IBM designed IMS with Rockwell and Caterpillar starting in 1966 for the Apollo program. IMS's challenge was to inventory the very large Bill of Materials for the Saturn V moon rocket and Apollo space vehicle.
The first "IMS READY" message appeared on an IBM 2740 terminal in Downey, California, on 14 August 1968. IMS is still going strong 40 years later and, over time, has seen some interesting developments as IBM System/360 technology evolved into the current z/OS and System z9 and z10 technologies. For example, IMS supports the Java programming language, JDBC, XML, and, since late 2005, Web services (though JDBC, XML or Web Services require licensing additional software from IBM). IMS Connect comes with version 9 and provides a TCP/IP interface to Message Processing Programs running in IMS Message Processing Regions.
Vern Watts is IMS's chief architect. Watts joined IBM in the late 1950s and, as of this writing in mid-2005, still works (at least "unofficially") at IBM's Silicon Valley development labs. He has continuously worked on IMS since the 1960s.
The IMS Database component stores data using a hierarchial model that is quite different from IBM's later released relational database, DB2. In IMS the hierarchical model is implemented using blocks of data known as segments. Each segment can contain several pieces of data referred to as fields. For example a customer database may have a root segment (or the segment at the top of the hierarchy) with fields such as phone, name, and age. Child segments may be added underneath another segment, for instance one order segment under each customer segment representing each order a customer has placed with a company. Likewise each order segment may have many item children segments for each item on the order. Unlike other databases you do not need to define all of the data in a segment to IMS. A segment may be defined with a size of 40 bytes but only define one field that is six bytes long as a key field that you can use to find the segment when performing queries. IMS will retrieve and save all 40 bytes as directed by a program but may not understand (or care) what the other bytes represent. Often in practice all data in a segment may map to a COBOL copybook. Besides DL/I query usage a field may be defined to IMS so that the data can be hidden from certain applications for security reasons. The database component of IMS is can be purchased alone, without the transaction manager component, and used by systems such as CICS.
There are three basic forms of IMS hierarchical databases:
Fast path DEDBs can only be built atop VSAM. DL/I databases can be built atop either VSAM or OSAM, with some restrictions depending on database organization. Although the maximum size of a z/OS VSAM dataset increased to 128 TB a few years ago, IMS still limits a VSAM dataset to 4 GB (and OSAM to 8 GB). This "limitation" simply means that IMS customers will use multiple datasets for large amounts of data. VSAM and OSAM are usually referred to as the access methods, and the IMS "logical" view of the database is referred to as the database "organization" (HDAM, HIDAM, HISAM, etc.) Internally the data are linked using 4-byte pointers or addresses. In the database datasets (DBDSs) the pointers are referred to as RBAs (relative byte addresses).
Collectively the database-related IMS capabilities are often called IMS DB. IMS DB has grown and evolved over nearly four decades to support myriad business needs.
IMS is also a robust transaction manager (IMS TM, also known as IMS DC) — one of the "big three" along with CICS and BEA 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.
Wang Laboratories Inc. of Lowell, Mass., has expanded its suite of software tools that provide document image services to mainframe applications.
May 20, 1991; Wang Laboratories Inc. of Lowell, Mass, has expanded its suite of software tools that provide document image services to...