# CrackingWalnuts: System Design & Engineering Knowledge Base

> CrackingWalnuts is a technical blog and resource hub covering system design, distributed systems, AI engineering, software architecture, and Staff+ engineering leadership. It provides cheat sheets, interactive tools, interview prep, case studies, in-depth articles, and practical references for senior and staff-level engineers.

## Cheat Sheets
Reference cards for system design concepts.
- [HTTP Status Codes](/cheat-sheets): 1xx-5xx status codes with descriptions
- [CAP Theorem](/cheat-sheets): Consistency, Availability, Partition Tolerance trade-offs
- [Database Comparison](/cheat-sheets): PostgreSQL vs MySQL vs MongoDB vs CockroachDB
- [System Design Acronyms](/cheat-sheets): ACID, BASE, PACELC, CRDT, LSM, SSTable, WAL
- [Networking Protocols](/cheat-sheets): TCP vs UDP vs QUIC comparison
- [Big-O Complexity](/cheat-sheets): Common data structure and algorithm complexities
- [Load Balancing Algorithms](/cheat-sheets): Round Robin, Weighted RR, Least Connections, IP Hash, Consistent Hashing
- [Caching Strategies](/cheat-sheets): Cache-Aside, Read-Through, Write-Through, Write-Behind, Refresh-Ahead
- [Message Queue Comparison](/cheat-sheets): Kafka vs RabbitMQ vs SQS vs Redis Streams
- [Consistency Models](/cheat-sheets): Strong, Eventual, Causal, Read-your-writes, Monotonic
- [API Design Patterns](/cheat-sheets): REST vs GraphQL vs gRPC vs WebSocket
- [Scaling Patterns](/cheat-sheets): Sharding, Read Replicas, CQRS, Event Sourcing
- [DNS & CDN](/cheat-sheets): Record types, TTL, CDN cache hierarchy, invalidation
- [Rate Limiting Algorithms](/cheat-sheets): Token Bucket, Leaky Bucket, Fixed Window, Sliding Window
- [Microservices Patterns](/cheat-sheets): API Gateway, Circuit Breaker, Saga, Sidecar, Bulkhead
- [Storage Types](/cheat-sheets): Block vs Object vs File storage, S3 classes, EBS types

## Interview Prep
System design interview questions with difficulty levels, hints, and key topics.
- [Design a URL Shortener](/interview-prep): Easy, Hashing, Base62, Key-Value Store
- [Design a Chat System with E2EE](/interview-prep): Medium, WebSocket, Signal Protocol, Key Exchange
- [Design a Rate Limiter](/interview-prep): Easy, Token Bucket, Sliding Window, Redis
- [Design a Distributed Cache](/interview-prep): Medium, Consistent Hashing, Eviction Policies, Replication
- [Design a Payment System](/interview-prep): Hard, ACID, Idempotency, Reconciliation
- [Design a News Feed](/interview-prep): Medium, Fan-out, Ranking, Push vs Pull
- [Design a Search Autocomplete](/interview-prep): Medium, Trie, Prefix Matching, Ranking
- [Design a Multi-Tenant SaaS](/interview-prep): Hard, Tenant Isolation, Schema Design, Billing

## Interactive Tools
Calculators and visualizers for system design estimation and learning.
- [Back-of-Envelope Calculator](/tools): Estimate QPS, storage, bandwidth from DAU
- [SLA / Uptime Calculator](/tools): Convert nines to downtime, compound SLA
- [Data Transfer Time Calculator](/tools): Transfer time across link speeds
- [Latency Numbers](/tools): Interactive latency reference chart
- [Database Selector Quiz](/tools): Workload-based database recommendation
- [Unit Converter](/tools): Throughput, storage, time, request unit conversion
- [Consistent Hashing Visualizer](/tools): Interactive hash ring visualization
- [Rate Limiter Visualizer](/tools): Token Bucket vs Fixed Window vs Sliding Window
- [Bloom Filter Visualizer](/tools): Membership testing with false positive demonstration
- [Cache Eviction Visualizer](/tools): LRU vs LFU vs FIFO step-through
- [Quorum Visualizer](/tools): N/W/R configuration with failure simulation
- [Leader Election Visualizer](/tools): Bully vs Ring vs Raft algorithm comparison
- [Circuit Breaker Visualizer](/tools): State machine visualization for circuit breaker pattern
- [DB Sharding Visualizer](/tools): Hash-based and range-based sharding visualization
- [Message Queue Patterns Visualizer](/tools): Fan-out, work queue, pub/sub pattern demonstration

## Architecture Decisions
ADRs, RFCs, migration strategies, and architecture patterns for Staff+ engineers.
- [ADR Template & Best Practices](/architecture-decisions): Decision record format, documentation workflow, team adoption
- [RFC Writing Guide](/architecture-decisions): Proposal structure, stakeholder alignment, anti-patterns
- [Trade-off Analysis Framework](/architecture-decisions): Weighted scoring matrices, reversibility classification, blast radius
- [Monolith to Microservices Migration](/architecture-decisions): Strangler fig pattern, domain decomposition, migration timelines
- [Event-Driven vs Request-Driven Architecture](/architecture-decisions): Async vs sync, hybrid approaches, when to use each
- [CQRS & Event Sourcing](/architecture-decisions): Command-query separation, event stores, projection patterns
- [LLM Integration Architecture Patterns](/architecture-decisions): Gateway pattern, semantic caching, output validation, model portability
- [ML Pipeline & Feature Store Architecture](/architecture-decisions): Feature stores, batch vs real-time pipelines, data versioning, model serving
- [API Versioning Strategy](/architecture-decisions): URL path vs header versioning, sunset policies, gateway translation
- [Database Selection Framework](/architecture-decisions): Query pattern analysis, operational complexity, PostgreSQL as default
- [Cache Invalidation Strategies](/architecture-decisions): Cache-aside, TTL, CDC-based invalidation, cache stampede prevention
- [Zero-Downtime Deployment Patterns](/architecture-decisions): Blue-green, canary releases, expand-contract database migrations
- [GraphQL vs REST Decision](/architecture-decisions): Over-fetching, caching trade-offs, federation, N+1 problem
- [Feature Flag Architecture](/architecture-decisions): Release, experiment, ops, and permission flags, progressive delivery
- [Strangler Fig Pattern](/architecture-decisions): Incremental legacy replacement, anti-corruption layers, parity verification
- [Data Mesh Architecture](/architecture-decisions): Domain ownership, federated governance, self-serve data platform
- [Serverless vs Containers Decision](/architecture-decisions): Cost crossover, cold starts, vendor lock-in, stateful workloads
- [Multi-Region Architecture](/architecture-decisions): Active-passive vs active-active, replication lag, data sovereignty
- [Cell-Based Architecture](/architecture-decisions): Blast radius reduction, cell sizing, cell-aware routing, operational tooling
- [Distributed Transaction Patterns](/architecture-decisions): Sagas, transactional outbox, compensating transactions, 2PC limitations
- [Schema Evolution Governance](/architecture-decisions): Protobuf vs Avro, compatibility modes, consumer-driven contracts
- [Idempotency & Exactly-Once Processing](/architecture-decisions): Idempotency keys, Kafka EOS, transactional outbox, dual-write problem

## Engineering Leadership
Technical vision, strategy, and executive communication for engineering leaders.
- [Building Technical Vision](/engineering-leadership): 2-3 year horizon, architectural principles, milestone planning
- [Technology Radar Creation](/engineering-leadership): Evaluation framework, quadrant placement, adoption lifecycle
- [Build vs Buy Framework](/engineering-leadership): TCO calculations, vendor evaluation, decision criteria
- [Tech Debt Negotiation](/engineering-leadership): Business language framing, prioritization, stakeholder buy-in
- [Engineering Roadmaps](/engineering-leadership): Rolling window model, outcome-based planning, dependency management
- [Communicating Strategy to Execs](/engineering-leadership): Pyramid principle, 5-slide format, handling pushback
- [AI Strategy for Engineering Leaders](/engineering-leadership): Use case prioritization, 70/20/10 portfolio, enabling team model
- [AI-Assisted Developer Productivity](/engineering-leadership): Copilot/Cursor ROI measurement, rollout strategy, IP policy
- [Engineering Culture at Scale](/engineering-leadership): Written digests, promotion criteria as values, subculture management
- [Managing Up as a Tech Leader](/engineering-leadership): Business impact translation, no-surprises rule, status updates
- [Incident Response Leadership](/engineering-leadership): Incident commander role, customer communication, post-incident reviews
- [Engineering Org Health Check](/engineering-leadership): Quarterly metrics, attrition analysis, developer satisfaction surveys
- [Managing Distributed Teams](/engineering-leadership): Async-first communication, time zone overlap, documentation culture
- [Hiring & Retaining Staff Engineers](/engineering-leadership): Scope of influence interviews, compensation, cross-cutting problems
- [Migration Program Management](/engineering-leadership): Last 20% budgeting, feature velocity targets, dedicated TPM
- [Developer Experience Strategy](/engineering-leadership): Time-to-first-PR, inner loop speed, DX as a product
- [Security Culture Building](/engineering-leadership): Shift-left security, security champions, secure defaults
- [Technical Due Diligence](/engineering-leadership): M&A evaluation, architecture review, integration planning
- [Building Engineering Brand](/engineering-leadership): Tech blog ROI, strategic OSS, conference speaking, recruiting impact
- [Leading Through Trust, Not Authority](/engineering-leadership): Psychological safety, empathy, giving credit, ethics under pressure, accountability

## Org Design
Team topologies, scaling organizations, and career structures.
- [Team Topologies Overview](/org-design): Stream-aligned, platform, enabling, complicated-subsystem teams
- [Conway's Law & Inverse Maneuver](/org-design): Org structure mirrors architecture, intentional restructuring
- [Scaling 10 to 50 Engineers](/org-design): Inflection points, first managers, process introduction
- [Scaling 50 to 200 Engineers](/org-design): Department formation, specialization, communication scaling
- [Incident Management Process](/org-design): Severity levels, role assignment, escalation paths
- [Engineering Career Ladders](/org-design): IC/management parity, level definitions, promotion criteria
- [ML & AI Team Structure Patterns](/org-design): Centralized vs embedded vs hybrid, handoff problem, scaling phases
- [AI Adoption & Change Management](/org-design): Champions program, displacement anxiety, governance framework
- [SRE Team Structure](/org-design): Embedded vs centralized, 1:8-10 ratio, production readiness reviews
- [Data Team Organization](/org-design): Data engineering vs data science vs analytics engineering, reporting structure
- [Security Team Integration](/org-design): AppSec vs SecOps vs GRC, security champions model, shift-left
- [Remote-First Org Design](/org-design): Async-first processes, time zone bands, onboarding structure
- [Engineering Manager to IC Ratio](/org-design): 5-8 direct reports, span of control, skip-level 1:1s
- [On-Call Rotation Design](/org-design): Minimum rotation size, follow-the-sun, compensation, escalation policies
- [Engineering Hiring Pipeline](/org-design): Referral programs, structured interviews, time-to-hire optimization
- [Cross-Functional Product Teams](/org-design): Team composition, embedded vs matrix, team APIs
- [Technical Program Management](/org-design): When to hire TPMs, 1:25-50 ratio, risk tracking, program plans
- [Chapters & Guilds](/org-design): Spotify model critique, chapter leads, guild survival patterns

## Platform Engineering
Internal developer platforms, golden paths, and infrastructure abstraction.
- [Internal Developer Platform Design](/platform-engineering): Assembly approach, capability mapping, adoption strategy
- [Golden Paths & Paved Roads](/platform-engineering): Opinionated defaults, guardrails over gates, adoption measurement
- [Developer Productivity Metrics](/platform-engineering): DORA + SPACE framework, developer satisfaction, flow metrics
- [Developer Portal & Backstage](/platform-engineering): Service catalog, build vs buy, plugin ecosystem
- [Self-Service Infrastructure](/platform-engineering): Crossplane vs Terraform, policy-as-code, approval workflows
- [Platform Team Operating Model](/platform-engineering): Product thinking, SLOs, customer feedback loops
- [ML Platform & AI Golden Paths](/platform-engineering): Feature stores, experiment tracking, model registry, self-service deployment
- [AI Infrastructure Cost Management](/platform-engineering): GPU optimization, LLM API cost controls, model selection economics
- [Service Mesh Implementation](/platform-engineering): Istio vs Linkerd vs Cilium, sidecar vs eBPF, mTLS, progressive rollout
- [Feature Flag Platforms](/platform-engineering): Local SDK evaluation, LaunchDarkly vs Unleash, stale flag cleanup
- [Secrets Management Platform](/platform-engineering): Dynamic secrets, HashiCorp Vault, External Secrets Operator
- [Internal API Gateway](/platform-engineering): East-west traffic, Kong, rate limiting, schema validation
- [CI/CD Pipeline Standardization](/platform-engineering): Shared templates, build caching, security scanning in pipeline
- [Observability Platform Design](/platform-engineering): OpenTelemetry, Grafana stack vs Datadog, sampling strategies
- [Container Runtime Platform](/platform-engineering): EKS/GKE management, spot instances, Karpenter, image management
- [GitOps Platform Patterns](/platform-engineering): ArgoCD vs Flux, drift detection, PR-based deployments
- [Cost Management Platform](/platform-engineering): Per-team attribution, Kubecost, show-back models, anomaly detection
- [Self-Service Database Provisioning](/platform-engineering): PgBouncer, automated backups, schema migration tooling

## Staff+ Interview Prep
Behavioral and system design interview preparation for Staff, Senior Staff, and Principal levels.
- [Ambiguity Resolution Exercises](/staff-interview-prep): Problem structuring, clarifying questions, avoiding premature solutions
- [Architecture Design Review](/staff-interview-prep): Systematic review framework, data flow analysis, failure modes
- [Cross-Team Project Leadership](/staff-interview-prep): Influence without authority, alignment mechanisms, escalation judgment
- [Legacy System Modernization](/staff-interview-prep): Strangler fig, business case building, risk assessment
- [System Design at Principal Level](/staff-interview-prep): 10-100x scale, cross-cutting concerns, operational depth
- [Technical Strategy Presentation](/staff-interview-prep): Pyramid principle, business impact focus, handling pushback
- [AI/ML System Design Interview](/staff-interview-prep): Cost-quality-latency tradeoffs, data flywheel, RAG architecture
- [Leading AI Transformation](/staff-interview-prep): Three-phase adoption model, resistance handling, success measurement
- [Engineering Metrics Deep Dive](/staff-interview-prep): DORA/SPACE knowledge, Goodhart's Law, data-driven process improvement
- [Security Architecture Review](/staff-interview-prep): Threat modeling, auth system design, zero-trust implementation
- [Technical Mentorship Scenarios](/staff-interview-prep): Structured mentorship approach, difficult coaching, high-potential identification
- [Platform Strategy Design](/staff-interview-prep): Internal platform scoping, adoption strategy, self-service vs guardrails
- [Organizational Design Questions](/staff-interview-prep): Team topology trade-offs, Conway's Law in practice, reorg communication
- [Conflict Resolution in Engineering](/staff-interview-prep): Technical disagreements, cross-team conflict, constructive pushback
- [Prioritization & Roadmap Defense](/staff-interview-prep): Tech debt vs features, roadmap building, executive communication
- [Incident Leadership Scenarios](/staff-interview-prep): Incident response, pattern recognition, systemic improvements
- [Cross-Team Dependency Management](/staff-interview-prep): Multi-team coordination, priority conflicts, API contracts
- [Migration Planning Interviews](/staff-interview-prep): Monolith decomposition, cloud migration, legacy modernization
- [API Design at Staff Level](/staff-interview-prep): Resource modeling, versioning, protocol selection, breaking changes
- [Cost Engineering & Cloud Economics](/staff-interview-prep): Cloud bill investigation, cost accountability, build-vs-buy decisions
- [Production Debugging at Scale](/staff-interview-prep): Systematic debugging, distributed tracing, cascading failure diagnosis

## Engineering Metrics
DORA, SPACE, SLOs, FinOps, and AI cost measurement for engineering organizations.
- [Cloud Cost Optimization](/engineering-metrics): Right-sizing, reserved instances, spot instances, storage lifecycle
- [DORA Metrics Deep Dive](/engineering-metrics): Four key metrics, team diagnostics, improvement sequence
- [Engineering Productivity Measurement](/engineering-metrics): McKinsey critique, SPACE framework, removing friction
- [FinOps Practices](/engineering-metrics): Chargeback/showback, unit economics, maturity model
- [SLO, SLA & SLI Budgeting](/engineering-metrics): Error budgets, burn rate alerts, release gating
- [SPACE Framework](/engineering-metrics): Five dimensions, activity vs satisfaction balance, implementation
- [AI Cost & Unit Economics](/engineering-metrics): Per-inference cost tracking, token optimization, model selection as economic decision
- [AI System Quality & Reliability Metrics](/engineering-metrics): Model quality SLOs, data drift detection, human evaluation sampling
- [Code Review Metrics](/engineering-metrics): Time-to-first-review, PR size impact, reviewer load balancing
- [Technical Debt Measurement](/engineering-metrics): Fowler's quadrant, static analysis baselines, debt budget allocation
- [Incident Metrics: MTTR & MTTD](/engineering-metrics): Detection vs response vs resolution, customer-minutes affected
- [API Performance Metrics](/engineering-metrics): P99 latency, Apdex scores, performance budgets, real user monitoring
- [Developer Satisfaction & DevEx](/engineering-metrics): DX Core 4, friction surveys, platform NPS, retention correlation
- [Release Quality Metrics](/engineering-metrics): Defect escape rate, rollback frequency, canary success rate
- [Test Coverage & Effectiveness](/engineering-metrics): Mutation testing, flaky test rate, test pyramid ratios
- [On-Call Health Metrics](/engineering-metrics): Pages per shift, sleep disruption, escalation frequency, toil tracking
- [Capacity Planning Metrics](/engineering-metrics): Resource utilization baselines, headroom calculation, cost per transaction
- [Security Metrics Dashboard](/engineering-metrics): MTTR by severity, vulnerability breakdown, AppSec pipeline coverage

## Compliance & Governance
SOC 2, GDPR, HIPAA, PCI DSS, ISO 27001, zero trust, data residency, and AI governance for engineers.
- [Audit Logging Architecture](/compliance-governance): Immutability, structured logging, hash chains, retention
- [GDPR & CCPA Data Architecture](/compliance-governance): PII indirection, deletion propagation, consent architecture
- [HIPAA Compliance for Engineers](/compliance-governance): PHI handling, three safeguard categories, BAAs, breach notification, cloud architecture
- [ISO 27001 & ISMS Implementation](/compliance-governance): ISMS lifecycle, Statement of Applicability, risk assessment, certification process
- [PCI DSS Compliance](/compliance-governance): Tokenization, network segmentation, v4.0 customized approach
- [Privacy by Design](/compliance-governance): Seven principles, data minimization, anonymization vs pseudonymization
- [Data Residency & Sovereignty](/compliance-governance): GDPR transfers, Schrems II, regional deployment, cross-border mechanisms
- [SOC 2 for Engineers](/compliance-governance): Five Trust Service Criteria, Type I vs II, evidence automation
- [Zero Trust & Access Governance](/compliance-governance): RBAC vs ABAC vs ReBAC, identity-aware proxies, JIT access, service-to-service auth
- [Software Supply Chain Security](/compliance-governance): SBOM, SLSA framework, artifact signing, dependency scanning
- [AI Governance & Responsible AI](/compliance-governance): EU AI Act risk classification, bias testing in CI/CD, model risk register
- [LLM Data Privacy & Security](/compliance-governance): Prompt injection defense, PII redaction pipelines, vendor DPA evaluation
- [NIST Cybersecurity Framework](/compliance-governance): Five core functions, implementation tiers, NIST CSF 2.0 Govern function
- [FedRAMP Compliance](/compliance-governance): 325+ NIST 800-53 controls, JAB P-ATO vs Agency ATO, continuous monitoring
- [Accessibility & WCAG Compliance](/compliance-governance): WCAG 2.2 Level AA, POUR principles, automated vs manual testing
- [Open Source License Governance](/compliance-governance): Permissive vs copyleft, AGPL risks, SBOM requirements, license compatibility
- [Third-Party Risk Management](/compliance-governance): Vendor security assessments, SOC 2 report evaluation, shadow IT
- [Data Classification Frameworks](/compliance-governance): Four-level classification, automated discovery, policy enforcement
- [Cross-Border Data Transfers](/compliance-governance): SCCs, Transfer Impact Assessments, EU-US Data Privacy Framework
- [Container Security Compliance](/compliance-governance): Image scanning, distroless images, Pod Security Standards, SLSA framework
- [EU AI Act Compliance](/compliance-governance): Risk classification, Annex III, conformity assessments, enforcement timeline
- [Incident Notification Requirements](/compliance-governance): GDPR 72-hour clock, SEC disclosure rules, breach notification runbooks

## Incident Patterns
Failure patterns, chaos engineering, post-mortems, and incident response for production systems.
- [Anatomy of a Production Incident](/incident-patterns): Timeline structure, role assignment, decision-making under pressure
- [Blameless Post-Mortem Guide](/incident-patterns): Five whys, action item quality, follow-through culture
- [Cascading Failure Patterns](/incident-patterns): Resource exhaustion, circuit breakers, bulkhead isolation
- [Chaos Engineering Principles](/incident-patterns): Steady-state hypothesis, blast radius control, game days
- [Deployment Rollback Patterns](/incident-patterns): Blue-green, canary analysis, database expand-contract
- [Thundering Herd & Retry Storms](/incident-patterns): Cache stampede, exponential backoff with jitter, retry budgets
- [AI Model Failure Patterns](/incident-patterns): Silent degradation, training-serving skew, concept drift, data quality cascades
- [LLM Production Incident Patterns](/incident-patterns): Provider outages, cost runaway, prompt injection as security incident
- [DNS Failure Patterns](/incident-patterns): SERVFAIL propagation, TTL-based recovery, secondary DNS failover
- [Certificate Expiry Incidents](/incident-patterns): TLS expiry detection, connection pool impact, automated renewal
- [Database Migration Failures](/incident-patterns): DDL lock contention, staging vs production divergence, rollback timing
- [Memory Leak Patterns](/incident-patterns): Gradual OOM, cascading pod kills, connection/transport leaks
- [Configuration Drift Incidents](/incident-patterns): Manual changes vs IaC, Terraform drift detection, undocumented fixes
- [Third-Party Dependency Failures](/incident-patterns): Circuit breaker activation, fallback payment processors, queue-based recovery
- [Data Corruption Patterns](/incident-patterns): Race conditions, event stream propagation, reconciliation and replay
- [Network Partition Handling](/incident-patterns): Partial partitions, split-brain detection, AZ failover, data divergence
- [Cloud Provider Outage Response](/incident-patterns): Cross-region failover, stale deployments, Route 53 health checks
- [Capacity Planning Failures](/incident-patterns): Traffic spikes, autoscaler lag, connection pool exhaustion, image size impact

## Distributed Systems Algorithms
Core algorithms and data structures that power distributed systems at scale.
- [Bloom Filters & Probabilistic Membership](/distributed-systems): False positive rates, optimal sizing, counting variants
- [Consistent Hashing & Ring Algorithms](/distributed-systems): Virtual nodes, bounded loads, ring-based partitioning
- [Erasure Coding & Reed-Solomon](/distributed-systems): Storage-efficient fault tolerance, RS(6,3), reconstruction costs, failure domains
- [Count-Min Sketch & Frequency Estimation](/distributed-systems): Sub-linear frequency counting, heavy hitters detection
- [CRDTs: Conflict-Free Replicated Data Types](/distributed-systems): G-Counter, LWW-Register, OR-Set, merge semantics
- [Distributed Snapshots & Chandy-Lamport](/distributed-systems): Global state capture, marker-based algorithm, consistent cuts
- [Failure Detection Algorithms](/distributed-systems): Phi-accrual, heartbeat, SWIM protocol, suspicion mechanisms
- [Gossip Protocols & SWIM](/distributed-systems): Epidemic dissemination, membership protocols, protocol periods
- [HyperLogLog & Cardinality Estimation](/distributed-systems): Probabilistic counting, register-based estimation, bias correction
- [Logical Clocks & Causality Tracking](/distributed-systems): Lamport timestamps, vector clocks, version vectors
- [LSM Trees vs B-Trees](/distributed-systems): Write amplification, compaction strategies, read/write trade-offs
- [Merkle Trees & Anti-Entropy Repair](/distributed-systems): Hash trees, consistency verification, range-based repair
- [Quorum Systems & NRW Notation](/distributed-systems): Read/write quorums, sloppy quorums, hinted handoff
- [Raft Consensus Protocol](/distributed-systems): Leader election, log replication, safety guarantees
- [Streaming Quantiles: T-Digest & DDSketch](/distributed-systems): Approximate percentiles, mergeable sketches, accuracy guarantees
- [Operational Transformation](/distributed-systems): Central-server conflict resolution, transform functions, Google Docs, Jupiter protocol
- [Write-Ahead Log](/distributed-systems): Durability, crash recovery, log-structured storage

## Concurrency
145-lesson curriculum covering concurrency in Java, Python, and Go with side-by-side code, classic interview problems, bug hunts, and Mermaid diagrams.
- [Core Mental Models](/concurrency): Concurrency vs parallelism, race conditions, critical sections, memory visibility & CPU reordering, deadlock/livelock/starvation, priority inversion, false sharing, when not to use concurrency
- [Core Patterns](/concurrency): Producer-consumer, worker pool, fan-out/fan-in, pipeline, futures & promises, async vs blocking, retry & backoff, circuit breaker, bulkhead, idempotency, backpressure strategies, colored functions, classic problems (dining philosophers, readers-writers, sleeping barber, cigarette smokers, Santa Claus, FizzBuzz multithreaded, Print FooBar, Print in Order, H2O molecule)
- [Go Concurrency](/concurrency): Goroutines, channels, select deep dive, context cancellation, leak prevention, sync.Mutex/RWMutex, sync.Map, sync/atomic, errgroup & semaphore, Go memory model, runtime tuning
- [Java Concurrency](/concurrency): Virtual threads (JEP 444), synchronized vs Lock, ExecutorService, CompletableFuture, ForkJoinPool, ConcurrentHashMap, BlockingQueue, atomics, ReadWrite/StampedLock, ThreadLocal, structured task scope, Java memory model, volatile, JMH benchmarking
- [Python Concurrency](/concurrency): The GIL & PEP 703, threading vs multiprocessing, asyncio patterns, concurrent.futures, queue module, ContextVar, sync/async bridge
- [Real-World Scenarios](/concurrency): Rate limiter, distributed lock, leader election & heartbeat, connection pool, idempotency key, scheduled executor, pub/sub bus, DAG task runner, web crawler, cache with TTL, API aggregator, retry, circuit breaker, job queue, inventory reservation, transactional outbox, saga, cancellation propagation
- [Performance & Tradeoffs](/concurrency): CPU vs I/O bound, context switch cost, memory overhead, throughput vs latency, lock contention, tail latency, Amdahl's law, USL (Universal Scalability Law), NUMA, benchmarking methodology, parallel merge sort, parallel map-reduce, parallel graph traversal
- [Lock-Free Programming](/concurrency): CAS basics, ABA problem, Treiber stack, MPSC queue, sharded hashmap (lock striping), lock-free Bloom filter, lock-free trie, lock-free priority queue, hazard pointers, RCU, spinlocks, progress taxonomy (wait-free / lock-free / obstruction-free), memory ordering
- [Debugging Concurrency](/concurrency): Race detection (TSan, Go race detector), debugging deadlocks & hangs, profiling concurrent code, CI race detection, production post-mortem patterns, bug hunts (counter race, broken DCL, false sharing, lost wakeup, goroutine leak, deadlock under load, x86 vs ARM memory ordering, RWMutex reentrancy)

## Low-Level Design
Object-oriented design problems with UML diagrams, design patterns, and code implementations in Python and Java.
- [API Gateway](/low-level-design): Proxy, Adapter, Facade patterns for microservice routing
- [ATM](/low-level-design): State pattern for lifecycle, Chain of Responsibility for bill dispensing
- [Car Rental](/low-level-design): Strategy for pricing tiers, State for vehicle lifecycle
- [Chat Room](/low-level-design): Mediator pattern for message routing
- [Chess](/low-level-design): Strategy for piece moves, Command for undo during check validation
- [Coffee Shop Ordering](/low-level-design): Decorator pattern for drink customization
- [Concert Ticket Booking](/low-level-design): Seat locking with TTL, optimistic/pessimistic concurrency
- [Course Registration](/low-level-design): Prerequisite validation, waitlist strategies, schedule conflict detection
- [CricInfo](/low-level-design): Observer for live scores, State for match lifecycle
- [Digital Wallet](/low-level-design): Double-entry bookkeeping, idempotency keys, Strategy for payment methods
- [Elevator System](/low-level-design): State machine per elevator, LOOK algorithm dispatcher
- [Expression Evaluator](/low-level-design): Interpreter for AST, Visitor for evaluation/printing
- [Food Delivery](/low-level-design): State machine for orders, Strategy for driver assignment, Observer for tracking
- [Game World (Tile Engine)](/low-level-design): Flyweight for terrain sharing, Bridge for rendering
- [HashMap](/low-level-design): Array of buckets, hash function, collision handling with linked lists
- [Hotel Management](/low-level-design): Strategy for seasonal pricing, Factory for room types, Observer for events
- [In-Memory File System](/low-level-design): Composite pattern for hierarchical file/directory structure
- [Library Management](/low-level-design): Catalog vs physical copy separation
- [Logging Framework](/low-level-design): Chain of Responsibility, Strategy for formatters, Singleton
- [LRU Cache](/low-level-design): Doubly linked list + hashmap for O(1) operations
- [Movie Ticket Booking](/low-level-design): TTL-based seat locks, dynamic pricing, Facade pattern
- [Music Streaming](/low-level-design): Iterator for playlists, Strategy for playback modes, Observer for UI updates
- [Notification System](/low-level-design): Multi-channel delivery, Strategy for channel selection, Template Method
- [Online Auction](/low-level-design): Time-based bidding, State machine for auction lifecycle, Observer for bidders
- [Online Shopping](/low-level-design): Cart, order pipeline, inventory management
- [Parking Lot](/low-level-design): Multi-floor, multi-vehicle-type with real-time availability
- [Payment System](/low-level-design): Strategy for methods, Chain of Responsibility for fraud checks, idempotency
- [Pub/Sub System](/low-level-design): Topic-based message broker, Observer + Strategy patterns
- [Rate Limiter](/low-level-design): Token bucket, sliding window, Strategy for algorithm swapping
- [Restaurant Management](/low-level-design): Strategy for table allocation, State for order lifecycle, Observer for kitchen
- [Ride Sharing](/low-level-design): Strategy for driver matching, State for ride lifecycle, surge pricing
- [Snake & Ladder](/low-level-design): Builder for board construction, Strategy for dice
- [Splitwise](/low-level-design): Multiple splitting strategies, greedy debt simplification
- [Stack Overflow](/low-level-design): Observer for reputation, Strategy for search, State for question lifecycle
- [Stock Brokerage](/low-level-design): Price-time priority order matching, per-stock order books
- [Task Management](/low-level-design): State machine, Observer, Command for undo
- [Text Editor](/low-level-design): Memento for state snapshots, Command for undo/redo
- [Tic Tac Toe](/low-level-design): O(1) win detection using row/column/diagonal counters
- [Traffic Signal](/low-level-design): State pattern for phase transitions, controller coordination
- [Cross-Platform UI Toolkit](/low-level-design): Abstract Factory for platform-specific widgets
- [URL Shortener](/low-level-design): Base62 encoding, Strategy for encoding schemes
- [Vending Machine](/low-level-design): State pattern for clean state transitions

## Blog Posts
In-depth technical articles on system design and engineering topics.
- Visit [crackingwalnuts.com](https://crackingwalnuts.com) for the full article archive

## Case Studies
Real-world system implementation deep-dives.

## Whitepapers
Curated collection of essential technical whitepapers.
- Visit [crackingwalnuts.com/whitepapers](https://crackingwalnuts.com/whitepapers)