Exchanges, bindings and queues: how a message finds a queue

Key points

• A publisher never writes to a queue - it publishes to an exchange with a routing key.
• A binding joins an exchange to a queue and carries the pattern the routing key is tested against.
• The exchange copies the message into every queue whose binding matches; no match means no copy.
• The default exchange has the empty name and binds every queue to its own name automatically.
• Consumers read from queues only, so one publish can feed one queue or twenty with no code change.
• Declarations are idempotent, but redeclaring with different arguments fails with 406 PRECONDITION_FAILED.

Example

// 1. the exchange: durable so it survives a restart
ch.exchangeDeclare("orders", BuiltinExchangeType.TOPIC, true);

// 2. the queue: durable, not exclusive, not auto-delete
ch.queueDeclare("invoicing", true, false, false, null);

// 3. the binding: this is what does the routing
ch.queueBind("invoicing", "orders", "order.created");

// 4. publish - note the exchange name, never the queue name
ch.basicPublish("orders", "order.created",
                MessageProperties.PERSISTENT_TEXT_PLAIN, body);

// the default exchange is the one exception: routing key = queue name
ch.basicPublish("", "invoicing", null, body);

Publishers address exchanges, consumers read queues, and bindings are the only thing that joins them.