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The batch processing facilities of the IMS/VS Data Base System permit users to write application programs in COBOL, PL/I, or System/370 assembler language; to define these programs to the system; and to execute them through batch executions of the system. Application programs may issue calls to the system to retrieve, modify, add, and delete data base data. For programming simplicity, application programs view data bases as sets of hierarchic records, and transformations between this view and the underlying data are carried out automatically. A system log and a set of utility programs are provided to facilitate the recovery of data bases from the effects of incorrect programs, and to facilitate the restarting of long-running programs, following a system outage. IMS/VS was one of the first generalized data base management systems to provide hierarchic data structures. Other early systems providing such structures were the FACT1 system for the Honeywell H800, the Generalized Information System (GIS),2 and to a limited extent the formatted file systems.3 The original motive for this type of structure was to eliminate redundancy inherent in flat files, when the entity being modeled has variably-occurring attributes or subordinate entities. (Engineering documents, a common entity in the early IMS milieu, have this property.) Since then, the hierarchic data structure has become a proper logical view or model of data, i.e., it is a natural way of looking at certain collections of data. IMS is frequently cited as the exemplar of the hierarchic model. The lesson from this experience and other similar experiences seems to be that if a data model is to be successful, it must simultaneously solve the data view problem and the implementation-performance problem.
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