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IMS began as a hierarchical database designed by IBM for Rockwell for the Apollo program. It was used to track the bill of materials for the Saturn V.

IMS is still running thirty five years later and over time has seen some interesting developments as IBM S/360 technology developed into the current z/OS operating system.

There are three basic forms of hierarchical database.

1. Full Function databases

2. IMS also has fast path databases - Data Entry Databases (DEDB) and Main Storage Databases (MSDB). These two types of database don't allow for any indexation, but are optimized for extremely high transaction rates. With modern releases of IMS MSDBs can be replaced by a Virtual Storage Option (VSO) DEDB.

3. And now High Availability Large Database (HALDB); which was introduced with IMS V7.1. This is an extension of IMS full function databases to provide better availability, better handling of extremely large data volumes (and with IMS Version 9 to provide online re-organization).

IMS is also a transaction manager. A transaction manager interacts with an end user (connected through VTAM or TCP/IP) and like a web server running a CGI program provides an interface to query or update IMS or DB2 databases. IMS uses an messaging an queuing paradigm. A transaction entered from a terminal is received by the IMS control program and stored on a message queue (in storage or on a dataset). When a transaction has been queued IMS invokes it's schedular to start the users application program in a message processing region. The message processing region retrives the transaction from the IMS message queue processes it reading and updating IMS and DB2 databases, then if required 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.

If you have ever withdrawn money from an automated teller machine (ATM), there is a fair chance that transaction will have run in an IMS system.

IMS can also stand for Irritable Male Syndrome, the male equivalent of PMS.