While covering the Outbox Pattern , I realized there's another side of event reliability to discuss — and that led me to write this article.

In event-driven systems, a lot of engineering discussions focus on publishing events reliably. That’s usually where the Transactional Outbox Pattern enters the conversation. And rightly so.

Reliable event publishing is hard.

But over time, I’ve noticed something in backend systems that:

publishing events reliably is only half the problem.

The other half is much harder.

Processing them reliably.

Because even if:

Kafka delivers the event,

RabbitMQ retries correctly,

the Outbox Pattern guarantees publication

distributed systems still face another uncomfortable reality:

duplicate processing is inevitable.

Consumers crash.

Retries happen.

Brokers redeliver events.

Deployments interrupt processing.

Offsets commit at the wrong time.

Network failures create uncertain states.

And suddenly you're staring at production wondering why:

a payment was processed…