Problems / Real world, hard
Asked at Meta (Facebook Live) and Twitch. Thirty thousand viewers a second poll a comments API that reads Postgres; posting writes to the same table. At 8 s a celebrity joins and traffic triples for 10 seconds.
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Success ≥ 99.5%, p99 ≤ 800 ms and Apdex ≥ 0.90, for at most $44,000 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 3; 1 of them are hidden until then.
30,000 requests a second for 20 seconds: 92% reads, 8% post (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 Comments API.Layer 4 balancers route by IP and port; layer 7 balancers read HTTP and can route by path or header. | – |
| Comments APIApp service | Load balancer forwards to it.Calls Comments (Postgres).Little's law: throughput ≈ concurrency ÷ latency. 200 slots at 50 ms serve about 4,000 req/s. | 20 replicas × 100 slots · timeout 1.0 s |
| Comments (Postgres)SQL database | Comments API calls it.Postgres runs one process per connection, which is why poolers like PgBouncer sit in front of busy databases. | 32 vCPU · 800 connections · 2 read replicas |