Working with RabbitMQ bindings from Ruby with Bunny

About This Guide

This guide covers bindings in AMQP 0.9.1, what they are, what role they play and how to accomplish typical operations using Bunny.

This work is licensed under a Creative Commons Attribution 3.0 Unported License (including images and stylesheets). The source is available on Github.

What version of Bunny does this guide cover?

This guide covers Bunny 2.6.x and later versions.

Bindings in AMQP 0.9.1

Learn more about how bindings fit into the AMQP Model in the AMQP 0.9.1 Model Concepts guide.

What Are AMQP 0.9.1 Bindings

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:

  • Queue is like your destination in New York city
  • Exchange is like JFK airport
  • Bindings are routes from JFK to your destination. There may be no way, or more than one way, to reach it

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.

Binding Queues to Exchanges

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")

Unbinding queues from exchanges

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

Exchange-to-Exchange bindings is a RabbitMQ extension to AMQP 0.9.1. It is covered in the RabbitMQ extensions guide.

Bindings, Routing and Returned Messages

How RabbitMQ Routes Messages

After a message reaches RabbitMQ and before it reaches a consumer, several things happen:

  • RabbitMQ needs to find one or more queues that the message needs to be routed to, depending on type of exchange
  • RabbitMQ puts a copy of the message into each of those queues or decides to return the message to the publisher
  • RabbitMQ pushes message to consumers on those queues or waits for applications to fetch them on demand

A more in-depth description is this:

  • RabbitMQ needs to consult bindings list for the exchange the message was published to in order to find one or more queues that the message needs to be routed to (step 1)
  • If there are no suitable queues found during step 1 and the message was published as mandatory, it is returned to the publisher (step 1b)
  • If there are suitable queues, a copy of the message is placed into each one (step 2)
  • If the message was published as mandatory, but there are no active consumers for it, it is returned to the publisher (step 2b)
  • If there are active consumers on those queues and the basic.qos setting permits, message is pushed to those consumers (step 3)

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.

Handling of 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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