Multi-Version Concurrency Control and CQRS and DDD

Question

In order to support offline clients, I want to evaluate how to fit Multi-Version Concurrency Control with a CQRS-DDD system.

Learning from CouchDB I felt tempted to provide each Entity with a version field. However there are other version concurrency algorithms like vector clocks. This made me think that maybe, I should just not expose this version concept for each Entity and/or Event.

Unfortunately most of the implementations I have seen are based on the assumption that the software runs on a single server, where the timestamps for the events come from one reliable source. However if some events are generated remotely AND offline, there is the problem with the local client clock offset. In that case, a normal timestamp does not seem a reliable source for ordering my events.

1. Does this force me to evaluate some form of MVCC solution not based on timestamps?

2. What implementation details must an offline-CQRS client evaluate to synchronize a delayed chain of events with a central server?

3. Is there any good opensource example?

4. Should my DDD Entities and/or CQRS Query DTOs provide a version parameter?

Answer 1

I manage a version number and it has worked out well for me. The nice thing about the version number is that you can make your code very explicit when dealing with concurrency conflicts. My approach is to ensure that my DTO's all have the version number of the aggregate they are associated with. When I send in a command it has the current version as seen on the client. This number may or may not be in sync with the actual version of the aggregate, ie. the client has been offline. Before the event is persisted I check the version number is the one I expected and if not I do a check against the preceding events to see any of them actually conflict. Only then if they do, do I raise an exception. This is essentially a very fine grained form of optimistic concurrency. If your interested I've written more detail including some code samples on my blog at: http://danielwhittaker.me/2014/09/29/handling-concurrency-issues-cqrs-event-sourced-system/

I hope that helps.

Answer 2

I suggest you to have a look at Greg's presentation on the subject. It might have answers you're looking for https://skillsmatter.com/skillscasts/1980-cqrs-not-just-for-server-systems

Answer 3

I guess you should rethink your domain, separate remote client logic in its own bounded context and integrate it with the other BC using the known principles of DDD for BC interop.