Handler - Processing routine

Enhanced Definition

A handler, in the context of z/OS, is a specialized processing routine or program module designed to intercept, manage, and respond to specific events, conditions, or requests within an application or the operating system environment. Its primary purpose is to provide a controlled and predefined response to anticipated or exceptional situations, ensuring system stability or customized behavior.

Key Characteristics

- Event-driven: Typically invoked in response to a specific trigger, such as an error, an I/O completion, a transaction request, a system interrupt, or a predefined system event.
- Specific Scope: Designed to perform a focused set of actions related to the event it handles, often involving error recovery, resource management, custom processing, or data transformation.
- Registered or Linked: Handlers are often registered with the operating system, a subsystem (e.g., CICS, IMS), or an application framework, allowing them to gain control when their associated event occurs.
- Language Agnostic: Can be written in various mainframe languages like COBOL, PL/I, Assembler, or C, depending on the specific context and the interface provided by the invoking system.
- Control Transfer: Receives control from the system or calling program, performs its designated function, and then typically returns control, often with status information or modified data.
- Predefined Interface: Adheres to a specific interface (e.g., parameter list, register conventions) dictated by the system or component that invokes it.

Use Cases

- Error Handling and Abend Recovery: An ESTAE (Extended Specified Task Abend Exit) or FRR (Functional Recovery Routine) is a type of handler that gains control during a program abend to attempt recovery, log diagnostics, or perform graceful termination.
- CICS Transaction Processing: A CICS application program acts as a handler for specific transaction IDs, processing incoming requests from terminals or other systems, interacting with databases (e.g., DB2, VSAM), and sending responses.
- IMS Message Processing: An IMS Message Processing Program (MPP) is a handler for messages placed on an IMS message queue, processing the message content and potentially generating a response message.
- User Exit Routines: Custom code modules (handlers) that are invoked by IBM products (e.g., DFSORT, JES2, DB2, SMF) at predefined points to extend or modify their standard behavior, such as data validation or security checks.
- Asynchronous Event Processing: Routines that handle asynchronous I/O completions, timer expirations, or external interrupts, often found in system-level programming or device drivers.

Related Concepts

Handlers are fundamental to the event -driven nature of many mainframe systems and applications. They are closely related to exit routines, which are specific types of handlers provided by IBM products for customization. They often interact with system services for error recovery (ESTAE, SETFRR), resource managers (like CICS, IMS, DB2) for transaction processing, and JCL which defines the execution environment for programs that might contain or invoke handlers. The concept underpins robust exception handling, system resilience, and the ability to customize standard product behavior.

Best Practices:

Keep Handlers Concise: Design handlers to be as small and efficient as possible, focusing only on the necessary actions for the event they manage to minimize overhead and improve performance.
Thorough Error Logging: Implement comprehensive logging within handlers, especially error handlers, to capture diagnostic information for problem determination and post-mortem analysis.
Avoid Recursive Calls: Be extremely cautious about actions within a handler that could inadvertently trigger the same handler or lead to infinite loops, potentially causing system instability or abends.
Test All Paths: Rigorously test handlers under all anticipated conditions, including normal operation, various error scenarios, and edge cases, to ensure reliability and correctness.
Document Interfaces: Clearly document the entry conditions, parameters, expected actions, and exit conditions for all handlers, especially user exits, to facilitate maintenance and future enhancements.