Data Language Interface: Difference between revisions

Revision as of 04:12, 6 April 2023

Revision as of 04:13, 6 April 2023

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In any full-function IMS database, the smallest element that can be retrieved is a ''segment''. Each segment is made up of ''fields'', one of which, typically, will be a key field. The segments are arranged hierarchically in the database, the highest level segment type being a root segment. 255 different segment types, on up to 15 levels, are allowed in any database. A database ''record'' consists of a specific root segment and all its dependent child segments{{snd}} there is no limit to the number of segments in a record, or to the number of records in a database (apart from physical limitations of storage space).

The structure of any database is presented to the application program as a ''PCB'' (Program Communication Block), and this is used as one of the [[parameterParameter (computer programming)|parameters]]s passed to the stub. Other types of PCB are used to send and receive [[transaction processing]] messages, access and write to the user’s [[computer display|VDU]] screen, and to print reports etc.

When accessing a database segment, the application program also uses an ''SSA'' (Segment Search Argument) as a parameter, to specify the segment or segments that it needs. This would typically contain the segment type required and the contents of any key fields.