9.1The Challenge of Event-Driven Testing

Testing event-driven systems is hard because:

1. Events are asynchronous (fire and forget)
2. Delivery is not guaranteed uniformly (at-least-once, at-most-once, exactly-once)
3. Order is not guaranteed (events may arrive out of sequence)
4. Side effects are distributed (one event triggers actions in multiple services)

Kafka consumer testing must verify that consumers correctly process events, handle duplicates (idempotency), survive malformed events, and maintain ordering guarantees.