Problems / Real world, hard
Tickets go on sale at 10 s: traffic jumps 5× for 10 seconds, mostly people refreshing the same few pages.
Free to open. Runs in your browser, no sign-up.
Success ≥ 99%, p99 ≤ 500 ms.
800 requests a second for 30 seconds: 95% reads, 5% 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 API.Layer 4 balancers route by IP and port; layer 7 balancers read HTTP and can route by path or header. | – |
| APIApp service | Load balancer forwards to it.Calls Redis cache (checked first), Postgres.Little's law: throughput ≈ concurrency ÷ latency. 200 slots at 50 ms serve about 4,000 req/s. | 2 replicas × 150 slots · timeout 1.0 s |
| Redis cacheCache | API checks it first for reads.An in-memory cache like Redis usually answers in well under a millisecond inside one region. | 95% hit rate |
| PostgresSQL database | API reads it when Redis cache misses. API calls it for writes.Postgres runs one process per connection, which is why poolers like PgBouncer sit in front of busy databases. | 4 vCPU · 60 connections |
Try it first. Open one hint at a time if you get stuck.
The spike is short and huge.
Reads of the same pages cache well; the API must still take the load.
Scale the API for the spike and keep the cache hit rate high.