ILE COBOL Programmer's Guide


OS/400 System Files

The OS/400 system has four categories of files:

Database files allow information to be permanently stored on the system. A database file is subdivided into groups of records called members. There are two types of database files:

A database physical file can exist on one OS/400 system or on multiple OS/400 systems. If a database physical file exists on more than one OS/400system, it is called a distributed physical file or a distributed file. Since a logical file is based on one or more physical files, if the underlying physical file is distributed, then the logical file is also a distributed file.

To access a distributed file from an ILE COBOL program, you OPEN the distributed file; no other intermediate file is required, and no knowledge is required of the OS/400 systems that have a part of the distributed file.

Contrast this to a Distributed Data Management (DDM) file which identifies the name of a database file that exists on a remote system. In ILE COBOL, to OPEN the remote database file, you actually open the local DDM file Thus, a DDM file combines the characteristics of a device file and a database file. As a device file, the DDM file refers to a remote location name, local location name, device name, mode, and a remote network ID to identify a remote system as the target system. The DDM file appears to the application program as a database file and serves as the access device between the ILE COBOL program and a remote file.

Since a DDM file identifies a remote database file, and since database files can be distributed files, a DDM file can refer to a distributed file.

For more information about DDM files and distributed files, refer to the DB2 Universal Database for AS/400 section of the Database and File Systems category in the iSeries 400 Information Center at this Web site - http://publib.boulder.ibm.com/pubs/html/as400/infocenter.htm.


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