Transaction Manager
IMS is also a robust transaction manager (IMS TM, also known as IMS DC) — one of the "big three" classic transaction managers along with CICS and BEA (now Oracle) Tuxedo. A transaction manager interacts with an end user (connected through VTAM or TCP/IP, including 3270 and Web user interfaces) or another application, processes a business function (such as a banking account withdrawal), and maintains state throughout the process, making sure that the system records the business function correctly to a data store. Thus IMS TM is quite like a Web application, operating through a CGI program (for example), to provide an interface to query or update a database. IMS TM typically uses either IMS DB or DB2 as its backend database. When used alone with DB2 the IMS TM component can be purchased without the IMS DB component.
IMS TM uses a messaging and queuing paradigm. An IMS control program receives a transaction entered from a terminal (or Web browser or other application) and then stores the transaction on a message queue (in memory or in a dataset). IMS then invokes its scheduler on the queued transaction to start the business application program in a message processing region. The message processing region retrieves the transaction from the IMS message queue and processes it, reading and updating IMS and/or DB2 databases, assuring proper recording of the transaction. Then, if required, IMS enqueues a response message back onto the IMS message queue. Once the output message is complete and available the IMS control program sends it back to the originating terminal. IMS TM can handle this whole process thousands (or even tens of thousands) of times per second.
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