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Async systems are hard to test. Timing matters. Failures are hard to repeat. Events arrive out of order.
Follow these steps to build reliable systems:
- Test producers and consumers separately. Check the data sent by the producer. Check how the consumer handles it.
- Use in-memory queues for unit tests. Skip Redis or RabbitMQ. This makes tests fast and predictable.
- Test timeouts and retries. Force the consumer to be slow. Break the queue. Check if the dead letter queue catches failures.
- Use real infrastructure for integration tests. Use Testcontainers. Test the full path from producer to database.
- Check for race conditions. Send events fast. Test idempotency. Ensure the system handles duplicate events.
- Control the clock for scheduled jobs. Move time forward to trigger the job.
Avoid these common mistakes:
- Do not over-engineer for scale. Build the simplest thing. Use data to find bottlenecks.
- Do not ignore observability. Add logging and metrics on day one.
- Do not ignore backpressure. Limit queue sizes. Set timeouts. Use circuit breakers.
Start simple. Measure everything. Use boring technology your team knows.