Triggering a domain event from Aggregate A within a service of Aggregate B

[SteveG-WeInvest]

Hello everyone,

I’m new to DDD principles and I’m having trouble understanding how to trigger a domain event for Aggregate A from within a service that belongs to Aggregate B.

I understand that domain events should ideally be raised from within the domain, and therefore are usually tied to an entity’s lifecycle. However, my use case is a bit different:

I retrieve data from a third-party API and use it to create multiple entities within Aggregate A. After that, I need to update entities in Aggregate B based on the entities created in Aggregate A (for example, calculating a score depending on how many A entities are related to each B entity).

I want to avoid triggering an event in Aggregate A's domain logic, because it would fire the event n times — once for each created entity. Instead, I need to trigger the event only once, after the data retrieval and creation process is complete. This would likely happen in a service or an event listener.

What would be the right way to approach this in DDD?

Thanks!

[Sairyss]

If you need to coordinate multiple aggregates, Domain Services can help with that (also Application Services if logic involves calling external APIs/databases).

I want to avoid triggering an event in Aggregate A's domain logic, because it would fire the event n times — once for each created entity

You can just make a method in Aggregate A that creates multiple entities at once and emits event only once. If your use case doesn't allow it, then just do it in the service IMO.