System Design

A full library for the system design interview — concepts, walkthroughs, and the reasoning framework that ties them together. Built for backend and distributed-systems engineers targeting mid-to-senior big-tech roles.

Orientation

Start here before the walkthroughs. The plan gives you a week-by-week order; the tradeoffs framework gives you the reasoning process; the decisions reference and checklist are the fast-lookup tools you return to throughout prep.

• Plan: 9-Week System Design Interview Prep — the week-by-week roadmap into the full library.
• Framework: Thinking About Tradeoffs in System Design — the four-step reasoning process (axes → context → cost → commit) that senior rounds actually score.
• Reference: Common System Design Decisions — a fast-lookup catalog of the recurring choices, each with a default and when to switch.
• Guide: Last-Minute System Design Review Checklist — a condensed pre-interview reference covering concepts, walkthroughs, and common mistakes.

Concept references

Short cheatsheets — one concept per page. Read these before, or alongside, the walkthroughs that use them. Each walkthrough links back to the relevant concept when it first reaches for it.

• Reference: API Design Patterns

  A short reference on REST vs gRPC, versioning, pagination, idempotency, and the small decisions that separate a designed API from an improvised one.

• Reference: Database Indexing

  A reference on B-Tree vs LSM-Tree storage engines, composite index column ordering, covering indexes, and where secondary indexes create hot shards.

• Reference: Common System Design Decisions

  A fast-lookup catalog of recurring system design choices — storage, performance, architecture, reliability — each with a default and when to switch away from it.

• Reference: Capacity Estimation

  A short reference on interview capacity math — QPS, storage, and bandwidth estimates, the numbers worth memorizing, and turning arithmetic into design conclusions.

• Reference: Consistency Models

  A short reference on consistency guarantees from linearizable to eventual — what each promises, what it costs, and how CAP and PACELC frame the choice.

• Reference: Partitioning Strategies

  A short reference on hash, range, and consistent-hash partitioning — how each distributes data, what breaks with each, and how to pick a partition key.

• Reference: Replication Strategies

  A short reference on leader-follower, multi-leader, and leaderless replication — failure behavior, lag anomalies, and quorum arithmetic for each.

• Reference: Queue Delivery Semantics

  A short reference on at-most-once, at-least-once, and exactly-once delivery — where each guarantee actually comes from, and what replay and idempotency add.

• Reference: NoSQL Hot Key Mitigation Patterns

  A short reference on hot key patterns in distributed NoSQL systems — what causes them, and the static and dynamic strategies for mitigating them.

• Reference: Consecutive Activity & Retention Aggregation

  A reference on computing streaks and retention over massive event streams — pre-aggregation, state slimming, and the bitmap, gaps-and-islands, and streaming techniques per time grain.

• Reference: Distributed Transaction Patterns

  A deep reference on distributed transaction patterns — 2PC, 3PC, and saga — their sequences, failure modes, and when to reach for each.

• Reference: Cache Access and Invalidation Patterns

  A reference on read/write caching patterns, invalidation strategies, and the classic failure modes — penetration, breakdown, avalanche — and how to avoid each.

• Reference: SQL vs NoSQL Schema Design

  A short reference on when to pick SQL vs NoSQL, how to model schemas in each, and how to avoid hot keys.

• Reference: Rate Limiting Algorithms

  A short reference on the four canonical rate limiting algorithms — token bucket, leaking bucket, fixed window, and sliding window — with when to pick each.

• Reference: Real-Time Connection Patterns

  A short reference on HTTP polling, long-polling, SSE, WebSocket, WebRTC, and push notifications — which to reach for and why.

Infrastructure walkthroughs

Foundational systems — KV stores, databases, caches, locks, queues, and CDNs — that appear as building blocks inside larger designs. The KV store, database, and cache form a natural triad: they are the two branches of the AP/CP CAP fork (KV store = leaderless/AP, database = consensus/CP) plus the cache that sits in front of both. Read them together.

• Walkthrough: Designing a Distributed Key-Value Store
• Walkthrough: Designing a Distributed Cache
• Walkthrough: Designing a Distributed Database
• Walkthrough: Designing a Distributed Lock
• Walkthrough: Designing a Distributed Queue
• Walkthrough: Designing a CDN

Classic product walkthroughs

The questions most commonly asked at FAANG-style interviews.

• Walkthrough: Designing a URL Shortener
• Walkthrough: Designing a News Feed
• Walkthrough: Designing a Chat System
• Walkthrough: Designing a Video Streaming Platform
• Walkthrough: Designing a Notification System
• Walkthrough: Designing a File Storage & Sync Service
• Walkthrough: Designing a Collaborative Editor

Search, ranking & recommendations

• Walkthrough: Designing a Post Search System
• Walkthrough: Designing a Recommendation System
• Walkthrough: Designing a Full Ranking System
• Walkthrough: Designing a Geospatial Matching System

Data & analytics walkthroughs

• Walkthrough: Designing an Ad Click Aggregation System
• Walkthrough: Designing a Metrics & Monitoring System
• Walkthrough: Designing a Top-K System

Transactional & commerce walkthroughs

• Walkthrough: Designing a Rate Limiter
• Walkthrough: Designing a Distributed Job Scheduler
• Walkthrough: Designing a Flash Deal System
• Walkthrough: Designing an Auction System
• Walkthrough: Designing an Order Processing System

AI/ML system walkthroughs

• Walkthrough: Designing an LLM Serving Platform
• Walkthrough: Designing a RAG System
• Walkthrough: Designing a Vector Database
• Walkthrough: Designing an AI Agent Orchestration System