This guide covers bindings in AMQP 0.9.1, what they are, what role they play and how to accomplish typical operations using Bunny.
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This guide covers Bunny 2.10.x and later versions.
Learn more about how bindings fit into the AMQP Model in the AMQP 0.9.1 Model Concepts guide.
Bindings are rules that exchanges use (among other things) to route messages to queues. To instruct an exchange E to route messages to a queue Q, Q has to be bound to E. Bindings may have an optional routing key attribute used by some exchange types. The purpose of the routing key is to selectively match only specific (matching) messages published to an exchange to the bound queue. In other words, the routing key acts like a filter.
To draw an analogy:
Some exchange types use routing keys while some others do not (routing messages unconditionally or based on message metadata). If an AMQP message cannot be routed to any queue (for example, because there are no bindings for the exchange it was published to), it is either dropped or returned to the publisher, depending on the message attributes that the publisher has set.
If an application wants to connect a queue to an exchange, it needs to bind them. The opposite operation is called unbinding.
In order to receive messages, a queue needs to be bound to at least
one exchange. Most of the time binding is explcit (done by
applications). To bind a queue to an exchange, use
Bunny::Queue#bind where the argument passed can be either an
Bunny::Exchange instance or a string.
q.bind(x)
The same example using a string without a callback:
q.bind("amq.fanout")
To unbind a queue from an exchange use Bunny::Queue#unbind:
q.unbind(x)
Trying to unbind a queue from an exchange that the queue was never bound to will result in a channel-level exception.
Exchange-to-Exchange bindings is a RabbitMQ extension to AMQP 0.9.1. It is covered in the RabbitMQ extensions guide.
After a message reaches RabbitMQ and before it reaches a consumer, several things happen:
A more in-depth description is this:
The important thing to take away from this is that messages may or may not be routed and it is important for applications to handle unroutable messages.
Unroutable messages are either dropped or returned to producers. RabbitMQ extensions can provide additional ways of handling unroutable messages: for example, RabbitMQ's Alternate Exchanges extension makes it possible to route unroutable messages to another exchange. Bunny support for it is documented in the RabbitMQ Extensions guide.
Bunny provides a way to handle returned messages with the
Bunny::Exchange#on_return method:
x.on_return do |basic_return, properties, payload|
puts "#{payload} was returned! reply_code = #{basic_return.reply_code}, reply_text = #{basic_return.reply_text}"
end
Exchanges and Publishing documentation guide provides more information on the subject, including full code examples.
The documentation is organized as a number of guides, covering various topics.
We recommend that you read the following guides first, if possible, in this order:
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