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1、1/2/2018,Distributed Computing, M. L. Liu,1,Message Service System,M. L. Liu,1/2/2018,Distributed Computing, M. L. Liu,2,The Message System Paradigm,The Message System or Message-Oriented Middleware (MOM) paradigm is an elaboration of the basic message-passing paradigm.In this paradigm, a message sy
2、stem serves as an intermediary among separate, independent processes. The message system acts as a switch for messages, through which processes exchange messages asynchronously, in a decoupled manner. A sender deposits a message with the message system, which forwards it to a message queue associate
3、d with each receiver. Once a message is sent, the sender is free to move on to other tasks.,1/2/2018,Distributed Computing, M. L. Liu,3,The Message System Paradigm,1/2/2018,Distributed Computing, M. L. Liu,4,Message System Models,Two subtypes of message system models:Point-to-pointPublish-subscribe,
4、1/2/2018,Distributed Computing, M. L. Liu,5,The Point-To-Point Message Model,In this model, a message system forwards a message from the sender to the receivers message queue. Unlike the basic message passing model, the middleware provides a message depository, and allows the sending and the receivi
5、ng to be decoupled. Via the middleware, a sender deposits a message in the message queue of the receiving process. A receiving process extracts the messages from its message queue, and handles each one accordingly.Compared to the basic message-passing model, this paradigm provides the additional abs
6、traction for asynchronous operations.,1/2/2018,Distributed Computing, M. L. Liu,6,The Publish/Subscribe Message Model,In this model, each message is associated with a specific topic or event. Applications interested in the occurrence of a specific event may subscribe to messages for that event. When
7、 the awaited event occurs, the process publishes a message announcing the event or topic. The middleware message system distributes the message to all its subscribers. The publish/subscribe message model offers a powerful abstraction for multicasting or group communication. The publish operation all
8、ows a process to multicast to a group of processes, and the subscribe operation allows a process to listen for such multicast.,1/2/2018,Distributed Computing, M. L. Liu,7,Toolkits based on the Message-System Paradigm,The MOM paradigm has had a long history in distributed applications.Message Queue S
9、ervices (MQS) have been in use since the 1980s. The IBM MQ*Series6 is an example of such a facility. http:/ Other existing support for this paradigm are Microsofts Message Queue (MSQ), Javas Message Service,1/2/2018,Distributed Computing, M. L. Liu,8,Sources of Information,Suns JMS Tutorialhttp:/ Message Queue(MSMQ) http:/ MQ-Series http:/
