Handoff
In mainframe computing, "handoff" or "transfer of control" refers to the act of one program, module, or system component relinquishing execution control to another. This fundamental mechanism allows different parts of a system to cooperate sequentially or asynchronously, enabling modularity and the invocation of services.
Key Characteristics
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- Explicit Mechanism: Often involves explicit programming constructs such as
CALLstatements in COBOL,LINKorXCTLcommands in CICS, orSVC(Supervisor Call) instructions in assembler. - Context Preservation: The state of the transferring program (e.g., registers, program status word, local storage) is typically saved to facilitate a potential return of control later.
- Parameter Passing: Data is commonly exchanged between the transferring and receiving components via parameter lists, common work areas, or shared data structures.
- Control Flow Management: Essential for managing the sequence of execution within a job step, between CICS transactions, or when invoking z/OS operating system services.
- Synchronous vs. Asynchronous: Can be synchronous (the caller waits for the called component to complete) or asynchronous (the caller continues processing while the called component runs independently, often as a new task).
- Explicit Mechanism: Often involves explicit programming constructs such as
Use Cases
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- Subprogram Invocation: A COBOL application program
CALLing a utility subprogram written in COBOL or assembler to perform a specific, reusable function like data validation or complex calculations. - CICS Program Linking: A CICS transaction program using
EXEC CICS LINK PROGRAM('PROGNAME')to temporarily transfer control to another CICS program, expecting control to return. - Operating System Services: An application program issuing an
SVCto z/OS to request system services such as I/O operations, memory allocation, or task creation. - Job Step Execution: The z/OS Job Entry Subsystem (JES) performing a handoff from one program specified in a
JCL EXEC PGM=statement to the next program in the job stream. - Cross-Memory Services: One z/OS address space using cross-memory services to invoke a routine or pass control to a program residing in a different address space.
- Subprogram Invocation: A COBOL application program
Related Concepts
Handoff is foundational to program modularity and structured programming, allowing complex applications to be broken into manageable, reusable components. It is central to transaction processing environments like CICS, where control is frequently passed between programs handling different stages of a transaction. It also underpins how operating system services are invoked by application programs and how multitasking and multiprogramming are managed by z/OS.
- Clear Interface Definition: Always define clear and consistent interfaces for programs that receive control, specifying expected parameters, their formats, and potential return codes.
- Robust Error Handling: Implement comprehensive error handling and recovery mechanisms in both the transferring and receiving components to gracefully manage unexpected conditions during a handoff.
- Resource Management: Ensure that resources (e.g., files, memory, database locks) acquired by a program are properly released, especially when control is transferred to another program that might not be aware of them.
- Performance Optimization: Minimize unnecessary or excessive handoffs in performance-critical paths, as each transfer of control can introduce overhead related to saving and restoring execution context.
- Security Context Awareness: Be mindful of the security context when transferring control, ensuring the receiving program has appropriate authorizations and that sensitive data is protected during parameter passing.