Can domain events be deleted?

Question

In order to make the domain event handling consistent, I want to persist the domain events to database while saving the AggregateRoot. later react to them using an event processor, for example let's say I want to send them to an event bus as integration events, I wonder whether or not the event is allowed to be deleted from the database after passing it through the bus? So the events will never ever be loaded with the AggregateRoot root anymore.

Answer 1

I wonder whether or not the reactor is allowed to remove the event from db after the reaction.

You'll probably want to review Reliable Messaging Without Distributed Transactions, by Udi Dahan; also Pat Helland's paper Life Beyond Distributed Transactions.

In event sourced systems, meaning that the history of domain events is the persisted history of the aggregate, you will almost never delete events.

In a system where the log of domain events is simply a journal of messages to be communicated to other "partners": fundamentally, the domain events are messages that describe information to be copied from one part of the system to another. So when we get an acknowledgement that the message has been copied successfully, we can remove the copy stored "here".

In a system where you can't be sure that all of the consumers have received the domain event (because, perhaps, the list of consumers is not explicit), then you probably can't delete the domain events.

You may be able to move them -- which is, instead of having implicit subscriptions to the aggregate, you could have an explicit subscription from an event history to the aggregate, and then implicit subscriptions to the history.

You might be able to treat the record of domain events as a cache -- if the partner's aren't done consuming the message within 7 days of it being available, then maybe the delivery of the message isn't the biggest problem in the system.

How many nines of delivery guarantee do you need?

Answer 2

Domain events are things that have happened in the past. You can't delete the past, assuming you're not Martin McFly :)

Domain events shouldn't be deleted from event store. If you want to know whether you already processed it before, you can add a flag to know it.

UPDATE ==> DESCRIPTION OF EVENT MANAGEMENT PROCESS

I follow the approach of IDDD (Red Book by Vaughn Vernon, see picture on page 287) this way:

1) The aggregate publish the event locally to the BC (lightweight publisher).

2) In the BC, a lightweight subscriber store all the event published by the BC in an "event store" (which is a table in the same database of the BC).

3) A batch process (worker) reads the event store and publish the events to a message queue (or an event bus as you say).

4) Other BCs interested in the event (or even the same BC) subscribe to the message queue (or event bus) for listening and react to the event.

Anyway, even the worker had sent the event away ok to the message queue, you shouldn't delete the domain event from the event store. Instead simply dont send it again, but events are things that have happened and you cannot (should not) delete a thing that have occurred in the past.

Message queue or event bus are just a mechanism to send/receive events, but the events should remain stored in the BC they were created and published.