Problems / Caching, hard

Flush during a spike

Traffic doubles at 8 s, and at 12 s someone flushes the cache. Two bad things at once.

Free to open. Runs in your browser, no sign-up.

Your goals

Success ≥ 99%, p99 ≤ 500 ms.

1,500 requests a second for 30 seconds: 90% reads, 10% 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.	90% 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.	2 vCPU · 40 connections

Try it first. Open one hint at a time if you get stuck.

Hint 1

Fix capacity first: the spike alone hurts.

Hint 2

Then the database must survive a cold cache at peak.

One way to solve it

Scale the API and give the database read replicas.