SystemCraftHigh-Level System Design notes for tech interviewshttps://systemcraft.in/2026-10-01T00:00:00+00:00SystemCrafthttps://systemcraft.in/about/Copyright 2026 SystemCrafthttps://systemcraft.in/favicon-192x192.pngString Algorithms - KMP, Z-Function, Rolling Hashhttps://systemcraft.in/dsa-fundamentals/string-algorithms/2026-10-01T00:00:00+00:002026-10-01T00:00:00+00:00KMP failure function, Rabin-Karp rolling hash, and the Z-function for substring search, plus expand-around-centre palindromes, in Python/Java/C++/JavaScript.Search Indexing - Inverted Index and BM25https://systemcraft.in/concepts/search-indexing/2026-10-01T00:00:00+00:002026-10-01T00:00:00+00:00How a search index actually works: inverted index, tokenisation and stemming, TF-IDF to BM25 ranking, immutable segments, and scatter-gather sharding.Prefix Sum and Hashing - Subarray Patternshttps://systemcraft.in/dsa-fundamentals/prefix-sum/2026-10-01T00:00:00+00:002026-10-01T00:00:00+00:00Prefix sums and hash maps: subarray sum equals K, divisible by K, 2D range queries, and difference arrays. Worked examples and Python/Java/C++/JavaScript code.Matrix and Grid Patterns - DSA Fundamentalshttps://systemcraft.in/dsa-fundamentals/matrix-grid/2026-10-01T00:00:00+00:002026-10-01T00:00:00+00:00Grid BFS and DFS, multi-source BFS, in-place 90 degree rotation via transpose and reverse, and spiral traversal. Code in Python, Java, C++ and JavaScript.LSM Trees and SSTables - Write-Optimised Storagehttps://systemcraft.in/concepts/lsm-trees/2026-10-01T00:00:00+00:002026-10-01T00:00:00+00:00How LSM trees beat B-trees on writes: memtables, SSTables, Bloom filters, size-tiered vs levelled compaction, tombstones, and the three amplifications.Change Data Capture - CDC with Debezium and the WALhttps://systemcraft.in/concepts/change-data-capture/2026-10-01T00:00:00+00:002026-10-01T00:00:00+00:00How CDC solves the dual-write problem: polling vs triggers vs log-based capture, Debezium and the WAL, snapshot-then-stream, and CDC versus outbox.Design Trending Topics - Top K in a Streamhttps://systemcraft.in/hld/TrendingTopics/2026-10-01T00:00:00+00:002026-10-01T00:00:00+00:00System design for trending topics - Count-Min Sketch, Space-Saving heavy hitters, two-tier top-K aggregation, sliding windows, and rate-of-change ranking.Design Spotify - Music Streaming System Designhttps://systemcraft.in/hld/Spotify/2026-10-01T00:00:00+00:002026-10-01T00:00:00+00:00System design for a music streaming service - pre-transcoded audio ladders, CDN delivery, instant playback prefetch, offline DRM, royalty-grade play counting.Design an E-Commerce Platform - Amazon and Flipkarthttps://systemcraft.in/hld/EcommercePlatform/2026-10-01T00:00:00+00:002026-10-01T00:00:00+00:00System design for an e-commerce platform - preventing oversell with conditional updates and reservations, order sagas, checkout idempotency, and CDC-fed search.Design a Distributed Cache - System Design Interviewhttps://systemcraft.in/hld/DistributedCache/2026-10-01T00:00:00+00:002026-10-01T00:00:00+00:00Design a distributed cache like Redis Cluster - consistent hashing with virtual nodes, approximated LRU eviction, hot key replication, and stampede control.QPS vs TPS vs RPS - System Designhttps://systemcraft.in/concepts/qps-tps-rps/2026-08-25T00:00:00+00:002026-08-25T00:00:00+00:00QPS vs TPS vs RPS explained: what each unit counts, which layer it belongs to, why transactions are always the smallest number, and how to size from one to another.Design a Freight Logistics Systemhttps://systemcraft.in/hld/FreightLogistics/2026-08-19T00:00:00+00:002026-08-19T00:00:00+00:00System design for a freight logistics platform with truck allocation, route optimization, real-time tracking, and ETA computation. Complete HLD with diagrams and data modeling.Design a Real-Time Bidding Platformhttps://systemcraft.in/hld/FreightBidding/2026-08-19T00:00:00+00:002026-08-19T00:00:00+00:00System design for a real-time freight bidding platform where carriers compete on loads posted by shippers. Complete HLD with auction mechanics, WebSocket updates, and data modeling.Design a Rules Engine - Business Ruleshttps://systemcraft.in/hld/RulesEngine/2026-08-17T00:00:00+00:002026-08-17T00:00:00+00:00System design for a Business Rules Engine with condition expression parsing, event evaluation, Rete algorithm, hot reload, versioning, and action triggering. Complete HLD with diagrams and deep dives.Design an Image Processing Microservicehttps://systemcraft.in/hld/ImageProcessing/2026-08-17T00:00:00+00:002026-08-17T00:00:00+00:00System design for an async image processing pipeline with resize, compress, format conversion, worker pools, idempotency, and auto-scaling. Complete HLD with diagrams and deep dives.Snowflake vs UUID v7 vs ULID - Unique IDshttps://systemcraft.in/concepts/unique-ids/2026-08-14T00:00:00+00:002026-08-14T00:00:00+00:00Snowflake vs UUID v7 vs ULID compared: sortability, bit layout, clock skew, and which to pick for distributed ID generation. Plus ticket servers and base62.UML Class Diagrams - Relationshipshttps://systemcraft.in/lld-fundamentals/uml-class-diagrams/2026-08-14T00:00:00+00:002026-08-14T00:00:00+00:00UML class diagrams for LLD interviews: association, aggregation, composition, inheritance, and dependency. Quick notation guide with Mermaid examples you can use in 90 minutes.Transaction Isolation Levels - ACID and MVCChttps://systemcraft.in/concepts/transactions-isolation/2026-08-14T00:00:00+00:002026-08-14T00:00:00+00:00Complete guide to database transactions and isolation levels for system design interviews. ACID properties, read committed vs repeatable read vs serializable, dirty reads, write skew, MVCC, and 2PC.System Design Terminology - 180+ Distinctionshttps://systemcraft.in/concepts/terminology/2026-08-14T00:00:00+00:002026-08-14T00:00:00+00:00Partition vs sharding, latency vs response time, linearizability vs serializability, authentication vs authorization. The precise vocabulary distinctions system design interviewers probe, in one rapid-fire reference.Template Method Pattern - Algorithm Skeletons and Hookshttps://systemcraft.in/lld-fundamentals/template-method-pattern/2026-08-14T00:00:00+00:002026-08-14T00:00:00+00:00Template Method pattern: define algorithm structure in base class, let subclasses override specific steps. ETL pipelines, order processing, and test frameworks in Java, Python, C++.Strategy Pattern - When and How to Use Ithttps://systemcraft.in/lld-fundamentals/strategy-pattern/2026-08-14T00:00:00+00:002026-08-14T00:00:00+00:00Strategy pattern explained with real examples: parking allocation, rate limiting, pricing. Code in Java, C++, Python. Know when to use it vs simpler alternatives.State Pattern - State Machine Implementationhttps://systemcraft.in/lld-fundamentals/state-pattern/2026-08-14T00:00:00+00:002026-08-14T00:00:00+00:00State pattern for LLD interviews: eliminate nested if-else with clean state transitions. Vending machine and order lifecycle examples in Java, Python, C++.State Machines in LLD - Enum-Based FSMhttps://systemcraft.in/lld-fundamentals/state-machines/2026-08-14T00:00:00+00:002026-08-14T00:00:00+00:00Implement state machines for LLD interviews: enum-based FSM, transition tables, guard conditions. Order lifecycle, elevator control, vending machines in Java, Python, C++.SOLID Principles with Code Exampleshttps://systemcraft.in/lld-fundamentals/solid-principles/2026-08-14T00:00:00+00:002026-08-14T00:00:00+00:00Master all 5 SOLID principles with real violation examples and fixes in Java, Python, C++. Practical guide for LLD interviews with before/after code.Singleton Pattern - Thread-Safe and Pitfallshttps://systemcraft.in/lld-fundamentals/singleton-pattern/2026-08-14T00:00:00+00:002026-08-14T00:00:00+00:00Singleton pattern done right: thread-safe implementations, double-checked locking, enum singleton. Learn when Singleton is appropriate and when it's an anti-pattern.Saga Pattern - Choreography vs Orchestrationhttps://systemcraft.in/concepts/saga-pattern/2026-08-14T00:00:00+00:002026-08-14T00:00:00+00:00Complete guide to the Saga pattern in system design interviews. Distributed transactions, choreography vs orchestration, compensating transactions.Rate Limiting - Token Bucket, Sliding Windowhttps://systemcraft.in/concepts/rate-limiting/2026-08-14T00:00:00+00:002026-08-14T00:00:00+00:00Complete guide to rate limiting for system design interviews. Token bucket, sliding window, leaky bucket algorithms. Distributed rate limiting with Redis.Performance Metrics - p99, Little's Law, SLOshttps://systemcraft.in/concepts/performance-metrics/2026-08-14T00:00:00+00:002026-08-14T00:00:00+00:00Complete guide to performance vocabulary for system design interviews. Latency vs throughput, p50/p99/p999, Little's Law, tail amplification, queueing theory, SLI/SLO/SLA, error budgets.Over-Engineering in LLD - YAGNI and When NOT to Use Patternshttps://systemcraft.in/lld-fundamentals/over-engineering/2026-08-14T00:00:00+00:002026-08-14T00:00:00+00:00Avoid over-engineering in machine coding rounds. Learn YAGNI, when patterns hurt, and how to keep designs simple without being simplistic. Practical LLD interview advice.Observer Pattern - Event-Driven Designhttps://systemcraft.in/lld-fundamentals/observer-pattern/2026-08-14T00:00:00+00:002026-08-14T00:00:00+00:00Observer pattern for LLD: decouple publishers from subscribers using event-driven design. Payment notifications, stock tickers, and real-time updates in Java, Python, C++.Observability - RED vs USE, Metrics vs Logshttps://systemcraft.in/concepts/observability/2026-08-14T00:00:00+00:002026-08-14T00:00:00+00:00Complete guide to observability for system design interviews. Metrics vs logs vs traces, the four golden signals, RED and USE methods, distributed tracing, cardinality explosion, and alerting on symptoms.Networking Basics - TCP, UDP, HTTP/3, DNS, TLShttps://systemcraft.in/concepts/networking-basics/2026-08-14T00:00:00+00:002026-08-14T00:00:00+00:00Networking fundamentals for system design interviews. TCP vs UDP, HTTP/1.1 vs HTTP/2 vs HTTP/3, QUIC, DNS resolution, TLS handshakes, keep-alive, head-of-line blocking, and OSI layers.Money Handling - BigDecimal and Ledgershttps://systemcraft.in/lld-fundamentals/money-handling/2026-08-14T00:00:00+00:002026-08-14T00:00:00+00:00Correct money handling in code: avoid floating point, use minor units or BigDecimal, implement proper ledger patterns. Java, Python, C++ examples for LLD interviews.Machine Coding Round - Format and Strategyhttps://systemcraft.in/lld-fundamentals/machine-coding-round/2026-08-14T00:00:00+00:002026-08-14T00:00:00+00:00Complete guide to machine coding interviews: format, grading rubric, 90-minute timing breakdown, and how it differs from DSA rounds. Prepare with real strategies.Load Balancing - Algorithms and L4 vs L7https://systemcraft.in/concepts/load-balancing/2026-08-14T00:00:00+00:002026-08-14T00:00:00+00:00Complete guide to load balancing for system design interviews. Round Robin, Least Connections, Consistent Hashing, L4 vs L7, health checks, sticky sessions.Leader Election in System Design - Raft, ZooKeeper, Bullyhttps://systemcraft.in/concepts/leader-election/2026-08-14T00:00:00+00:002026-08-14T00:00:00+00:00Complete guide to leader election in system design interviews. Bully algorithm, Raft consensus, ZooKeeper ephemeral nodes, split-brain prevention.Inheritance vs Composition - When to Use Which in OOPhttps://systemcraft.in/lld-fundamentals/inheritance-vs-composition/2026-08-14T00:00:00+00:002026-08-14T00:00:00+00:00Inheritance vs composition with real tradeoffs: when IS-A makes sense, when HAS-A is better, and how to refactor from inheritance to composition. Java, Python, C++ examples.Idempotency Keys - Patterns and Retry Safetyhttps://systemcraft.in/concepts/idempotency/2026-08-14T00:00:00+00:002026-08-14T00:00:00+00:00Complete guide to idempotency in system design interviews. Idempotency keys, Redis implementation, at-least-once delivery with exactly-once semantics.Heartbeat vs Health Check - System Designhttps://systemcraft.in/concepts/heartbeat/2026-08-14T00:00:00+00:002026-08-14T00:00:00+00:00Heartbeat vs health check: who initiates, push vs pull, liveness vs readiness probes, timeout tuning, and gossip-based failure detection.Fan-Out on Write vs On Read - System Designhttps://systemcraft.in/concepts/fan-out/2026-08-14T00:00:00+00:002026-08-14T00:00:00+00:00Fan-out on write vs fan-out on read: push at post time or pull at read time, the celebrity problem, and the hybrid approach news feeds actually use.