Problems / Async and streaming, hard
Asked at Meta and Google. Every ad click is written as a row to one Postgres table, and advertisers’ dashboards read counts from the same table. At 8 s a big campaign doubles clicks for 6 seconds. Clicks time out and dashboards stall.
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Success ≥ 99.9%, p99 ≤ 300 ms through the campaign spike, for at most $18,000 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.
20,000 requests a second for 20 seconds: 97% click (writes), 3% report (reads).
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 Click API, Reporting API.Layer 4 balancers route by IP and port; layer 7 balancers read HTTP and can route by path or header. | – |
| Click APIApp service | Load balancer forwards to it for /click.Calls Clicks table (Postgres).Little's law: throughput ≈ concurrency ÷ latency. 200 slots at 50 ms serve about 4,000 req/s. | 10 replicas × 100 slots · timeout 1.0 s |
| Reporting APIApp service | Load balancer forwards to it for /report.Calls Clicks table (Postgres).Little's law: throughput ≈ concurrency ÷ latency. 200 slots at 50 ms serve about 4,000 req/s. | 2 replicas × 100 slots · timeout 4.0 s |
| Clicks table (Postgres)SQL database | Click API calls it. Reporting API calls it.Postgres runs one process per connection, which is why poolers like PgBouncer sit in front of busy databases. | 16 vCPU · 600 connections |