NServiceBus queue concept

Question

Just started learning NServiceBus and trying to understand the concept. When it talks about queues, are we talking about MSMQs on both publisher and subscriber? So, if I have an application that generates a list of something (say, name of animals), then it dumps the list into publisher’s queue. The publisher polls the queue every minute and if there is something in the queue, it will publish to subscriber’s queue for further processing. Does this make sense?

Thanks.

Answer 1

The sequence of events for a publish is as follows:

• The Publisher will start up(Windows Service)
• A Subscriber will start up and place a message into the Publisher's input queue(MSMQ)
• The Publisher will take that message, read the address of the Subscriber and place that into storage(subscription storage: memory, MSMQ, or RDBMS)
• When it is time to publish and event, the Publisher will inspect the type of message and then read subscription storage to find Subscribers interested in that message
• The Publisher will then send a message to each of the Subscribers found in subscription storage
• The Subscriber receives the message in its input queue(MSMQ) and processes it

You can leverage other messaging platforms instead of MSMQ, but MSMQ is the default. There really is no polling done, all the endpoints are signaled when a message hits the queues.

Answer 2

MSMQ is a transport layer. It passes the messages around.

The application will publish something using a NServiceBus queue. If you configured it to use MSMQ, that's what it will use for its transport layer and this is what the subscribers will be looking at.

Answer 3

NServiceBus follows the publisher/subscriber model as you have correctly stated. However your confusion is based on the use of two queues. This is incorrect. The server (publisher) will maintain the queue which is interfaced via the MSMQ protocol and so your application would communicate directly with this possibly remotely or locally.

You would typically use a WCF service which would raise an event upon a new message being pushed onto the queue. Your application can then make use of this new message as desired. See the NServiceBus documentation for examples: http://www.nservicebus.com/ArchitecturalPrinciples.aspx