Concept guides.
582interactive tutorials on the ideas behind the questions — learn the concept, then practice the questions built on it.
Interview concepts
AI, data & design
A structured design critiqueAccessibility basics: keyboard, contrast, announceCatching AI's confident mistakesDebugging as hypothesis testingDelegating to AI agents safelyEvery screen has six statesJoins that keep the right rowsPrecision vs recall in product termsPrompt craft for everyday workReview comments that landTelling a project: context, constraint, decision, outcomeVisual hierarchy: directing the eyeWhat data can go into which AI toolWhich tasks to hand to AIWhy your query is slow: reading the planWindow functions: running totals, ranks and gaps
Business & analytical
A/B test design: hypothesis, randomization, primary metricAnswer-first: delivering findings under time pressureCohort analysis and retention curvesExperiment pitfalls: peeking, novelty, multiple comparisonsFermi estimation: decompose, anchor, sanity-checkFunnel metrics and conversion decompositionGuardrail metrics and healthy growthInterference and choosing the randomization unitMarket sizing: top-down vs bottom-upMetric anomaly triage: real move or artifact?North-star metrics and metric treesPrioritization: RICE and impact vs effortProduct sense: user, problem, solution, tradeoffsProfitability trees and MECE decompositionSegmenting a metric move: mix shift and Simpson's paradoxStatistical power and sample sizeUnit economics: CAC, LTV, and paybackWhen you can't A/B test: quasi-experiments and holdouts
Education & teaching
Backward design: objective, evidence, then activitiesBehavior change is data: diagnose, document, escalateChecking for understanding — and acting on itDe-escalation in the momentI do, we do, you do: the lesson arcOne lesson, many levelsOpenings that earn attentionParent conversations that keep the partnershipRestorative conversations and repairing harmRoutines, norms, and getting adults alignedSequencing, spiraling, and pacingTeaching against misconceptionsTeaching when every student has AIUnpacking standards and proving alignment
Healthcare & clinical
Aviate, navigate, communicate: task priority in an emergencyBreaking bad news with SPIKESBriefing the cabin: NITS and passenger announcementsConfidentiality and its limitsDecision-making capacity: the four abilitiesDeclaring an emergency: MAYDAY, PAN-PAN, and working ATCDocumentation that stands up laterGoals-of-care conversations and family meetingsHypothesis-driven history and the differentialInformed consent: the elements that make it validMemory items and QRH checklist disciplineMotivational interviewing: behavior change without lecturingRed flags and the first five minutes: can't-miss diagnoses and ABCDERisk stratification: scores, pretest probability, and dispositionWhen to call ethics — and when not toWho decides when the patient can't: surrogates and advance directives
Legal & policy
BLUF: the one-page decision briefComparing policy options with a criteria matrixContract anatomy: where the risk lives and how to move itImplementation feasibility: capacity, cost, and rolloutIssue-spotting sweep: parties, duties, breaches, remediesJurisdiction, venue, and forum selectionMurder-boarding the brief: anticipating the questionsNegotiating clauses: fallback ladders and trade-offsProof problems: oral deals, hearsay, spoliation, privilegeSequencing evidence in a brief: strong first, caveats ownedStakeholder mapping: who's affected, who decides, who fightsStatute of limitations: accrual, discovery, and tollingTightening vague standards: from 'reasonable' to measurableTranslating for non-experts: plain-language summariesUnintended consequences: modeling behavioral responsesWeighing policy evidence: validity and transferability
Marketing & sales
Attribution models vs incrementalityBudget allocation and the media mixCAC three ways: blended, paid, and marginalCampaign brief anatomy: audience, insight, message, measurementClosed loops and shared mental modelsDifferentiation maps: finding open spaceDiscovery questions that ladder to impactMessage hierarchy: choosing what leadsMutual close plans and deal momentumOwning errors: reporting and the blame-free debriefPositioning statements and proof pointsPrice pressure: value framing, anchors, and give-getQualifying with BANT and MEDDICSpeaking up: graded assertiveness in the cockpitStakeholder maps: champion, economic buyer, blockersThe objection loop: acknowledge, isolate, reframe, confirmUnit economics: ROAS, margin, and LTVWorkload management and task shedding
Operations & industrial
Anchors, concession ladders & multi-issue tradesBATNA & ZOPA: know your walk-away before you sit downBlameless postmortems & debriefs that change somethingBottlenecks & queues: why waits explode near capacityChannel economics: OTA commissions vs directChecklists, runbooks & the pre-shift briefingCrisis communications: what to say before you know anythingIncident command: severity, roles & the first hourLean diagnosis: eight wastes & value-stream mapsLetter vs intent: prescriptive & performance-based complianceLoads, margins & factors of safetyMaterial selection: environment, lifecycle & failure modesPacing, pickup & displacement: forecast-driven decisionsPreserve the evidence, reconstruct the sequencePrevention that sticks: CAPA & the hierarchy of controlsRevPAR & rate fences: selling the same room at many pricesRoot cause: five whys, fishbone & causal layersService recovery: own it, fix it, follow upSignal vs noise: is this spike real?Which code applies? Editions, adoption & the AHJ
People & leadership
Answer first: the pyramid principle for executivesBlameless postmortems: separating the person from the systemCalibration: defending ratings against drift and biasDecision rights: RACI and who really holds the vetoEscalating well: when to raise it and howEthical gut-checks: front-page, harm, and spirit-of-the-rule testsFive conflict styles and when each winsHard conversations: name it in the first 30 secondsHiring debriefs: evidence over vibesInfluence without authorityManaging up: pushing back, then committingOne-way vs two-way doors: deciding at 60% informationPIPs and paper trails: performance intervention done fairlyPrioritizing when everything is urgentSBI: feedback that names behavior, not characterStakeholder mapping: the power–interest gridSTAR and CARL: structuring your storyThe first hour of a crisis: stabilize, learn, communicate
Code Room concepts
Code review
Coding
Access Control RbacAdvanced DP patternsAdvanced graph algorithmsAnalyticsAnomaly DetectionAnti Entropy convergence rounds across a replica setAnycast Edge Routing ProtocolApi Key Issuance AuthApplied crypto & authArrays & in-place tricksAsync Concurrency BackpressureAudit Log SecurityAuthentication SsoAuthorization rbac abacBackup Restore StorageBackup Silent FailureBad Data DeployBinary searchBinary search trees & balancingBinary trees & traversalBit manipulationBitmask DPBlob Storage DedupBlock Storage Volume Detach Stale MountBloom FiltersCache Eviction Storm Storage OriginCarrier Sms Routing ProtocolCdn Cache HierarchyCdn Cache Miss StormCdn Invalidation StormCdn Stale Content TtlCdn Video Edge StorageChange Data Capture ReplicationCold StorageCold Storage Logs MetricsCold Storage Retrieval TimeoutCold Tier Storage Retrieval SlaComment SystemConcurrency Atomic Increment InterleavingConcurrency Await Holding LockConcurrency Barrier Synchronization RoundsConcurrency Check Then Act ToctouConcurrency Data RaceConcurrency Deadlock VictimConcurrency Dining Philosophers Lock OrderingConcurrency Event Loop SchedulingConcurrency Fire And Forget AsyncConcurrency Iterate While ModifyConcurrency Lazy Init RaceConcurrency Missing Lock CompoundConcurrency Non Atomic CounterConcurrency primitivesConcurrency Producer Consumer Bounded BufferConcurrency Race Condition DetectionConcurrency Semaphore Permit SimulationConcurrency Shared Mutable RustConcurrency Stale Read VisibilityConcurrency Thread Unsafe Lazy CppConfig ErrorConnection Pool LeakConnection Pool Lock StarvationConnection Pool RaceConnection Pooling Proxy GatewayConsistent Hashing replica set for N Way replicationCorrupted Sstable DurabilityCoupon ServiceCRDT OR Set replica mergeCron FailureData ProcessingDatabase AggregationDatabase Autovacuum Falling BehindDatabase Connection Pool ExhaustionDatabase Connection Pool ProxyDatabase Covering IndexDatabase Dedup LatestDatabase Disk SaturationDatabase Hot Partition ShardDatabase Index IntersectionDatabase internalsDatabase Lsm CompactionDatabase Partition Maintenance GapDatabase Query OptimizerDatabase Read Replica RoutingDatabase ReplicationDatabase Runaway MigrationDatabase Secondary IndexDatabase Table Index BloatDatabase Wal ReplayDb UnreachableDdos Mitigation SecurityDeadlock Lock OrderingDesigning data structuresDisk Inode ExhaustionDistributed algorithmsDistributed CounterDistributed FilesystemDns Lookup Hot LoopDns Traffic Routing GatewayDurability Bitrot ScrubDynamic programming: 1DDynamic programming: 2D & stringsEdge Realtime Sync ProtocolEgress Proxy GatewayEmbedded Kv TuningEnd To End EncryptionEnv MisconfigEvent Loop BlockedFalse Sharing Cache ContentionFeature FlagsFeature Pipeline Batch Scoring NullsFeature Pipeline Data QualityFeature Pipeline Flag Bad InputsFeature Pipeline Label DelayFeature Pipeline Late DataFeature Pipeline Online Offline SkewFeature Pipeline Point In Time LeakageFeature Pipeline StallFeature Pipeline Store TimeoutFile UploadFraud Detection SecurityFsync Durability Commit LatencyGame Netcode NetworkingGame theory & GrundyGeo Partitioned DatabaseGlobal Photo StorageGraph Database ShardingGraph Db StorageGraph traversal (BFS & DFS)Greedy algorithmsGrpc No DeadlineHash maps & setsHealth FlappingHeaps & priority queuesHttp Client TimeoutHttp Connection LeakHttp Keepalive ChurnHttp No Retry BackoffHttp Retry IdempotencyHttp Status IgnoredHttp Streaming Partial ResponseHttp Unbounded ReadHybrid Logical Clock update for replica orderingImage Video Storage DeliveryIndex CorruptionInference Canary Nan FeatureInference Embedding Index StalenessInference Embedding Store LatencyInference Gpu AutoscalerInference Gpu Memory FragmentationInference Gpu OomInference Output Contract BreakInference Queue BacklogInference Vector Db LatencyInjectionInsecure Deserialization VulnerabilityInterval DPIntervals & sweep lineInventoryJwt Auth VulnerabilityKey rebalancing when a node joins a consistent Hashing clusterKey Value StoreKnapsack & subset-sumLeaderboardLibrary CheckoutLinked listsLivelockLoad Balancer TcpLock Acquisition Deadlock CycleLock Ordering Deadlock AvoidanceLockingLog VolumeLoggingMatchmaking Realtime NetworkingMath & number theoryMetadata Storage Hot Partition SaturationMinimum spanning treesMl Ads BiddingMl Content ModerationMl Cross ValidationMl Fraud DetectionMl Infra ServingMl Online LearningMl Online Learning Feedback LoopML primitives from scratchMl Target LeakageMl Time Series ValidationMl Train Serve SkewMl Training Job OomModel Drift CalibrationModel Evaluation PytorchModel Serving Ab Variant RegressionModel Serving Bad VersionModel Serving Cold StartModel Serving Dependency FallbackModel Serving Recommendation Stale CacheModel Serving Registry OutageModel Serving Shadow Traffic OverloadModel Serving Tokenizer Version MismatchModel Training PytorchMqtt Iot Broker ProtocolMulti Region Traffic Failover GatewayMulti Tenant DatabaseMutex ContentionNetwork & protocol parsingNetwork Flow Telemetry Collector ProtocolNetworking CdnNetworking Mac AddressNetworking Percent DecodeNetworking Rate LimitNetworking TlsNetworking User AgentNotification SenderNotificationsObject Storage 429 ThrottleObject Storage Multipart StuckObject Storage Read After Write ConsistencyObject Storage Strong ConsistencyOltp To Olap ReplicationOnline Schema Migration DatabaseOom KillP2p File Transfer ProtocolParking LotPastebinPii Tokenization SecurityPki Certificate AuthPolling FeedPool ExhaustionPrefix sums & difference arraysPresence Status Service NetworkingPrivileged Access SecurityProtocol Accept LanguageProtocol Csv ParsingProtocol Dns EncodingProtocol Http RangeProtocol TcpPrototype Pollution VulnerabilityPush Notification Gateway NetworkingQr TicketingQuery Cost Based OptimizerQuery Of DeathQuery Predicate EvaluatorQuery Top NQueue BacklogQueues, deques & ring buffersRace Condition LockRate LimiterRate Limiting SecurityRead Repair reconciliation across replicasReadabilityReal Time PersonalizationRecursion & backtrackingRedos Security VulnerabilityResumable Large File Upload StorageReverse Proxy Rate Limiting GatewayRtp Media Relay ProtocolScheduling Critical Path DependenciesSecondary Index MaintenanceSecondary Index Service DatabaseSecondary IndexesSecrets Management SecuritySecurity Access Control CorsSecurity Access Control IdorSecurity Breach Lateral MovementSecurity Broken AuthenticationSecurity Command InjectionSecurity Compromised Ci TokenSecurity Credential StuffingSecurity cryptography weak hashSecurity CsrfSecurity Data ExfiltrationSecurity Dependency CveSecurity Hardcoded SecretSecurity injection ldapSecurity Injection NosqlSecurity Injection Sql OrderbySecurity Injection SstiSecurity Input Validation XxeSecurity Mass AssignmentSecurity misconfiguration debugSecurity misconfiguration headersSecurity Missing Authorization AdminSecurity missing rate limiting apiSecurity Missing Rate Limiting AuthSecurity Oauth Token LeakSecurity Open RedirectSecurity Phishing Session HijackSecurity Privilege EscalationSecurity S3 Bucket ExposureSecurity Secret Logged At ScaleSecurity Sensitive Data LoggingSecurity Sql InjectionSecurity Tls Cert ExpirySecurity Vulnerability SsrfSecurity Waf Sqli SpikeSegment trees & Fenwick treesSemaphore Lock Permit LeakService Mesh NetworkingShort Poll PresenceShortest paths (Dijkstra & friends)Single Threaded SaturationSip Session Routing ProtocolSliding windowSloppy quorum with hinted handoff replica selectionSnapshot Bloat Disk PressureSocial FeedSocket Accept No TimeoutSocket Short WriteSorting algorithmsSql Data AccessSql Index DesignSql Query PerformanceSse Longpoll Fanout ProtocolSsrf VulnerabilitySsti Injection VulnerabilityStacks & monotonic stacksStale LeaderStorageStorage AllocatorStorage Atomic WriteStorage Backup VerifyStorage CacheStorage Cache EvictionStorage ChecksumStorage Cluster Split BrainStorage Compaction StallStorage Corruption PitrStorage data structuresStorage DurabilityStorage EncodingStorage Fd LeakStorage File IoStorage FlushStorage Latency Failing DiskStorage Mmap Large FileStorage MultipartStorage Node Down RebalanceStorage Prefix CompressionStorage QuotaStorage Quota Exceeded Mid WriteStorage RaceStorage Rebalance StuckStorage Replica Divergence SilentStorage Resource LeakStorage SnapshotStorage Snapshot Restore StuckStorage Streaming DownloadStorage Temp File LeakStorage ToctouStorage Versioned StoreStorage Write Amplification CompactionStorage Write ErrorString matching (KMP, Z)Strings & parsingTcp Socket FramingTeam ChatTeam File ShareTest CoverageThread Pool Task Assignment OrderThundering Restore Storage StormTime Series DatabaseTime Series DbTime Series StorageTls Ingress Protocol RouterTls Security VulnerabilityTls Socket No ShutdownTodo BackendTopological sortTraffic SpikeTransaction Deadlock StormTransfer Locking Final BalanceTries & suffix structuresTwo pointersUdp Recv Buffer TruncationUnion-Find (DSU)Url RoutingUrl ShortenerVersioned Document StorageVideo Conferencing Sfu NetworkingVoip Sip Signaling ProtocolVpn Tunnel Service ProtocolWaf Bot Detection SecurityWal Disk Fsync Tombstone Read AmplificationWebhook Delivery GatewayWebhook Ordering Retry ProtocolWebsocket Partial FrameWorker Thread Pool Exhaustion
On-call
System design
API designCaching strategiesCDNs & the edgeConsensus & coordinationData modelling & indexingData pipelines & warehousingDelivery semantics & idempotencyDistributed transactionsFeature stores & MLOpsGeospatial & location systemsMedia & streaming systemsML system designObject & blob storageObservability systemsPayments & ledgersQueues & streamingRate limitingReal-time & fan-outRecommendation & rankingReplication & consistencyScaling & load balancingSearch & ranking systemsSecurity architectureSharding & partitioningStorage engines (LSM & B-tree)Vector search & embeddings