# Domain event

From \[\[Domain-Driven Design]] (\[\[DDD]]).

## Context

A service often needs to publish events when it updates its data. These events might be needed, for example, to update a [CQRS view](https://github.com/ducminhgd/notes/blob/main/computer/microservices/cqrs.md). Alternatively, the service might participate in an choreography-based [saga](https://gia-duong-duc-minh.gitbook.io/notes/computer/microservices/saga), which uses events for coordination.

## Problem

How does a service publish an event when it updates its data?

## Solution

Organize the business logic of a service as a collection of DDD [aggregates](https://gia-duong-duc-minh.gitbook.io/notes/computer/microservices/aggregate) that emit domain events when they created or updated. The service publishes these domain events so that they can be consumed by other services.

## Related patterns

* The \[\[Saga]] and \[\[CQRS]] patterns create the need for this pattern
* The \[\[Aggregate]] pattern is used to structure the business logic
* The \[\[Transactional outbox]] pattern is used to publish events as part of a database transaction
* \[\[Event sourcing]] is sometimes used to publish domain events

## See also

My book [Microservices patterns](https://microservices.io/book) describes this pattern in a lot more detail.


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