In CQRS (event-sourced), do you need a global sequence counter in the event store?

Question

In trying to get my head around CQRS (and DDD in general) I have come across situations when two events occur on different aggregates but the order of them has domain meaning. If so then they could happen so close together that a timestamp (as used by the sample implementations I have seen) cannot differentiate them, meaning the event store doesn't contain a 'complete' representation of the domain as there is ambiguity over the order in which events occurred.

As an example, the domain could fire a CustomerCreatedEvent which applies to the Customer aggregate, and then a CustomerAssignedToAgent event on the Agent aggregate. The CustomerAssignedToAgent event doesn't make sense if it occurs before the CustomerCreatedEvent, but typically both of these might be fired as a result of one operation which makes it likely that the timestamps would effectively be the same.

So am I just modelling things badly? Should there ever be a situation where the sequence of events across different aggregates is important? Or should you keep a global sequence number on your event store, so that you can identify the exact sequence in which events occurred?

Answer 1

In general it is bad idea to enforce global order. Aggregates are meant to form ACID-semantics boundaries.

This means that two aggregates should not be updated in one transaction and there is no other means of enforcing global order.

In your case it could make sense that along with creating CustomerCreatedEvent Customer should send a message to Agent aggregate telling it to assign itself. Generally inter-aggregate communication should be done via messaging.

Answer 2

I also found this post by Greg Young on the domaindrivendesign yahoo list:

Order is only assured per a handler within an aggregate root boundary.

There is no assurance of order between handlers or between aggregates.

Trying to provide those things leads to the dark side.