Information Management System

IMS (Information Management System) is a joint hierarchical database and information management system.

IMS was designed by IBM working with Rockwell and Caterpillar in 1966 for the Apollo program. It was used to track the very large Bill of Materials for the Saturn V.

The first "IMS READY" message was displayed on an IBM 2740 terminal in Downey, California on 14 August 1968. 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 databases:

1. Full Function databases

• Full function, which is basically the same Data Language/1 (DL/I) databases as developed for Apollo. Full function databases can have primary and secondary indexes and are accessed using DL/I calls from your application program.
• Full function databases can have a variety of access methods, although Hierarchical Direct (HDAM) and Hierarchical Indexed Direct (HIDAM) prevail.
• The other formats are Simple Hierarchical Indexed Sequential (SHISAM), Hierarchical Sequential (HSAM) and Hierarchical Indexed Sequential (HISAM).
• Data in full function databases can be stored using VSAM (a native MVS access method) or Overflow Sequential (OSAM), an IMS specific access method that optimizes the channel program for IMS access. OSAM has the advantage that there is special handling in IMS for sequential access of OSAM databases (OSAM Sequential Buffering) which has a performance benefit.

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 its scheduler to start the user's application program in a message processing region. The message processing region retrieves 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.

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

Fast path databases DEDB can only be built on top of VSAM. DLI databases can be built on top of either VSAM or OSAM (with some restrictions depending on database organization) VSAM has a physical addressable limit of 4GB (note: the maximum size of a VSAM dataset has been increased to 128TB in recent versions of z/OS but IMS still limits a VSAM database dataset to 4GB) and OSAM has a physical limit of 8GB. VSAM and OSAM are usually referred to as the Access Method, the IMS "logical" view of the database is referred to as the database "organization" (HDAM, HIDAM, HISAM, ...). Internally the data is linked using 4 byte pointers or addresses in the database datasets (DBDS) the pointers are referred to as RBA's (relative byte address).

External links

• IMS Family - IBM Software (http://www-306.ibm.com/software/data/ims/index.html)
• Description (http://www.cbi.umn.edu/shp/entries/ims.html) from the Software History Dictionary
• History of IMS (http://www.research.ibm.com/journal/sj/374/blackman.html)sv:IMS