CODEWITHSUNDEEP
domain-driven-designcqrsevent-sourcing
Patrick
Patrick

Reputation: 876

Why do we need command model in CQRS

So I was watching this video Event Sourcing You are doing it wrong by David Schmitz at the 15:17 he was talking about eventual consistency in event-sourcing. At first, I was like oh I got it so this is why CQRS is helpful with Event-sourcing because we can validate this things through command model before publishing an event right? but after I did a few research I was wrong. So I wonder why command model even exists since we can just retrieve the request body (suppose it's http request) put some business logic and then publish event.

Upvotes: 0

Views: 187

Answers (1)

Mattias Holmqvist
Mattias Holmqvist

Reputation: 832

With Event Sourcing we store events. A model exists in your application to support applying your business logic before deciding to save new event(s). To be able to make that decision, it must be possible to consistently read/write events to materialize your model before applying your business logic on it when a command is processed.

You need to store the events consistently to be able to make decisions further on. If you only publish your events to other parts of the system your model cannot use them in a consistent way.

The publishing of events to other systems is something that potentially can happen as a side-effect to use these events to also create read-models/projections or to react to them in other ways.

It does not have to be a push/publish though. It is perfectly valid to have a pull-based solution where downstream systems poll for events.

For example, in Serialized we store events in Aggregates. Feeds are used to provide a poll-based (eventually-consistent) downstream view of these events, but there is no publishing at all.

Upvotes: 1

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