What all functionality are there in queue which can't be achieved by topic??

Question

What all functionality are there in queue which can't be achieved by topic??

Answer 1

The main requirement that I run into is that consumers cannot compete for a single message on a topic. For example, I have a client who publishes call center events. Several systems subscribe to these events. One of these systems is the actual call routing application which has multiple instances running. If each instance subscribes then the call is routed to all of them. However, if the message is dropped onto a queue and all the instances consume off the same queue then only one will receive the message and the call goes to that operator. If the publishing application converts from topics to a queue, the call center works but all the other subscriber apps don't get the message.

The solution (as implemented in WebSphere MQ) was to create an administrative subscription on the topic and deliver the messages to a queue that all application instances consume from. So the producer apps are still publishers, all the dynamic subscribers still get copies of the message and the call center app instances compete for a single instance of each published message.

Also, you can't use browse semantics on a topic whereas you can on a queue. With topics you can specify selectors to filter the messages that are returned but that's about it. With queues you can browse, reset the browse pointer and then browse some more.

If you put a message on a queue and nothing is there to receive it, the message remains queued up. If you put a message to a topic and there are no active subscribers or durable subscriptions, the message is discarded. Therefore messages in a queue are naturally durable whereas messages on a topic may or may not be.

From a pure JMS perspective, queue and topic are both instances of destination and are interchangeable if you don't try to browse. An application may not know whether the destination it opens is a queue or a topic unless it uses instanceOf() at run-time to find out.