after an API change with practical .NET data-access examples and production trade-offs.Debugging scenarioHard~12 min.NETC#EF CorePractice 22Connection Pool ExhaustionExplains database connection pool exhaustion with practical .NET data-access examples and production trade-offs.Debugging scenarioHard~12 min.NETC#EF CorePractice 23Query TimeoutsFits database command timeouts into endpoint latency budgets while diagnosing slow SQL, blocking, pool pressure, and uncertain write outcomes.Concept questionMedium~7 min.NETC#EF CorePractice 24Singleton CaptureReviews a captive scoped dependency as both a lifetime and concurrency bug, then replaces it with explicit per-operation scopes and minimal immutable work data.Code reviewHard~12 min.NETC#Practice 25Unbounded CacheExplains cache memory growth, eviction, size limits, stale data, and why cache design needs explicit bounds.Architecture scenarioMedium~10 min.NETASP.NET CoreC#Practice 26Batch Memory SafetyCovers chunking, bounded concurrency, scoped dependencies, progress, retries, and avoiding full-dataset memory spikes.Architecture scenarioHard~12 min.NETASP.NET CoreC#Practice 27Memory Growth InvestigationGives a practical incident flow for memory growth using metrics, traffic correlation, heap evidence, and safe mitigation.Debugging scenarioHard~12 min.NETASP.NET CoreC#Practice 28Heap Dump ComparisonExplains when to capture dumps, what to compare, and how to use roots without turning dumps into guesswork.Debugging scenarioHard~12 min.NETASP.NET CoreC#Practice 29High GC CPUCovers allocation rate, collection frequency, latency symptoms, and safe production steps for GC-heavy services.Debugging scenarioHard~12 min.NETASP.NET CoreC#Practice 30OOM ResponsePractices incident response after an OOM: restore service, collect evidence, identify trigger, and prevent recurrence.Debugging scenarioHard~12 min.NETASP.NET CoreC#Practice 31Memory MetricsIdentifies useful memory and GC signals for alerting without overwhelming the team with noisy telemetry.Concept questionMedium~7 min.NETASP.NET CoreC#Practice 32Dump SecurityExplains why memory diagnostics need access control, retention limits, and careful handling of secrets and user data.Concept questionMedium~7 min.NETASP.NET CoreC#Practice

Practice order

32 drillsOne at a time

1Middleware OrderingUp next / Medium / ~7 min
2CancellationToken in APIsMedium / ~7 min
3AuthN vs AuthZEasy / ~5 min
4Idempotent POSTHard / ~12 min
5Correlation IDsMedium / ~10 min
6Dependency TimeoutsMedium / ~10 min
7Slow API DiagnosisHard / ~12 min
8401 vs 403 DebuggingMedium / ~10 min

+24 more drills in this section