The Outbox Pattern and Change Data Capture

Sagas often rely on events to communicate between services. For example, when the Order Service creates an order, it publishes an OrderCreated event so the Inventory Service can reserve the required products.

But this introduces an important reliability problem. What happens if the Order Service successfully saves the order in its database but crashes before publishing the event? The order exists, but the Inventory Service never learns about it. The workflow gets stuck.

This is a common problem in event-driven microservices, and it's known as the Dual-Write Problem. The Transactional Outbox Pattern solves this problem by ensuring that database changes and outgoing events are recorded atomically.

Change Data Capture (CDC) provides another important capability: detecting committed database changes and streaming them to other systems without requiring the application to publish each change directly.

Let's understand how these mechanisms work and how they help build reliable event-driven microservices.