Messages are Domain Events

Principle

Emit domain events instead integration events which represents a state change to multiple entities or aggregates inside the domain.

Description

Use domain events over integration events. The difference being that a domain event represents a state change to multiple entities or aggregates inside a domain (i.e. business concepts). Therefore, it does not represent the fact that an entity (e.g. database model) itself was updated. The consumer of the event may not hold the same entity in its codebase and be unaware.

Consequences

• Producers are responsible for the articulation of an event that has happened. An updated entity may lead to multiple attributes being updated, which could each become a domain event.
• Consumers need to support partial updates to their entities/domain model after receiving an event that indicates one of its attributes has changed.

Example

In the example, the current mileage of a vehicle is updated - which is a domain event “mileage updated”. The fact that therefore the vehicle model that holds the mileage attribute has changed, is of no relevance to the consumer of a message.

Correct

NEW_MILEAGE_READ_EVENT {
	mileage: 12345,
	date: 2019-01-01
}

Incorrect

VEHICLE_UPDATED_EVENT {
	colour: "black",
	mileage: 12345
}

References

See for reference this article that explains the difference well.