I'm building my first event sourced system. It will have multiple domains using projects with a publication lifecycle at it's core. How can I effectively replay or re-apply events of two domains to a new aggregate inside a third domain?
To be more specific. Imagine 4 domains each with their own bounded context and purpose. A short description of these contexts:
- Project - A
projectis a complex object at the core of the system, almost every domain requires project data to operate. A project has one or more ProductTypes which contain the limited supply of Products. - Media - The media domain covers operations around images, documents and generated reports and functions as a file server.
- Delivery - Delivery allows for the configuration of which content channels to publish all publications to.
- Publication - The publication domain handles the complex tasks of verifying if a project can be published to the requested status in it's current state.
The states of publication follow the lifecycle: concept (not yet published) > announced (optional) > sale > sold-out (publication ended). In my description I focus on the announced status. Concept is not actually a thin for the publication domain since a project is always in concept if publication does not know about it yet.
My first attempt was setting up a normal aggregate which handled the incoming event AnnouncementPublishedEvent. This requires a project to meet some basic requirements like 'it has a name', 'it has a description', 'it has at least one image' and so on. This means I need to validate this information before the event is applied and therefore I somehow need to supply a project instance in the command.
While doing this I suspected this method breaks the purpose of CQRS and I should be looking at the real data source: events. My next attempt was creating a Saga that starts when the event AnnouncementPublicationRequestedEvent. This saga needs to review which events occured around the given projectId and apply those to this new 'published project' projection in order to (at least) validate if the request can be accepted.
I researched and experimented with tracking processors but could not get a good example how this is done in version 4 of Axon. I also started to read several other questions on Stackoverflow that made me think I might need to reconsider my approach.
Unfortunately, the exact code can not be shared as it's not open source and even if I could it's far from a working state. I can use example code to show what I'm trying to do.
@Saga
@ProcessingGroup("AnnouncementPublication")
public class AnnouncementPublicationSaga {
private static int NUMBER_OF_ALLOWED_IMAGES
private PublicationId publicationId;
private ProjectId projectId;
private int numberOfImages = 0;
//...other fields
@StartSaga
@SagaEventHandler(associationProperty = "projectId")
public void handle(AnnouncementPublicationRequestedEvent event) {
publicationId = generatePublicationId();
//set parameters from event for saga to use
projectId = event.getProjectId();
targetPublicationStatus = event.getPublicationStatus();
date = event.getDate();
//initialize the 'publicated project' aggregate
//start a replay of associated events for this @ProcessingGroup
}
...
@SagaEventHandler(associationProperty = "projectId")
public void handle(ProjectCreatedEvent event) {
//Verify the project exists and has a valid name
}
...
/* Assumption* on how AssociationResolver works: */
@SagaEventHandler(AssociationResolver=MediaProjectAssociator.class )
public void handle(ProjectImageAdded event) {
numberOfImages += 1;
}
/* Assumption* on how AssociationResolver works: */
@SagaEventHandler(AssociationResolver=MediaProjectAssociator.class )
public void handle(ProjectImageRemoved event) {
numberOfImages -= 1;
}
...
/* In my head this should trigger if all events have been played
up to the PublicationRequestedEvent. Or maybe
*/
@SagaEventHandler(associationProperty = "publicationId")
public void handle(ValidationRequestCompleted event) {
//ValidationResult result = ValidationResult.builder();
...
if (numberOfImages > NUMBER_OF_ALLOWED_IMAGES) {
//reason to trigger PublicationRequestDeniedEvent
//update validationResult
}
...
if (validationResult.isAcceptable()) {
//Trigger AnnouncementPublicationAcceptedEvent
} else {
//Trigger AnnouncementPublicationDeniedEvent
}
}
...
@EndSaga
@SagaEventHandler(associationProperty = "publicationId")
public void handle(AnnouncementPublicationDeniedEvent event) {
//do stuff to inform why the publication failed
}
@EndSaga
@SagaEventHandler(associationProperty = "publicationId")
public void handle(AnnouncementPublicationAcceptedEvent event){
//do stuff to notify success to user
//choice: delegate to delivery for actual sharing of data
// or delivery itselfs listens for these events
}
}
*The associationResolver code is an assumption to it's actual working as I'm not even close to that part yet. My media context uses a file id as aggregate identifier as not every event is bound to a project. But all the media events this saga needs to replay will have a projectId as field in them. Any feedback on this is welcome but it's not my main problem now.
In the end the result should be: a record of the publication or a record of the attempt and why it failed.
The record of the publication contains all data from project or media events that are relevant to a publication. This is mostly information that potential buyers need to make a decision.
For the purpose of this question I don't expect the above to be solved completely. I just want to know if I'm on the right track with thinking in events, if my approach on replaying relevant events is the right way to go and if so how this can be done in Axon4.
