System Design
How real systems are actually built at scale, one architecture at a time: the constraints, the tradeoffs, and the decisions behind each one. Read them in any order.
1 → 2 → 3 → 4 → 5 → 6 → 7 → 8 → 9 → 10 → 11 → 12 → 13 → 14 → 15 → 16 → 17 → 18 → 19 → 20 →
How WhatsApp Works: The Architecture Behind 100B Messages a Day
How Slack Works: Workspace-Sharded Architecture at Scale
How YouTube Works: Transcode Once, Serve From Edge
How Stripe Works: Idempotent API Over an Append-Only Ledger
How Spotify Works: Audio CDN Plus Nightly Recommendation
How Instagram Works: Fanout-on-Write With Celebrity Exception
How Twitter Timeline Works: Hybrid Fanout Architecture
How Uber Finds Nearby Drivers: H3 Hex Grid Architecture
How ChatGPT Works: KV-Cache, GPU Cluster, and the Router
How Reddit Works: Python Monolith Plus 90% Varnish Cache
How Airbnb Works: Calendar as a Consistency Boundary
How Zoom Works: SFU Beats MCU at Scale
How Amazon Lambda Works: Firecracker MicroVMs at Scale
How Google Docs Works: Operational Transformation at Scale
How Tinder Works: Asymmetric Match Graph at Scale
How Apple AirTags Work: Encrypted Crowd-Sourced Finding
How Bluesky Works: AT Protocol and Portable Identity
How Cloudflare Scaled Postgres: SQL at the Edge
How Meta Serverless Works: TwSF and Bin ACL at FB Scale
How Scion Works: Source Control for 30,000 Merges a Day
Frequently asked questions
What does each system design explainer cover?
One real system at a time: the constraints, the tradeoffs and the decisions behind the architecture, with diagrams.
Do I need to read them in order?
No. Each explainer is self-contained — read them in any order.
Are they beginner-friendly?
Yes. Every explainer starts from the problem before the boxes and arrows.