5.2.6. Alternate Exchange
An alternate exchange provides a delivery alternative for messages that cannot be delivered via their initial routing.
For an alternate exchange specified for a queue, two types of unroutable messages are sent to the alternate exchange:
- Messages that are acquired and then rejected by a message consumer (rejected messages).
- Unacknowledged messages in a queue that is deleted (orphaned messages).
For an alternate exchange specified for an exchange, one type of unroutable messages is sent to the alternate exchange:
- Messages sent to the exchange with a routing key for which there is no matching binding on the exchange.
Note that a message will not be re-routed a second time to an alternate exchange if it is orphaned or rejected after previously being routed to an alternate exchange. This prevents the possibility of an infinite loop of re-routing.
However, if a message is routed to an alternate exchange and is unable to be delivered by that exchange because there is no matching binding, then it will be re-routed to that exchange's alternate exchange, if one is configured. This ensures that fail-over to a dead letter queue is possible.
