RabbitMQ: exchanges, bindings and routing

On the live broker, the second lane holds the exchanges and the lines to the queues are bindings, labelled with their keys. Publish a message with a key nobody bound and watch it vanish.

The path of a message

A publisher sends a message to an exchange with a routing key (a short string like order.paid). The exchange stores nothing. It compares the key with its bindings, the rules "send messages matching this to that queue", and puts one copy into each queue that matches. Queues hold the messages until a consumer acknowledges them.

This split is the main difference from Kafka: in Kafka the producer picks a topic and the topic is the storage. In RabbitMQ the publisher only names an exchange, so the routing can change without touching the publisher.

The four exchange types

Type	A queue gets a copy when…
direct	its binding key equals the routing key exactly
fanout	it is bound at all; the key is ignored
topic	its binding pattern matches the key, word by word (words are separated by dots)
headers	the message headers match the binding's arguments (x-match = all or any)

A queue gets at most one copy per message, even if several of its bindings match.

Topic patterns

In a topic binding, * matches exactly one word and # matches zero or more words.

Key	*.orange.*	*.*.rabbit	lazy.#
quick.orange.rabbit	yes	yes	no
lazy.orange.male.rabbit	no	no	yes
lazy	no	no	yes
quick.brown.fox	no	no	no

A binding of just # turns a topic exchange into a fanout; a binding with no wildcards makes it behave like a direct exchange.

The default exchange

Every broker has a direct exchange with an empty name, and every queue is bound to it automatically with the queue's own name as the key. That is why the first tutorial "publishes to a queue": it publishes to the default exchange with the key hello.

When nothing matches

By default an unroutable message is dropped silently, and the publisher never knows. Two ways to catch it:

• mandatory flag on the publish: the broker sends the message back with basic.return, and the publisher can log or retry.
• Alternate exchange (alternate-exchange argument on the exchange): unroutable messages go to that exchange instead, usually a fanout into a queue someone inspects. No publisher has to change.

Try it

In the terminal on /rabbitmq:

rabbitmqadmin declare queue name=mine
rabbitmqadmin declare binding source=orders destination=mine routing_key=order.#
rabbitmqadmin publish exchange=orders routing_key=order.refund payload=hi
rabbitmqctl list_bindings