Problems / Async and streaming, hard
The order API writes the order, then publishes an event (dual write). At 10 s the queue is down for 5 seconds. The workers that send confirmation emails lose 2% of their acknowledgements and are not idempotent.
Free to open. Runs in your browser, no sign-up.
No lost events, no duplicate side effects, success ≥ 99%.
600 requests a second for 30 seconds: 100% submit (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 Orders DB, Job queue (SQS) (publishes).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 |
| Orders DBSQL 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 |
| Job queue (SQS)Message queue | API publishes to it.Calls Workers.If a consumer does not finish within the visibility timeout, the message is delivered again. | – |
| WorkersWorker | Job queue (SQS) consumes from it.Calls Results DB.Queues deliver at least once, so a worker can see the same message twice. | 16 concurrent jobs |
| Results DBSQL database | Workers 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.
Two contracts are missing: one on the API, one on the workers.
Events vanish when the publish fails after the write; emails repeat when a message is redelivered.
A transactional outbox on the API and an idempotency key on the workers.
Switch the API to a transactional outbox and give the workers an idempotency-key registry.