The IMS/DC (Data Communications) system provides users an online "real time" access to information in IMS databases. MPP transaction programs are NOT submitted with OS JCL. The On-line IMS Control Region automatically schedules the MPP program necessary to process a transaction.
IMS/TM, formerly IMS/DC (IMS/Data Communications), runs each transaction in its own address space and allows for more precise tuning than CICS, which runs all transactions in a region. IMS/TM is also used to access DB2 databases, and Java applications can access IMS databases.
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.
IBM's Hierarchical DBMS and Transaction Manager (IMS DC).