Invoke
In the mainframe context, "invoke" refers to the act of initiating the execution of a program, subroutine, function, or service from another program or system component. It signifies transferring control to the called entity, often with parameters, to perform a specific task or set of operations.
Key Characteristics
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- Control Transfer: The invoking entity temporarily relinquishes control to the invoked entity, which then begins its execution.
- Parameter Passing: Data can be passed from the invoker to the invoked entity (and sometimes back) using predefined mechanisms such as the
LINKAGE SECTIONin COBOL, registers, or parameter lists. - Return Mechanism: Upon completion, the invoked entity typically returns control to the exact point of invocation in the calling program, often with a return code indicating success or failure.
- Dynamic vs. Static: Invocation can be static (resolved at link-edit time, e.g., a
CALLstatement to a subroutine within the same load module) or dynamic (resolved at run-time, e.g.,CALL identifierin COBOL,EXEC CICS LINK). - Context Preservation: The state of the invoking environment (e.g., registers, program status word, working storage) is generally preserved and restored upon the invoked entity's return.
Use Cases
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- Subroutine Calls in COBOL/PL/I: A COBOL program
CALLing anotherPROGRAM-IDorENTRYpoint to perform a specific calculation, data validation, or I/O operation. - CICS Program Linking: An online CICS transaction using
EXEC CICS LINK PROGRAM('PROGA')to transfer control to another CICS program, maintaining the same logical unit of work and passing data via theCOMMAREA. - DB2 Stored Procedure Execution: An application program
CALLing aDB2 STORED PROCEDUREto encapsulate complex database logic, improve performance by reducing network traffic, and enhance security. - JCL Procedure Invocation: A JCL job step
EXEC PGM=MYPROGto execute a specific program orEXEC PROC=MYPROCto invoke a cataloged or in-stream procedure containing a sequence of job steps. - System Service Calls: An application program invoking z/OS system services (e.g.,
SVCcalls,CALLs to system routines) for tasks like memory management, security checks, or advanced I/O operations.
- Subroutine Calls in COBOL/PL/I: A COBOL program
Related Concepts
Invocation is a cornerstone of modular programming and system architecture on z/OS. It directly relates to load modules, which are the executable units containing programs or subroutines that can be invoked. Concepts like the LINKAGE SECTION in COBOL, DFHEIBLK in CICS, and parameter lists are crucial for defining the interface and passing data during an invocation. It also ties into program management and library management (PDS/PDSE), as invoked programs must be accessible in the system's search order.
- Clear Interfaces: Define clear, stable, and well-documented interfaces (parameters, return codes, expected side effects) for invoked programs/subroutines to ensure maintainability and reduce coupling.
- Robust Error Handling: Implement comprehensive error handling in both the invoking and invoked entities, including checking return codes and handling exceptional conditions gracefully.
- Performance Optimization: Be mindful of the overhead associated with frequent invocations, especially across different address spaces or system components. Consider grouping related functions to reduce invocation frequency.
- Reentrancy and Thread Safety: For frequently invoked modules in shared environments like CICS or IMS, ensure they are
reentrantorthread-safeto prevent data corruption and maximize concurrency. - Security Considerations: When invoking programs, especially those that perform sensitive operations, ensure proper authorization checks are in place and that data passed between modules is handled securely.