Topic exchange

Topic exchanges route messages to queues based on wildcard matches between the routing key and something called the routing pattern specified by the queue binding. Messages are routed to one or many queues based on a matching between a message routing key and this pattern.

The routing key must be a list of words, delimited by a period (.); examples are agreements.us and agreements.eu.stockholm, which, in this case, identifies agreements that are set up for a company with offices in lots of different locations. The routing patterns may contain an asterisk (*) to match a word in a specific position of the routing key (for example, a routing pattern of agreements.*.*.b.* will only match routing keys where the first word is agreements and the fourth word is b). A pound symbol (#) indicates a match on zero or more words (for example, a routing pattern of agreements.eu.berlin.# matches any routing keys beginning with agreements.eu.berlin).

The consumers indicate which topics they are interested in (such as subscribing to a feed for an individual tag). The consumer creates a queue and sets up a binding with a given routing pattern to the exchange. All messages with a routing key that match the routing pattern will be routed to the queue and stay there until the consumer consumes the message.

The following diagram shows three example scenarios:

SCENARIO 1:

Consumer A is interested in all the agreements in Berlin:

• Exchange: agreements
• Queue A: berlin_agreements
• Routing pattern between exchange (agreements) and Queue A (berlin_agreements): agreements.eu.berlin.#
• Example of message routing key that will match: agreements.eu.berlin and agreements.eu.berlin.headstore

SCENARIO 2:

Consumer B is interested in all the agreements:

• Exchange: agreements
• Queue B: all_agreements
• Routing pattern between exchange (agreements) and Queue B (all_agreements): agreements.#
• Example of message routing key that will match: agreements.eu.berlin and agreements.us

Topic Exchange: Messages are routed to one or many queues based on a matching between a message routing key and the routing pattern.

SCENARIO 3:

Consumer C is interested in all agreements for European head stores:

• Exchange: agreements
• Queue C: headstore_agreements
• Routing pattern between exchange (agreements) and Queue C (headstore_agreements): agreements.eu.*.headstore
• Example of message routing keys that will match: agreements.eu.berlin.headstore and agreements.eu.stockholm.headstore