Exit

Enhanced Definition

In the mainframe context, an "exit" refers to a predefined point within an IBM operating system (like z/OS), a subsystem (such as CICS, DB2, or IMS), or an application program where control can be temporarily passed to a user-written routine. This mechanism allows for extending or modifying standard system or application behavior without altering the original IBM code.

Key Characteristics

- Predefined Points: Exits are specific, documented locations within IBM-supplied code where the system is designed to relinquish control to a customer-supplied module.
- User-Written Routines: The code executed at an exit point is developed by the customer, often in languages like Assembler, COBOL, or PL/I, to perform custom processing.
- Customization and Extension: They provide a powerful way to customize system functions, add specific business logic, implement enhanced security, or integrate with other applications.
- Control Flow: The system passes control to the exit routine, which performs its designated function and then returns control to the system, often with a return code influencing subsequent system actions.
- Types: Exits can be synchronous (executed inline as part of the normal processing flow) or asynchronous (triggered by events or conditions), and can be global or specific to a particular resource.
- Interface Stability: IBM typically maintains stable interfaces for exit points across system releases, ensuring that user-written exit routines remain compatible, although new exits or interface enhancements may occur.

Use Cases

- Security Enforcement: Implementing custom security checks for resource access, such as validating user credentials for CICS transactions or DB2 database access.
- Data Validation and Manipulation: Modifying or validating data before it is written to a database or file, or after it is retrieved, using exits in IMS or DB2.
- Performance Monitoring and Logging: Capturing detailed performance metrics, logging specific events for auditing, or generating custom reports using System Management Facilities (SMF) or CICS global user exits.
- Workload Management: Influencing job scheduling, resource allocation, or message processing based on custom criteria through JES or MVS message exits.
- System Automation: Triggering automated actions, alerts, or external processes in response to specific system events or console messages.

Related Concepts

Exits are a fundamental mechanism for system customization and extensibility on z/OS, akin to "hooks" in other computing environments. They complement JCL procedures and system parameters by providing dynamic, programmatic control over system behavior, whereas JCL and parameters offer static configuration. Exit routines often interact with system services (e.g., SVCs, macros) and application programs (e.g., COBOL, PL/I, Assembler) by providing a bridge for user-defined logic to integrate directly into the operating system or subsystem flow.

Best Practices:

Thorough Testing: Rigorously test all exit routines in controlled development and test environments, as errors can significantly impact system stability and performance.
Minimal Logic: Keep exit routine logic concise and efficient, as they often execute frequently and in critical system paths, minimizing overhead.
Robust Error Handling: Implement comprehensive error handling and recovery mechanisms within exit routines to prevent system crashes or data corruption.
Clear Documentation: Document the purpose, logic, dependencies, and expected behavior of all custom exit routines for future maintenance, troubleshooting, and knowledge transfer.
Performance Monitoring: Continuously monitor the performance impact of exit routines and optimize them for efficiency, especially those in high-frequency execution paths.
Version Control: Manage exit routine source code and load modules under strict version control, treating them as critical system components.