4.2. Publishing

As a message producer there are a number of different publishing strategies that you can use with AMQP messaging.

You can publish messages to an exchange, and message consuming applications can subscribe to the exchange, creating their own queues. There are a number of different exchange types that you can use, depending on how you want to distribute the information your application produces. One thing to note when publishing to an exchange is that if your message falls in the woods while no-one is listening, it doesn't make a sound: if no consumers are subscribed to the exchange when you send a message to it, the message disappears into the ether. It is not stored. If you need your messages to be stored whether consumers are listening or not, then you want to publish to a queue.

You can publish messages to a queue by creating a queue and subscribing it to an exchange. You then send messages to that exchange routed to that queue, and consuming applications can connect to your published queue and collect messages. This method of publishing is the one to use when your messages need to be stored on the broker whether someone is listening or not. Using this method of publishing, you can still allow consumers to create their own subscriptions to the exchange, or you can publish exclusively to your queue.

To publish to an exclusive queue, you would publish to a Direct Exchange, and bind your publishing queue to the exchange with an exclusive binding key. This means that you can route messages directly to your queue, and no-one else can bind a queue to the exchange that can receive those messages.

To publish to a queue and also allow consumers to create their own queues that receive your messages, you could publish to a Fanout or Topic exchange, and create and bind a queue with the appropriate binding key to receive your messages. Consumers can then subscribe to your queue, and can also create their own queues and bind them to the exchange.