In eventsourcing, I am having bit confusion on where exactly have to apply Business logic? I have already searched in google, but all examples are very basic ie., Updating state of an object inside Handler from an event object, but in my other scenario, had some confusion didnt understood on where exactly have to apply Business logic.
For eg: lets take a scenario to update status of IntervieweeVO, which exists inside Interview aggregate class as below:
class Interview extends AggregateRoot {
private IntervieweeVO IntervieweeVO;
}
class IntervieweeVO {
int performance;
String status;
}
class IntervieweeSelectedEvent extends BaseEvent {
private IntervieweeVO IntervieweeVO;
}
I have a business logic, ie., if interviewee performance < 3, then status = REJECTED, otherwise status should be SELECTED.
So, my doubt is: where should I keep above business logic? Below are 3 scenarios:
1) Before Applying an Event: Do Business Logic, then apply(IntervieweeSelectedEvent) and then eventstore.save(intervieweeSelectedEvent)
2) Inside EventHandler: Apply Business logic inside EventHandler class, like handle(IntervieweeSelectedEvent intervieweeSelectedEvent) , check Business logic and then update Object state in ReadModel table.
3) Applying Business Logic in both places ie., Before Applying an event and also while handing the event (combining above 1 + 2)
Please clarify me on above.
You should be able to reconstruct the entity's state as of a specific point in time from the event stream.
This implies that applying events should NOT contain any logic other than state mapping logic. All state necessary to project the AR's state from the events must be explicitly defined in those events.
Events are an expressive way to define state changes, not operations/commands. For instance, if IntervieweeRejected means IntervieweeStatusChanged(rejected) then that meaning can't ever change. The IntervieweeRejected event can't ever imply anything else than status = rejected, unless there's some other state captured in the event's data (e.g. reason).
Obviously, the way the state is represented can always change, but the meaning must not. For example the AR may have started by only projecting the current status and later on projected the entire status history.
apply(IntervieweeRejected) => status = REJECTED //at first
apply(IntervieweeRejected) => statusHistory.add(REJECTED) //later
I have a business logic, ie., if interviewee performance < 3, then status = REJECTED, otherwise status should be SELECTED.
Business logic would be placed in standard public AR methods. In this specific case you may expect interviewee.assessPerformance(POOR) to yield IntervieweePerformanceAssessed(POOR) and IntervieweeRejected events. Should you need to reevaluate that smart screening policy at a later time (e.g. if it has changed) then you could implement a reevaluateSmartScreeningPolicy operation.
Also, please note that such logic may not even belong in the Interviewee AR itself. The smart screening policy may be seen as something that happend after/in response to the IntervieweePerformanceAssessed event. Furthermore, I can easily see how a smart screening policy could become very complex, AI-driven which could justify it living in a dedicated Screening bounded context.
Your question actually made me think about how to effectively capture the context or why events occurred and I've asked about that here :)