What result does a Command request return in CQRS design?

Question

I've been looking at CQRS but I find it restricting when it comes to showing the result of commands in lets say a Web Application.

It seems to me that using CQRS, one is forced to refresh the whole view or parts of it to see the changes (using a second request) because the original command request will only store an event which is to be processed in future.

In a Web Application, is it possible that a Command request could carry the result of the event it creates back to the browser?

Answer 1

The answer to the headline of this question is quite simple: nothing, void or from a webbrower/rest point of view 200 OK with an empty body.

Commands applied to the system (if the change is successfully committed) does not yield a result. And in the case that you wish to leave the business logic on the server side, yes you do need to refresh the data by executing yet another request (query) to the server.

However most often you can get rid of the 2nd roundtrip to the server. Take a table where you modify a row and press a save button. Do you really need to update the table? Or in the case a user submits a comment on a blog post just append the comment to the other comments in the dom without the round trip.

If you find yourself wanting the modified state returned from the server you need to think hard about what you are trying to achieve. Most scenarios can be changed so that a simple 200 OK is more than enough.

Update: Regarding your question about queuing incoming commands. It's not recommended that incoming commands are queued since this can return false positives (a command was successfully received and queued but when the command tries to modify the state of the system it fails). There is one exception to the rule and that is if you are having a system with an append only model as state. Then is safe to queue the mutation of the system state till later if the command is valid.

Udi Dahans article called Clarified CQRS is always a good read on this topic http://www.udidahan.com/2009/12/09/clarified-cqrs/