A direct exchange delivers messages to queues based on a message routing key. The routing key can be seen as an address that the exchange is using to decide how to route the message. A message goes to the queues whose binding key exactly matches the routing key of the message.
The direct exchange type is useful when you would like to distinguish messages published to the same exchange using a simple string identifier.
Imagine that Queue A (create_pdf_queue) in the following diagram is bound to a direct exchange (pdf_events) with the binding key pdf_create. When a new message with the routing key pdf_create arrives at the direct exchange, the exchange routes it to the queue where the binding_key = routing_key is, in this case, to Queue A (create_pdf_queue).
SCENARIO 1:
- Exchange: pdf_events
- Queue A: create_pdf_queue
- Binding a key between exchange (pdf_events) and Queue A (create_pdf_queue): pdf_create
SCENARIO 2:
- Exchange: pdf_events
- Queue B: pdf_log_queue
- Binding a key between exchange (pdf_events) and Queue B (pdf_log_queue): pdf_log
EXAMPLE:
A message with the routing key pdf_logis sent to the exchange pdf_events. The message is routed to pdf_log_queue because the routing key (pdf_log) matches the binding key (pdf_log). If the message routing key does not match any binding key, the message will be discarded, as seen in the direct exchange diagram:
A message goes to the queues whose binding key exactly matches the routing key of the message.