6.8.3. Message Group Consumer Requirements

The correct handling of group messages is the responsibility of both the broker and the consumer. When a consumer fetches a message that is part of a group, the broker makes that consumer the owner of that message group. All of the messages in that group will be visible only to that consumer until the consumer acknowledges receipt of all the messages it has fetched from that group. When the consumer acknowledges all the messages it has fetched from the group, the broker releases its ownership of the group.

The consumer should acknowledge all of the fetched messages in the group at once. The purpose of message grouping is to ensure that all the messages in the group are dealt with by the same consumer. If a consumer takes grouped messages from the queue, acknowledges some of them and then disconnects due to a failure, the unacknowledged messages in the group will be released and become available to other consumers. However, the acknowledged messages in the group have been removed from the queue, so now part of the group is available on the queue with the header redelivered=True, and the rest of the group is missing.

For this reason, consuming applications should be careful to acknowledge all grouped messages at once.