3.2. Messaging styles

With this type of messaging you send a message to a queue. The message is then typically persisted to provide a guarantee of delivery. Some time later the messaging system delivers the message to a consumer. The consumer processes the message and acknowledges the message when it is done. Once the message is acknowledged it disappears from the queue and is not available to be delivered again. If the system crashes before the messaging server receives an acknowledgment from the consumer, the message will be available to be delivered to a consumer again, upon recovery.

With point-to-point messaging, there can be many consumers in the queue but a particular message will only ever be consumed by one of them. Senders (also known as producers) to the queue are completely decoupled from receivers (also known as consumers) of the queue; that is, they do not know of each other's existence.

A classic example of point-to-point messaging would be an order queue in a company's book ordering system. Each order is represented as a message which is sent to the order queue. Let us imagine there are many front end ordering systems which send orders to the order queue. When a message arrives on the queue it is persisted; this ensures that if the server crashes the order is not lost. Let us also imagine there are many consumers on the order queue; each representing an instance of an order processing component - these can be on different physical machines but consuming from the same queue. The messaging system delivers each message to only one of the ordering processing components. Different messages can be processed by different order processors, but a single order is only processed by one order processor - this ensures orders are not processed twice.

As an order processor receives a message, it fulfills the order, sends order information to the warehouse system and then updates the order database with the order details. Once the order processor updates the order database, it acknowledges the message to tell the server that the order has been processed and can be forgotten about. Often the send to the warehouse system, update in database and acknowledgment will be completed in a single transaction to conform with atomicity, consistency, isolation, durability(ACID) properties.