Queued Transaction Processing

Queued transaction processing (TP) is an alternative to direct TP that uses a persistent queue between client and server programs. The client enqueues requests and dequeues replies. The server dequeues a request, processes the request, enqueues a reply, and commits; if the transaction aborts, the request is replaced in the queue and can be retried. Clients can determine whether or not a request is executed by examining its queues. An unexecuted request is still in the client's request queue. An executed request has a reply in the client's reply queue. If the queue manager remembers the unique ID of the last request enqueued and reply dequeued by a client, then the client can recover from a failure by synchronizing its state with the state known to the queue manager. A queue manager is needed to support the queued communication model. It may be an independent product or an integral component of a transactional middleware product. It provides operations for the storage abstraction of queues. Publish-subscribe is a messaging style where a publisher sends a message to a broker that is responsible for forwarding the message to many subscribers. Subscribers register interest in certain message types and receive all published messages of that type. Other types of message-oriented middleware are available to integrate independent TP applications that support different communications protocols, different application functions, and different message formats.