DDD About a design decision

Question

I have to solve a domain problem and I have some doubts about what is the better solution. I am going to present the problem:

I have Applications and each Application has many Process. An Application has some ProcessSettings too. I have some business rules when I have to create a Process, for example, based on the process settings of application, I have to apply some rules on some process properties.

I have considered Application as aggregate root and Process as other aggregate root, and ProcessSettings as a value object inside Application aggregate.

I have a use case to create processes, and the logic is to create a valid instance of process and persist it with ProcessRepository. Well, I think I have two options to apply the process settings:

1. In the use case, get the process settings from Application aggregate by ApplicationId through a domain service in Application aggregate, and pass ProcessSettings to process create method.
2. In the use case, to create the process and through a domain service in Application aggregate pass a copy of process (a value object) to apply the process settings.

What approach do you believe is most correct to use?, or do you implement it in another way?

Thanks in advance!

Answer 1

Our product owner told us that if the client paid for some settings in a moment and created a process that settings will be valid for that process if the client does not update it. If the client leave to paid some settings then, when the client want to update that process our system will not allow update it because the actual settings will not be fit to the process data

That makes the implementation much easier, given that process settings-based validation only has to occur in process creation/update scenarios. Furthermore, I would guess that race conditions would also be irrelevant to the business, such as if settings are changed at the same time a process gets created/updated.

In light of this, we can assume that ProcessSettings and Process can be in distinct consistency boundaries. In other words, both can be part of separate aggregate roots.

Furthermore, it's important to recognize that the settings-based validation are not Process invariants, meaning the Process shouldn't be responsible for enforcing these rules itself. Since these aren't invariants you also shouldn't strive for an always-valid strategy and use a deferred validation strategy instead.

From that point there are many good ways of modeling this use case, which will all boil down to something like:

//Application layer service
void createProcess(processId, applicationId, data) {
    application = applicationRepository.applicationOfId(applicationId);
    process = application.createProcess(processId, data);
    processRepository.add(process);
}

//Application AR
Process createProcess(processId, data) {
    process = new Process(processId, this.id, data);
    this.processSettings.ensureRespectedBy(process);
    return process;
}

If ProcessSettings are part of the Application AR then it could make sense to put a factory method on Application for creating processes given it holds the necessary state to perform the validation, like in the above example. That removes the need from introducing a dedicated domain service for the task, such as a stand-alone factory.

If ProcessSettings can be it's own aggregate root you could always do the same, but introduce a lookup domain service for settings:

//Application AR
Process createProcess(processId, data, settingsLookupService) {
    process = new Process(processId, this.id, data);
    processSettings = settingsLookupService.findByApplicationId(this.id);
    processSettings.ensureRespectedBy(process);
    return process;
}

Some might say your aggregate is not pure anymore however, given it's performing indirect IO through calling the settingsLookupService. If you want to avoid such dependency then you may introduce a domain service such as ProcessDomainService to encapsulate the creation/update logic or you may even consider the lookup logic is not complex enough and put it directly in the application layer.

//Application layer service
void createProcess(processId, applicationId, data) {
    processSettings = processRepository.findByApplicationId(applicationId);
    process = application.createProcess(processId, data, processSettings);
    processRepository.add(process);
}

There's no way for us to tell which approach is better in your specific scenario and sometimes there isin't even a perfect way and many various ways could be equally good. By experience it's a good idea to keep aggregates pure though as it's easier for unit tests (less mocking).