Command Validation in DDD with CQRS

Question

I am learning DDD and making use of the CQRS pattern. I don\'t understand how to validate business rules in a command handler without reading from the data store.

For

Answer 1

It depends if the operation is async or not i.e does the user expect some immediate response. Gift ownership is basically a security feature and that can be done as a 'prep' operation before invoking the actual service or sending the GiveGiftCommand.

The only command validation you can do is to make sure it contains the data in the required format (UI validation) and that the user has the permissions to do that action. But after the command is sent it's up to the Domain to decide if other business constraints are respected.

If the user expects some immediate feedback, you can actually 'wait' until the command is completed and for that you can use an approach where a command handler can provide a result to the sender using a mediator . But this implies that at least some commands are executed in an immediate manner and that might not be the case in your app. However, this is the simplest approach if you want to just return a message error as opposed to implementing compensations and other stuff. Some use cases are simple.

About command handlers and business logic, I disagree with Tomasz Jaskuλa . A command handler is a function, a technical detail. You can put business logic in a command handler or a static function, it doesn't matter. Messages and their handlers are infrastructural components that can be used to implement functionality. For example, in an app you can have Domain Events, Application Events etc . They're all events i.e notification that something changed and you can have event handlers that reside in Domain or in other places.

There's no rule preventing you to 'read' from the db, however at least the read model theoretically is stale. However this might not be such a problem in 99% of cases. For the rest 1% you need very specific solutions.