A channel is an ephemeral destination within a queue that can be used to improve message processing fairness, specify message selection, and enable request-response patterns. Messages published to a channel are available for consumption from both the queue or the channel level.
Channels increase the number of individual, addressable destinations within a single queue, enabling better scaling.
Channels don't require explicit creation or deletion. Channels share the same permissions, encryption method, and limits of their parent queue, but the queues can configure channel capacity, limiting the maximum amount of the queue's overall capacity that a single channel can use at any time.
Message Processing Fairness
When many producers are publishing to the same queue, a sudden spike in volume from one producer can cause delays in processing messages from other producers. In some scenarios you might need to protect message consumption from skewing in favor of consumers with more traffic, or noisy neighbors.
Without channels, you'd need to maintain individual queues each for each producer and implement a load balancing mechanism to consume messages from all producers evenly.
With channels, each producer can publish messages to a specific channel, but you can consume messages at the queue level. The Queue service returns messages from a random channel that has available messages, resulting in improved fairness.
Consumers can include an optional parameter in requests to select messages that were published with the matching the channel ID. The Queue service only returns available messages from one of the channels that matches the ID.
If multiple channels IDs match the selection in a request, messages are returned from a random channel that has available messages. If messages are published to a specific channel, but a consumption request doesn't include a channel, messages are returned from a random channel with available messages.
For more information, see Consuming Messages from a Channel.
A request-reply ephemeral pattern is a well-established integration pattern that enables a sender application to send a request and provides a way for the receiver to correctly send a response back to the sender application. This pattern typically needs only a short-lived queue for the transaction.
Using queues and channels, multiple applications can send messages to a single queue and specify a channel ID that uniquely identifies itself. A consumer application can send a response to the message producer and include its ID. Once the producer receives the response that includes the same ID, the transaction is considered complete. After the messages are processed successfully and deleted by the consumer, and if no more messages exist in the channel, the channel is automatically deleted.