Problems / Databases, hard
Two requests read a counter, both add one, both write it back: one increment disappears. Popular rows are updated constantly.
Free to open. Runs in your browser, no sign-up.
No lost updates, success ≥ 99%.
1,200 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. | 8 vCPU · 60 connections |
Try it first. Open one hint at a time if you get stuck.
Read-then-write races under concurrency.
Let the database do the check-and-set.
Use atomic conditional updates (UPDATE … WHERE version = ?).