RabbitMQ "Presence Exchange" Plugin

Extends RabbitMQ Server with support for a new experimental exchange type, x-presence. An exchange of type x-presence notifies queues that are bound to it when other bindings appear and disappear.

Prebuilt Binary Downloads

From time to time, I make a snapshot binary download of the plugin available: see http://eighty-twenty.org/tech/rabbitmq/binary-plugins.html. Expect plugins to have a GPG signature from [email protected], key available here, fingerprint 630A 8781 4B1F A5BA C30F A95D 6141 87C4 CEB5 3E0C.

Overview

Messages are sent out from an x-presence exchange to queues attached with a binding key of the empty string when a new binding appears or an existing binding disappears. The message sent out has an empty body, with all the interesting information in the headers property of the message headers:

Key       Type  Description
-----------------------------------------------------------------
action    str   Either "bind" or "unbind"
exchange  str   The name of the exchange being bound/unbound
queue     str   The name of the queue being bound/unbound
key	      str   The binding key supplied at the time of binding

No messages are published for bindings with a key of the empty string. That means there are two categories of bindings:

• bindings made with the empty string "" as their binding key, which receive presence messages, but do not produce them; and

• bindings made with any other binding key, which produce presence messages but do not receive them.

Do not publish messages to an x-presence exchange. Right now, messages published to x-presence exchanges are dropped, rather than sent through to bound queues, but this behavior may change in the future.

Initial summary of present bindings

When a new presence listener is bound to a presence exchange (i.e. a new binding is created with binding_key being the empty string), an initial summary of the available bindings is sent to the newly-bound queue.

The summary is sent as a sequence of messages, each formatted as a bind message as described above.

The initial summary transmission can be disabled by binding with a special argument in the args table of the Queue.Bind command. Supplying an argument named x-presence-exchange-summary with a value of 0 or false will disable the initial summary transmission; if no such argument is present, or if it has any other value than a numeric value or boolean false, the initial summary transmission will be enabled.

Binding to an x-presence exchange

To receive presence (bind and unbind) notifications

Bind to a presence exchange with the empty string as the binding's binding_key. Optionally, supply an x-presence-exchange-summary argument to Queue.Bind to control the initial presence summary, as described above.

To generate presence events

Bind to a presence exchange with any binding_key other than the empty string. The chosen binding key will be propagated to all presence listeners in a message formatted as described above.

Why aren't messages published to the exchange passed on?

Because it would let arbitrary clients forge presence messages.

Messages could be forwarded if they could be marked by the exchange in a way that made it clear that the passed-on messages didn't originate from within the exchange itself. One way to do that might be to have the exchange rewrite messages to be similar to presence messages, but with an action of message and a body containing the encapsulated properties and body of the relayed message. At the moment, the RabbitMQ pluggable exchange API makes it a bit tricky to rewrite messages on their way through an exchange, so for now messages published to presence exchanges are simply dropped.

Licensing

This plugin is licensed under the MPL 2.0. The full license text is included with the source code for the package. If you have any questions regarding licensing, please contact [email protected].