Exit Routine
An exit routine is a user-written program or module that gains control at specific, predefined points within an IBM z/OS operating system component, subsystem (like CICS, DB2, IMS), or application. It allows users to customize or extend the standard behavior of the system without modifying the vendor-supplied code.
Key Characteristics
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- User-written: Developed by the customer or a third-party to meet specific business requirements or operational needs.
- Predefined Entry Points: Invoked by the system at specific, documented "exit points" or "hooks" that are designed for customization.
- Parameter Passing: Typically receives control with parameters passed from the invoking system component, providing context, data, and return codes.
- Specific Functionality: Performs a specialized task, such as data validation, security checks, logging, resource management, or custom processing.
- Language Agnostic (often): Can be written in various languages like
Assembler,COBOL,PL/I, orC/C++, depending on the subsystem and the specific exit type. - Performance Critical: Must be efficient and well-tested as it runs within the system's address space and can significantly impact overall system performance and stability.
Use Cases
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- Security Enhancements: Implementing custom security checks for resource access, such as a
RACFexit to enforce additional authorization rules beyond standard profiles. - Data Transformation/Validation: Modifying or validating data before it's written to a
DB2table, passed to aCICStransaction, or sent over a network. - Auditing and Logging: Recording specific system events, user actions, or transaction details for compliance, chargeback, or troubleshooting purposes.
- Resource Management: Customizing how resources are allocated or managed, for example, a
CICSglobal user exit for transaction routing, storage management, or terminal control. - Error Handling and Recovery: Intercepting and customizing error messages, recovery procedures, or system abends to provide specific actions or notifications.
- Security Enhancements: Implementing custom security checks for resource access, such as a
Related Concepts
Exit routines are fundamental to the extensibility of z/OS and its major subsystems. They work in conjunction with system components like SVCs (Supervisor Calls) and PC routines (Program Call) which provide the mechanisms for transferring control. They are often configured via system parameters (PARMLIB), subsystem definitions (e.g., CICS CSD, DB2 DSNZPARM, IMS PROCLIB), or installation exits, and their proper functioning is critical for the stability and customization of the entire z/OS environment.
- Keep it Simple and Efficient: Exit routines run in critical paths; minimize processing time, resource consumption, and avoid complex logic or unnecessary I/O.
- Thorough Testing: Rigorously test exit routines in all possible scenarios, including error conditions, across various environments (DEV, QA, PROD) before deployment.
- Robust Error Handling: Implement comprehensive error handling and recovery logic within the exit to prevent system crashes and ensure graceful degradation.
- Clear Documentation: Document the purpose, logic, parameters, return codes, and potential side effects of each exit routine for future maintenance and troubleshooting.
- Version Control: Manage exit routine source code and load modules under strict version control, tracking changes and ensuring proper fallback procedures.