Page History

General Information

The Qpid M3 release introduces priority queues into the Java Messaging Broker, supporting JMS clients who wish to make use of priorities in their messaging implementation.

There are some key points around the use of priority queues in Qpid, discussed in the sections below.

Defining Priority Queues

Under the covers, a priority queue is an instance of a different class from a vanilla queue. Thus, you You must define a priority queue specifically before you start to use it. You cannot subsequently change a queue to/from a priority queue (without deleting it and re-creating).

You define a queue as a priority queue in the virtualhost configuration file, which the broker loads at startup. When defining the queue, add a <priority>true</priority> element. This will ensure that the queue has 10 distinct priorities, which is the number supported by JMS.

...

So, you need to set the prefetch values for your client (consumer) to make this sensible. To do this set the java system property XXXXXXXXXXXXXXXXXX max_prefetch on the client environment (using -D) before creating your consumer.

Setting the Qpid pre-fetch to 1 for your client means that message priority will be honoured by the Qpid broker as it dispatches messages to your client. (Coming soon: see JIRA). A default for all client connections can be set via a system property:

-Dmax_prefetch=1

The prefetch can be also be adjusted on a per connection basis by adding a 'maxprefetch' value to the connection url

amqp://guest:guest@client1/development?maxprefetch='1'&brokerlist='tcp://localhost:5672'

There is a slight performance cost here if using the receive() method and you could test with a slightly higher pre-fetch (up to 10) if the trade-off between throughput and prioritisation is weighted towards the former for your application. (If you're using OnMessage() then this is not a concern.)

...