Step 1: Use Cases, Constraints, Assumptions, Back-of-Envelope

Use cases

We'll scope to handle only the core flows that expose the bottlenecks; everything else is deferred.

• User posts a tweet — 280 chars + media refs, persisted, then fanned out to followers' timelines.
• User reads home timeline — GET merged, ranked feed (paginated, 1M followers (0.01% users).
• Read:write 100:1 for timeline; peak read QPS 70k, write QPS 700.
• Tweet 300 bytes + media pointers; timeline cache per user ~800 tweets.
• 99.99% read availability, 99.9% write, search staleness "
  -H "Content-Type: application/json"
  -d '{"body":"hello world #design","media_ids":[]}'

201 {"tweet_id":"1829938319231","created_at":"2026-09-02T00:00:00Z","author_id":42}

curl "https://api.twitter.example/api/v1/timeline?cursor=eyJ...&limit=20"

200 {"tweets":[...],"next_cursor":"eyJ..."}

Generation