Problems / Real world, hard
Asked at Google (Docs) and Figma. Every keystroke is a transaction that locks the document row in Postgres, cursors are database writes, and opening a document replays it from the database. At 8 s a company all-hands starts and traffic triples for 10 seconds.
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Success ≥ 99.5%, p99 ≤ 400 ms and Apdex ≥ 0.90 through the meeting, for at most $13,000 a month.
Like a coding problem, a solution has to pass every case, not just the one it was built for. Submit in the lab runs all 4; 1 of them are hidden until then.
8,000 requests a second for 20 seconds: 60% edit (writes), 30% open (reads), 10% cursor (writes).
What each component does in this design, and what calls it. The simulator mimics each one from these settings.
| Component | Its job here | Settings |
|---|---|---|
| Load balancerLoad balancer | Traffic enters here.Calls Docs API.Layer 4 balancers route by IP and port; layer 7 balancers read HTTP and can route by path or header. | – |
| Docs APIApp service | Load balancer forwards to it.Calls Documents and edits (Postgres, row lock per document).Little's law: throughput ≈ concurrency ÷ latency. 200 slots at 50 ms serve about 4,000 req/s. | 8 replicas × 100 slots · timeout 1.0 s |
| Documents and edits (Postgres, row lock per document)SQL database | Docs API calls it.Postgres runs one process per connection, which is why poolers like PgBouncer sit in front of busy databases. | 16 vCPU · 400 connections |