Constrain Domain Events to be raised only by the Domain Model

[subhashb]

Protean allows publishing new events with the domain.publish() method:

@classmethod
def add_new_role(cls, params):
    """Factory method to add a new Role to the system"""
    new_role = Role(name=params['name'])

    # Publish Event via the domain
    domain.publish(RoleAdded(role_name=new_role.name))

    return new_role

This approach disconnects the event from its source, allowing any code anywhere to be able to raise an event (in Application services, Command handlers, or even Subscribers for example). But ideally, we want events only to be raised on state change, specifically in the aggregate method performing the state change.

So it is better to make it impossible to raise events unless they are bubbled by the aggregates undergoing the change. An API like this would make more sense:

@classmethod
def add_new_role(cls, params):
    """Factory method to add a new Role to the system"""
    new_role = Role(name=params['name'])

    # Publish Event via the domain
    self.events.add(RoleAdded(role_name=new_role.name))

    return new_role

Each aggregate would have a built-in events array that is populated within state-change methods, The events are consumed by the UnitOfWork on commit and dispatched to the broker.

[subhashb]

As of v0.9.0, Domain.publish is doubling as a mechanism to persist events to the message store and push events to message brokers. It is probably better to separate the message store persistence from pushing to message brokers and allow applications to decide when they want to push a message to a broker.