CQS and CRUD operation

Question

I working on high-scalability web site for learning purpose. I decided to use CQS pattern and some ideas of CQRS. I have separate write and read layers used by command handlers and event handlers which system sends and receives from message buses (two separate message- buses).

I have some problem dealing with commands. I read that command shouldn't return anything. And now the point is: for example I have a form on with user can create an event or for example change something in his profile (photo or name). After user clicked save i want to show him, his profile or add a new event to his wall. But how can I know that his profile has been already updated when command is only send to the bus ? I How connect idea of command and CRUD operations ? Or maybe this wrong idea at all ?

Answer 1

Well first off, the split should not be between commands and events, but rather between domain and read models. You can't really map CQRS commands to CRUD operations as a general rule, although most of the commands in your system will change the state of your repositories. I will give you a general overview of how this works. Say you want to add a user, you create a command AddUserCommand and assign an id to that message. On the back end, you have an handler for that command and you're right that the command does not return nothing. Once that command is handled you should publish and event reflecting the change: UserWasAddedEvent. The id of this message will be unique, but it can and should have an id related to the command which you created in the UI. Your read models should handle the event and update a read model with the command status (waiting, processing, completedOnError, completedSuccessfully) depending on the event you published. On the UI after you submited the command, you should start querying the read models whith the ID of the command you created to get the status and then update your UI accordigly.

Your right that CQRS handlers return void, but you should bear in mind that typically in an architecture like this, the backend should return the validation results of the submited commands, not the handler itself but the infrastucture around your cqrs handlers.

Answer 2

Just update te UI on the assumption the command succeeds - which most of the time it will.

If validation is required on the user input, you could run validation as the user types or tabs to increase the likelyhood the command will succeed.