CQRS：在CommandHandlers中注入messageBus

Question

Does it sound like a correct approach to inject the whole Bus in commandHandlers so the latter could call $this->messageBus->dispatch($events);

class OneCommandHandler
{
   private $messageBus;

   //.....

   public function handle(Command $command)
   {
     //..... will at some points hopefully returns DomainEvents
   }

   private function dispatch(DomainEvents $events)
   {
     $this->messageBus->dispatch($events);
   }
}

Isn’t it too much coupling?

Answer 1

I would even say it is recommended since in your command handler you most probably will want to reply back to the originator with the processing status and the only way to do this is to engage the bus reply/response functionality.

As it was discussed in the DDD/CRQS Google group, there are no such thing as "one way (or "fire and forget") commands" since command means action, a state change and this can fail, leaving the system in undefined state if the failure is not handler properly, comparing to a query that is idempotent.