Introduction to Exchanges

• Exchanges are AMQP entities where messages are sent to.

• Exchanges take a message and route it into zero or more queues.

• The routing algorithm used depends on the exchange type and rules called bindings.

• AMQP 0-9-1 brokers provide four exchange types:

  Exchange Type	Default Pre-declared names
  Direct exchange	(Empty string) and amq.direct
  Fanout exchange	amq.fanout
  Topic exchange	amq.topic
  Headers exchange	amq.match (and amq.headers in RabbitMQ)

• Exchanges are declared with a number of attributes, the most important of which are:

  • Name

  • Durability (exchanges survive broker restart)

  • Auto-delete (exchange is deleted when last queue is unbound from it)

  • Arguments (optional, used by plugins and broker-specific features)

• Exchanges can be durable or transient.

  • Durable exchanges survive broker restart whereas transient exchanges do not (they have to be redeclared when broker comes back online).

  • Not all scenarios and use cases require exchanges to be durable.

Default Exchanges

• The default exchange is a direct exchange with no name (empty string) pre-declared by the broker.

• This makes it ideal for simple applications, every queue that is created is automatically bound to it by the messsage broker with a routing key which is the same as the queue name.

• The default exchange makes it seem like it is possible to deliver messages directly to queues, even though that is not technically what is happening.

Direct Exchanges

• A direct exchange delivers messages to queues based on the message routing key.

• A direct exchange is ideal for the unicast routing of messages. They can be used for multicast routing as well.

• Working of direct exchanges,

  • A queue binds to the exchange with a routing key K.

  • When a new message with routing key R arrives at the direct exchange, the exchange routes it to the queue, if K = R.

  • If multiple queues are bound to a direct exchange with the same routing key K, the exchange will route the message to all queues for which K = R.

Fanout Exchanges

• A fanout exchange routes messages to all of the queues that are bound to it and the routing key is ignored.

• If N queues are bound to a fanout exchange, when a new message is published to that exchange a copy of the message is delivered to all N queues. Fanout exchanges are ideal for the broadcast routing of messages.

Topic Exchanges

• Topic exchanges route messages to one or many queues based on matching between a message routing key and the pattern that was used to bind a queue to an exchange.

• The topic exchange type is often used to implement various publish/subscribe pattern variations.

• Topic exchanges are commonly used for the multicast routing of messages.

• Whenever a problem involves multiple consumers/applications that selectively choose which type of messages they want to receive, the use of topic exchanges should be considered.

Headers Exchanges

• A headers exchange is designed for routing on multiple attributes that are more easily expressed as message headers than a routing key.

• Headers exchanges ignore the routing key attribute.

• The attributes used for routing are taken from the headers attribute.

• A message is considered matching if the value of the header equals the value specified upon binding.

• Headers exchanges can be looked upon as "direct exchanges on steroids".

• Because they route based on header values, they can be used as direct exchanges where the routing key does not have to be a string; it could be an integer or a hash (dictionary) for example.

Learn more

• Understanding Exchanges