Callback
In the mainframe context, a **callback** refers to a function or routine that is passed as an argument to another program or system service, to be invoked by that program or service at a later time, typically when a specific event occurs or a certain condition is met. It allows a calling program to provide a custom routine for the called program to execute, extending its functionality without modifying its core logic.
Key Characteristics
-
- Dynamic Invocation: The callback routine is not directly called by the initial program but is invoked dynamically by another system component or program when a predefined event or state change occurs.
- Extensibility Mechanism: It serves as a powerful mechanism for extending the capabilities of system services, middleware (like CICS or IMS), or application frameworks by allowing users to inject custom logic.
- Function Pointer/Address: Often implemented by passing the entry point address (a function pointer in languages like C/C++ or an entry address in assembler/COBOL) of the user-defined routine to the system or service.
- Contextual Execution: The callback routine executes within the context (e.g., address space, task, transaction) of the program or system that invokes it, which implies considerations for resource access and error handling.
- Synchronous or Asynchronous: Callbacks can be invoked synchronously (the calling program waits for the callback to complete) or asynchronously (the calling program continues processing while the callback runs independently).
Use Cases
-
- CICS User Exits: Global User Exits (GLUEs) or Program-Level Exits (PLEXs) in CICS allow user-written programs to gain control at specific points in CICS processing (e.g., before/after program execution, terminal I/O, transaction initiation) to perform custom validation, logging, or security checks.
- IMS User Exits: Similar to CICS, IMS provides various user exits (e.g., for database calls, message processing, security) where custom routines can be invoked to modify behavior or gather statistics.
- z/OS System Exits: Operating system services often provide exit points (e.g., SMF exits for system activity recording, JES exits for job processing) where user-written routines can intercept and process system events.
- Application Programming Interfaces (APIs): Many APIs, especially those dealing with asynchronous operations (e.g., network communication, event queues), accept callback routines to be executed upon completion of an operation or receipt of data.
- Event-Driven Frameworks: Custom application frameworks might use callbacks to notify registered components when specific business events occur, enabling modular and decoupled application design.
Related Concepts
Callbacks are a fundamental concept closely related to User Exits, which are specific, predefined points in system or application code where user-written routines (callbacks) can be invoked. They are integral to Event-Driven Programming, where actions are triggered by events, and callbacks define the response to those events. Many APIs on z/OS leverage callbacks to provide extensibility or handle asynchronous operations. The mechanism of passing and invoking callbacks relies heavily on Program Linkage conventions and the ability to manage Function Pointers or entry addresses within the z/OS memory model.