I have a situation where data is coming from a third party service. It is being passed through to a function that formats the data and then saves it to a view model in a way that I can visualize for my system.
In an Event driven approach, should I save the payload of the request (as this can easily be repayable) in the Event stream, or the formatted changes it produces to the view model (a more accurate representation of the current state of the data)?
Or something else completely?
Thanks
The incoming data can be viewed as a command expressing the intent to ultimately update some state. In this case the command is from outside our system, but commands can also be internal to our system. Especially for external commands, one critical thing to remember is that a command can be rejected.
In event sourcing, however, events are internal and express that the change has occurred and cannot be denied (at most it can be ignored). Thus it's probably best to store them in the format that is the most convenient for that internal use.
I would characterize the requests as commands and the formatted changes as events. Saving the payload is command sourcing, saving the formatted changes is event sourcing (confusingly, Fowler's earliest descriptions of event sourcing are more like command sourcing) and both are valid approaches. Event sourcing tends to imply a commitment to replay to a similar state while command sourcing leaves open the ability for replay to depend on something in the outside world. I've seen (and developed even) applications which used both techniques (e.g. incoming data is dumped to Kafka, a consumer treats those messages as commands against aggregates whose state is persisted as a stream of events, which gets projected back into Kafka).
If you (in CQRS/ES fashion) consider the read-side of your application to be a separate autonomous component from the write-side, then you reach the interesting conclusion that when the write-side publishes events, from the read-side's perspective it's publishing commands to the read-side. "One component's events are often another component's commands".
