System Design Case Studies 2

A blog series

System Design Case Studies, Part 2

Twelve more system design case studies, each with requirements, back-of-envelope numbers, API, data model, architecture, deep dives, failure modes and a runnable simulation: web crawler, key-value store, Dropbox-style sync, collaborative editing, CDN, top-K, unique IDs, multi-region, matching engine, sales ranking, finance pipeline and scaling on AWS.

Started 
2026
Lessons 
12
Reading 
~4 h

Contents

Every lesson has the same steps: what it is, how it works, a real example, common mistakes, then interview questions with answers you can say out loud.

  1. Lesson 00Design a Web CrawlerThe URL frontier, politeness, Bloom-filter dedup and trap defence, with a crawler simulation you can run.25 min
  2. Lesson 01Design a Key-Value Store (Like Redis or Dynamo)A write-ahead log and snapshots on one node, then a consistent-hash ring, N/R/W quorum and version vectors across many.22 min
  3. Lesson 02Design a File Sync Service Like DropboxContent-defined chunking, dedup, journal-based sync and conflicted copies.22 min
  4. Lesson 03Design Collaborative Editing Like Google DocsOperational transformation versus CRDTs, cursors and presence, and an operation log with snapshots.24 min
  5. Lesson 04Design a CDNEdge caches, cache keys and TTL, purging, request coalescing and hit-ratio arithmetic, with an edge-cache simulation.22 min
  6. Lesson 05Design Trending Topics and Top-K RequestsStreaming counts, sliding windows, a count-min sketch with a heap, and merging top-K across partitions.22 min
  7. Lesson 06Design a Unique ID GeneratorAuto-increment limits, UUID v4 and v7, ULID, Snowflake bit maths, clock skew and ticket servers, with a tested generator.14 min
  8. Lesson 07Design a System Across Multiple Data CentresActive-passive versus active-active, DNS steering, sync versus async replication, split brain and failback, with an RPO simulation.15 min
  9. Lesson 08Design a Stock Exchange Matching EngineA price-time priority order book, a single-threaded sequencer, event-log replay, market data and risk checks, with a tested engine.20 min
  10. Lesson 09Design Sales Ranking by CategoryHourly buckets and a sliding-window aggregator feeding per-category sorted sets, with freshness, hot categories and backfill.18 min
  11. Lesson 10Design a Personal-Finance Data Pipeline (Mint-Style)Idempotent ingest, dedup, rules-first categorisation, budget alerts, reconciliation and token security.22 min
  12. Lesson 11From One User to Millions: Scaling a Web App on AWSTen scaling stages with trigger, change and cost, the metric that tells you, AWS services in plain words, and a queueing arithmetic demo.20 min

Glossary

19 terms used in this series. Inside any lesson, hover or tap a dotted term to see its meaning without leaving the page. Browse all terms →