CQRS and DDD boundaries

Question

I've have a couple of questions to which I am not finding any exact answer. I've used CQRS before, but probably I was not using it properly.

Say that there are 5 services in the domain: Gateway, Sales, Payments, Credit and Warehouse, and that during the process of a user registering with the application, the front-end submits a few commands, the same front-end will then, once the user is registered, send a few other commands to create an order and apply for a credit.

Now, what I usually do is create a gateway, which receives all pubic commands, which are then validated, and if valid, are transformed into domain commands. I only use events to store data and if one service needs some action to be performed in other service, a domain command is sent directly from one service to the other. But I've seen in other systems that event handlers are used for more than store data. So my question is, what are the limits to what event handlers can do? And is it correct to send commands between services when a specific service requires that some other service performs an action or is it more correct to have the initial event raise and event and let the handler in the other service perform that action in the event handler. I am asking this because I've seen events like: INeedCreditAproved, when I was hoping to see a domain command like: ApprovedCredit.

Any input is welcome.

Yevhen Bobrov Yevhen Bobrov · Accepted Answer · 2012-09-19T23:52:12

You're missing an important concept here - Sagas (Process Managers). You have a long-running workflow and it's better expressed centrally.

Sagas listen to events and emit commands. So OrderAccepted event will start a Saga, which then emit ApproveCredit and ReserveStock commands, to be sent to Credit and Warehouse services respectively. Saga can then listen to command success/failure events and compensate approprietely, like say emiting SendEmail command or whatever else.

CQRS and DDD boundaries

2 Answers