Can a domain event be emitted without being part of an aggregate state change (DDD)

Question

I would like to know how can a domain event be implemented (in DDD) if it is not a natural result of an aggregate's state change.

I will borrow the example of this post.

So lets say that we need an event like: temperatureWasMeasured which could be a product of some modelling we have done (e.g. in an event storming session).

This event feels like a product of some read action and not an actual state change of some aggregate.

So the question is: Where and how this event could make sense to be emmited?.

We could create an entity or an aggregate, which could have a state to count the number of reads in order to be emmited as a state change. But is this really necessary?

In the resources out there, the definition of the "domain event" does not seem to mention state change, but just as something which has happened in the domain.

So an action of a measurement (in a non quantum mechanical environment 😝), is considered to be a domain event, like the temperatureWasMeasured event?

It would be great if someone could clarify these questions, since it seems there is no clear answer out there.

Lectureship

The purpose of the domain model change event is to let others know that the domain is changed so that they can update their own models. It does not appear that this event has anything to do with that. – Anubis

What do you mean exactly by the term "domain model change event"? I have not seen this term in the bibliography, do you mean state change?. So you mean these kind of events could not exist in the DDD universe? So if it could not exist how this can be my modelled in a DDD way? Or there are part of the software solution that are part of the domain and others are not part of it. If we for example wanted to model this in an event storming session this could be a product of a time event if the domain experts, would say: every minute the system will inform for its temperature.So we should ignore it? – Lectureship

Answer 1

Nice question, my understanding is that domain events should occur when domain state changes and not in a simple query.

In the above example if we want to measure temperature we just query using a repository directly from application layer that has nothing to do with domain, so no domain events should occur.

If someone else is interested in the temperature changes like the example they should probably listen to temperature changed domain event and get their initially state from a query.

If there is a need to know the measurements for this domain a TemperatureMeasure aggregate might be missing from the design that will persist all the measurements and also emit domain events.

Gamete

Answer 2

Interesting question... I'm trying to get to grips with DDD as well.

Wouldn't you use a Boolean value object within an aggregate to define if its true to false in this case? If temperature was measured or not?

I've always thought you would need to associate any event to an aggregate....

Alden

Yeah, though if the aggregate method is just a read, would it make sense to emit domain events without an actual state change? Would it make sense to create a different entity for the measurement only? Or we can just emit an event when the actual read method of the aggregate is invoked? With this solution you are proposing it seems that the measurement in this case would be a mutation (write) since it would change the part of state of the aggregate which has to do with its measurement. In this case it seems it could be modelled as a state change. – Lectureship

Answer 3

Of course I don't know the context, but ** temperatureWasMeasured** feels for me like some kind of action, like I actually need to go through some business logic to measure that temperature (maybe there are few sensors and we want to get average value or something ). If it's a simple read then I would probably return data through some read command + handler and thats all. Maybe If I had more context on which you guys came up with an event like that It would be easier to help. If it was introduced in event storming session then it's something you probably want to trace, but its hard to guess why and by whom.

Maybe it is more like an analytical event for the business? Does not having this event actually affect your business in any way?

Pas