1. MW Tech IS 8040 Data Comm and Networking Dr. Hoganson Middleware Technology Communication Mechanisms Synchronous – process on client side must stop and wait until server reply is received. –Classic programming model Asynchronous – process on client side may proceed to do other work while waiting for server reply –Modern multi-threaded languages –Java, VB, Delphi –Adds another “wrinkle” – communication is not just between machines, but between specific processes on specific machines – TCP/IP protocol accommodates this concept – Port #

2. MW Tech IS 8040 Data Comm and Networking Dr. Hoganson Remote Procedure Call Procedural languages – Cobol, Pascal, C, etc The client “calls” a subroutine that happens to reside on the server rather than the local machine Synchronous – stop and wait for reply DCE “open” standard (Distributed Computing Environment)

3. MW Tech IS 8040 Data Comm and Networking Dr. Hoganson Message-Oriented Message Passing Requires a Message Protocol (a standard) 1.Client packages request into message format 2.Client transmits message 3.Server received message 4.Server un-packs request from message 5.Server computes/retrieves formulates reply 6.Server packages reply into a message format 7.Server transmits message 8.Client receives message 9.Client un-packs reply from message 10.Etc.

4. MW Tech IS 8040 Data Comm and Networking Dr. Hoganson Object-Oriented Object to Object, rather than procedure to procedure Object sends a message (a method call) to another object Objects can be multithreaded, so supports both synchronous and asynchronous CORBA: Common Object Request Broker Architecture DCOM: Distributed Common Object Model (Microsoft proprietary)

5. MW Tech IS 8040 Data Comm and Networking Dr. Hoganson Concurrent Processing Multiple concurrent processes A form of parallel processing Introduces synchronization and communication issues A useful model: Petri Nets Process State Transition Process State Transition Process State Resource Transitions may require resources Token

6. MW Tech IS 8040 Data Comm and Networking Dr. Hoganson Client/Server Petri Net Synchronized Client/Server Comput ing Idle WaitingProcess Reply WaitingProcess Request Reply