RabbitMQ and message priority

The answers on this question are out-of-date. As of RabbitMQ 3.5.0, there is now in-core support for AMQP standard per-message priorities. The documentation has all the gory details, but in short:

• You need to define the queue's priority range at the time the queue is created;
• Messages without a priority set get a priority of 0;
• Messages with a numeric priority higher than the maximum set on the queue get the highest priority the queue supports.

More interesting caveats are in the docs. It's well worth reading them.

Rabbit has no concept of priority other than, as Brian succinctly puts it, the one in front gets there first. ;-)

I would suggest implementing a set of queues that serve to service your particular messaging need and have these queues model your prioritisation need by, say, calling them 'MyQueueP1', 'MyQueueP2' and so on and then have our consumer(s) check P1 before P2 (etc.) and service messages from there first.

If you then have a message that is high priority you would publish it to the appropriate priority queue by way of a suitable routing key and voila.

[update] Check this question: In a FIFO Qeueing system, what's the best way the to implement priority messaging

[update] As per recent RabbitMQ release 3.5.0 this answer is now outdated and should be considered valid for only versions prior to this release. https://stackoverflow.com/a/29068288/489888

IIRC RabbitMQ still uses the AMQP protocol version 0.9.1 (get the spec here). The spec definitely mentions message priority:

Messages may have a priority level. A high priority message is sent ahead of lower     priority messages
waiting in the same message queue. When messages must be discarded in order to maintain a specific
service quality level the server will first discard low-priority messages.

And:

Note that in the presence of multiple readers from a queue, or client transactions, or use of priority fields,
or use of message selectors, or implementation-specific delivery optimisations the queue MAY NOT
exhibit true FIFO characteristics.

The spec says priority is a MUST, so I guess RabbitMQ should implement it, but you may want to consult its documentation.

Yes, RabbitMQ supports priority queues.

To make a queue work as a priority queue, supply property x-max-priority when declaring the queue.

Property x-max-priority defines the maximum priority number the queue supports.

In Java, you can do like below:

Map<String, Object> props = new HashMap<>();
props.put("x-max-priority", 10); // max priority number as 10
channel.queueDeclare(QUEUE_NAME, durable, false, false, props);

To publish messages of a particular priority, do like below:

String message = "My message with priority 7";
AMQP.BasicProperties.Builder basicProps = new AMQP.BasicProperties.Builder();
basicProps.contentType("text/plain")
            .priority(7);
channel.basicPublish("", QUEUE_NAME, basicProps.build(), message.getBytes());