The present invention enables “unit-of-order”, which allows a message producer to group messages into a single unit. It guarantees that messages are not only delivered to consumers in order, they are also are processed in order. The unit-of-order will be delivered to consumers as one unit and only one consumer will process messages from the unit at a time. The processing of a single message is complete when it is acknowledged, committed, recovered, or rolled back. Until message processing for a message is complete, the remaining unprocessed messages for that unit-of-order are blocked.