as always in DDD / CQRS related scenarios, it all depends on the details. But here are some sensible default ways to deal with this.
The CommandBus allows for Interceptors to be configured. Some of these interceptors run in the calling thread, some others in the executing thread. In case of the SimpleCommandBus, the calling thread and executing thread are the same. The main different is that the first type of interceptors is execute before the method returns, allowing them to throw an exception, which is propagated to the caller. This is typically the place where you do structural validation. Your rest API can translate this exception to a 40x error.
As for command handling, when some business rule is violated, you can either throw an exception or use a return value to indicate success/failure. In your rest API, you can deal with this in 2 ways: return immediately giving a 302 while providing a URL where the client can request the result (as soon as it is available). Alternatively, you can wait for the result and return a 200 or 40x depending on the result. Do note that waiting is not always a good thing. When possible, use Servlet 3 async to send the response, as it prevents you from blocking a HTTP thread.
Third option, which could be a little more complex, is to subscribe to the Event Bus for the result of an action. When the result is detected, you return the same way as you would have done using the callback method described earlier (either immediate 302 with request for result, or wait and send a 200/40x).
If you’re planning to use the send-and-wait method, you may want to consider using the CommandGateway concept, as it allows you to easily configure if and how long you want to wait. (See http://www.axonframework.org/docs/2.0/command-handling.html#d4e221).
In the end, it’s all about deciding what the semantics of the rest api are. Does 200 mean “thanks for the message, we’ve got it in order”, or does it mean “thanks, we’ve processed your request, and it’s ok”. The first is faster, the second more reliable.
Hope that helps.