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 run time. 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.
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.