Problems / Real world, hard

Core banking at month end

Month-end batch traffic doubles writes at 10 s. The database runs hot and crashes under CPU pressure.

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Your goals

Success ≥ 99%, p99 ≤ 500 ms.

1,000 requests a second for 30 seconds: 50% reads, 50% 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 Postgres.Little's law: throughput ≈ concurrency ÷ latency. 200 slots at 50 ms serve about 4,000 req/s.	3 replicas × 200 slots · timeout 1.0 s
PostgresSQL database	API calls it.Postgres runs one process per connection, which is why poolers like PgBouncer sit in front of busy databases.	2 vCPU · 60 connections

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

Hint 1

Writes all hit the primary.

Hint 2

Replicas don’t take writes.

One way to solve it

Scale the primary up (more vCPU).