Outbox Pattern Solves Publishing. Inbox Pattern Solves Processing.

Venkatesan Ramar Posted on May 30           Outbox Pattern Solves Publishing. Inbox Pattern Solves Processing. # systemdesign # eventdriven # microservices # distributedsystems 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 twice, inventory was deducted twice, customers received three confirmation emails, some workflow executed multiple times. That's where the Inbox Pattern enters the conversation. The Outbox Pattern solves reliable event publishing . The Inbox Pattern solves reliable event processing . And if you're building serious event-driven systems, you usually need both. 1. The Problem Starts With At-Least-Once Delivery Most messaging systems don't promise exactly-once delivery, they promise at-least-once delivery . This includes Apache Kafka, RabbitMQ and many cloud messaging platforms. Note: Some might think, I've missed to consider Kafka's Exactly-Once Semantics. By default, Kafka operates on an at-least-once model. Kafka is famous for introducing true Exactly-Once Semantics (EOS). It achieves EOS