APC - Asynchronous Procedure Call
An Asynchronous Procedure Call (APC) in the z/OS environment is a mechanism allowing a program or address space to request the execution of a routine in another program or address space without waiting for its completion. It facilitates asynchronous processing and inter-address space communication by queuing a request for later dispatch.
Key Characteristics
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- Asynchronous Execution: The calling program does not wait for the requested routine to complete, allowing it to continue its own processing concurrently.
- Inter-Address Space Communication: Often used to schedule work or pass control between different z/OS address spaces or within complex subsystems like CICS.
- Queued Processing: The request for the APC is typically placed in a queue and processed by the target environment's dispatcher when resources become available.
- Context Switching: The routine invoked by an APC executes in the context (address space, task, or TCB) of the target environment, not the caller.
- Event-Driven: Can be triggered by system events or explicit requests, leading to deferred execution of specific code paths.
Use Cases
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- CICS Asynchronous Processing: CICS uses internal APC-like mechanisms to schedule tasks, handle asynchronous I/O completions, or dispatch user-written exit routines.
- Cross-Memory Services: A program in one address space might use an APC-like mechanism (e.g., via
PCroutines orSVCs) to request a service from another privileged address space. - Workload Management: Scheduling low-priority background tasks or deferred processing that doesn't require immediate completion from the caller's perspective.
- Resource Management: Releasing resources or performing cleanup operations in a different task or address space context after a primary operation has completed.
Related Concepts
APCs are fundamental to understanding asynchronous processing in z/OS, contrasting with synchronous calls like CALL or LINK where control is immediately transferred and the caller waits. They are often implemented using z/OS supervisor services, cross-memory services, or internal subsystem dispatchers (like CICS's dispatcher). APCs enable robust inter-address space communication and efficient resource utilization by decoupling request initiation from execution.
- Minimize Data Transfer: When passing data with an APC, keep the amount of data small or use shared memory techniques (e.g.,
LFAREA,CSA,ECSA) to avoid excessive overhead. - Error Handling: Implement robust error handling in both the caller (for request failure) and the target routine (for execution errors) to ensure system stability.
- Resource Management: Be mindful of the resources (TCBs, storage) consumed by the target routine invoked by the APC, especially in high-volume scenarios.
- Avoid Deadlocks: Design APC interactions carefully to prevent situations where multiple address spaces are waiting for each other, leading to system hangs.
- Performance Monitoring: Monitor the performance of APC-driven routines to identify bottlenecks and ensure timely processing of asynchronous requests.