Glossary

AMQP is a complicated protocol that comes with its own terminology. If you are lost, perhaps the glossary below may be of use.

For more detail about these terms, see the specification documents, hosted by RabbitMQ.

message

A message in AMQP usually consists of some data (the payload), an arbitary set of application-defined headers, and some basic properties that may control message delivery.

queue

A queue of messages waiting to be consumed. Queues may be configured to save the messages they contain to disk so that the messages will survive a broker restart.

exchange

The entry point for messages into an AMQP system. Messages are published to an exchange, and are immediately routed to one or more queues based on routing keys and bindings. There are three types of exchange: direct, fanout, and topic. Each type interprets the queue bindings a different way.

routing key

When a message is published, the routing key it is published with defines which queues will receive a copy of the message. Effectively this is like specifying the addressee of the message, though the exact configuration of the exchanges and queues will define which queues actually receive a copy.

binding

The link from an exchange to a queue by which the queue may receive messages from that exchange. Each binding also contains a routing key “pattern” that acts as a filter. Messages published to that exchange, whose routing key “matches” the binding, will be put in the queue. If the exchange is a topic exchange, the routing key binding may contain wildcards.

publisher

A program that sends messages.

consumer

A program that receives messages.

broker

A network service that talks the AMQP protocol, hosts exchanges and queues, and ultimately passes messages from publishers to consumers.

channel

A single TCP connection to an AMQP service can be used for multiple operations at the same time. Each such operation must be performed on a different channel. Channels are multiplexed together so that messages can be received on one channel even while another channel is receiving a very large message. This means that clients rarely need to make multiple connections to an AMQP server.

fanout exchange

A fanout (fan out, because of the shape of the diagram when this is drawn) exchange disregards the routing key when delivering messages. All queues that are bound to the exchange receive a copy of the message.

direct exchange

A direct exchange delivers messages to all queues whose binding routing key exactly matches the message routing key.

topic exchange

A topic exchange delivers messages to all queues whose binding routing key is a pattern match for the message routing key. # is a wildcard meaning “zero or more dot-separated parts``. * is a wildcard meaning “exactly one dot-separated part. So if the binding routing key is profile.update and a queue is bound with the routing key profile.# it will receive the message, as well as any other messages whose topic starts with profile.. Likewise a queue could be bound with a routing key *.update to receive all messages of the form something.update (but not something.something.update).

default exchange

A pre-existing exchange, whose name is the empty string. When a queue is first created, it is bound to the default exchange, with the queue’s name as the routing key.