Exist
Enhanced Definition
In the context of z/OS, "exist" refers to the state of an entity (such as a dataset, program, job, task, or system resource) being present, identifiable, and potentially available for use within the operating system or its subsystems. Its existence is a fundamental prerequisite for any interaction or operation involving that entity.
Key Characteristics
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- Verifiable State: The existence of an entity can often be programmatically verified through system services, utility commands, or API calls (e.g., checking for a dataset's presence before allocation).
- Resource Dependency: Many z/OS operations and applications depend on the prior existence of specific resources (e.g., a COBOL program requires the existence of its load module in a
STEPLIBorJOBLIB). - Dynamic vs. Static: Entities can exist statically (e.g., a pre-allocated
VSAM KSDSdataset) or dynamically (e.g., a CICS transaction task that exists only for the duration of its execution). - Scope of Existence: An entity's existence might be system-wide (e.g., a started task), within a specific address space (e.g., a loaded module), or within a subsystem (e.g., a DB2 table).
Use Cases
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- JCL Conditional Processing: Using JCL
IFstatements to check for the existence of a dataset (e.g.,IF (DSNAME EXISTS) THEN ...) or the successful completion of a previous step (implying the existence of expected output) before proceeding. - Programmatic Resource Checking: A COBOL or REXX program verifying the existence of an input file or a specific member in a
PDS/PDSEbefore attempting to open or access it, to preventabends(e.g.,S0C4,S013). - System Monitoring: Operations teams monitoring the existence and health of critical started tasks (
STCs), CICS regions, or DB2 subsystems to ensure system availability and proper functioning. - Security Authorization:
RACFor other security managers checking for the existence of
- JCL Conditional Processing: Using JCL