IDMS Essentials

Data Dictionaries and the DDS

CA-IDMS enables you to build one or more databases using a data dictionary. A data dictionary is itself a CA-IDMS database that contains all the data and system definitions for one or more databases.

A data dictionary is divided logically into areas. The information is organized into entity types, which correspond to the main data processing components, such as elements, records, files, programs, and users. Data dictionaries monitor most aspects of the database environment, from tracking the status of terminals, systems, and users to being a central resource of information about the system and providing security. Some large information systems use multiple dictionaries; for example, a system might have one dictionary for each division of a company.

A database administrator (DBA) manages and maintains the data dictionaries and the entire CA-IDMS system. DBA duties often include programming systems, managing resources, monitoring the system's performance, and overseeing its security. The DBA has a key role in the SAS/ACCESS interface to CA-IDMS, which is explained in more detail in this section.

Within a CA-IDMS data dictionary are the definitions for a database's schema and subschema. A schema describes the contents and structure of a single database, including all of the records and sets that are necessary to define its data elements and data relationships.

A subschema is a subset of a schema that is used by programs at runtime. It consists of all the data elements, records, sets, and areas that are defined in the schema or a subset thereof. It includes database records and can include logical records as well as logical-record paths (defined below). The DBA defines logical records and their paths in the subschema before application programs are coded and executed.

The following figure illustrates the relationships among the data dictionary, schemas, and subschemas.

Data Dictionary, Schemas, and Subschemas

[Data Dictionary, Schemas, and Subschemas]

CA-IDMS provides two operating environments, or modes, for accessing data dictionaries and databases. In the central version, multiple concurrently executing programs access the database(s) through one shared copy of the database management system (DBMS). The central version controls concurrent updating of the database by multiple users in order to maintain database integrity.

In local mode, one program at a time accesses the database through a dedicated copy of the DBMS. You cannot run local mode against a database at the same time that the central version is accessing it.

A Distributed Database System (DDS) distributes data storage and processing functions among several systems. These systems can execute on one or more computers and at one or more sites. Each system is a node in the DDS configuration. A central version specifies which node within the DDS system to access.

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