Lets assume rebus could not publish message to rabbitmq or some other queue, what is the best practice to handle this exception. I stopped rabbitmq service and rebus threw Aggregate exception. I can manually cacth this exception in try - catch block but is there a better solution to catch exceptions when such situations happened ?
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First off: If you get an exception when initially sending/publishing a message (e.g. while handling a web request), there's nothing you can do, really. Sorry ;)
You should probably log - thoroughly - all the information you can, and then be sure to set up logging so that the information ends up in a file or in some other persistent log. And then you should have some kind of notification or a process in place that ensures that someone will at some point look at the log.
You should probably have this kind of logging in place, regardless of the type of work you do.
Depending on how important your information is, you could also set up some kind of retry mechanism (although you should be careful that you do not consume threads and too much memory while retrying). Also, since your web application should be able to be recycled at any time, you probably should not rely (too much) on retries.
You can do some things, though, in order to minimize the risk of ending up in a situation where you can't send/publish.
I can recommend that you use some kind of high-availability transport, like MSMQ (because it has local outgoing queues), RabbitMQ (with a shovel on each machine), or Azure Service Bus or Azure Storage Queues if you're in Azure.
Moreover - if you were using MSMQ, and you want to publish an event - I would recommend that you await bus.Send(theEvent) first, and then when you handle the message, you await bus.Publish(theEvent). This is because Rebus (with the MSMQ transport) needs to do a lookup in the subscription storage in order to get all subscribers for the given event. This is not a problem with RabbitMQ though, because Rebus will use Rabbit's topics to do pub/sub and will be just as safe as doing an ordinary send.
When you're sending/publishing from within a Rebus message handler, there is of course no problem, since the receive operation will be rolled back, and eventually the incoming message will end up in an error queue.
I hope that cast some light on the situation :)
