Temporary versus Permanent Files

For every disk-based or memory-based file you define in the APPX dictionary, you must designate whether the file is temporary or permanent. This distinction affects how and/or when a file is created, opened, and scratched (see Table 2-1-5).

• Temporary files provide working space that is unique to a task and automatically managed by APPX during processing. At its maximum, the existence of a temporary file is limited to a single APPX session. If, based upon a file’s specifications, it is not shared between processes, the existence of a file may be considerably less than the length of a session. Temporary files are database-independent; there is only one copy of a file for all databases that access it. Temporary files are never shared between users.
• Permanent files, on the other hand, are always shareable between processes, and are managed by APPX’s database management. APPX never automatically scratches permanent files. If a file is disk-based, each database has its own copy of it; permanent, disk-based files are database-dependent. They are always shared between processes, regardless of file specifications.

Permanent files are created by APPX’s database management. See the System Management Manual for more information.

Temporary files are created when a process that references the file begins. By using a CREATE statement, you exercise explicit control over the time when a temporary file is created. For example, you can define a CREATE statement in a Start of Process event point to force APPX to create the file at the beginning of the process.

APPX never automatically opens temporary or permanent files at the beginning of a process. To enhance perceived performance (and quite often actual performance), APPX distributes file openings throughout a process. A file is physically opened when APPX first needs to access the file. By using an OPEN statement, you exercise explicit control over the time when a file is opened. For example, you can define an OPEN statement in a Start of Process event point to force APPX to open a file at the beginning of the process.

Any process can be invoked as DETACHED or related, or as a SUBPROCESS. When working with temporary files, it is important to know how the file type and the process invocation are related and interact. See Chapter 1-5 for information about Interprocess Communication in APPX.

If APPX does not know for sure that a file exists, it attempts to open the file during the execution of a process. File Does Not Exist error messages can therefore occur at any time during process execution, unless all files are explicitly opened at the beginning of the process. The obvious trade-off is decreased performance.