Installation Exit

Enhanced Definition

An Installation Exit, often referred to as a "site exit" or "user exit," is a specific point within an IBM mainframe operating system (like z/OS) or a major subsystem (such as CICS, DB2, IMS, or JES) where a customer can insert their own custom-written code. These exits allow an organization to modify or extend the standard behavior of IBM-supplied software to meet unique site-specific requirements, without altering the original IBM code.

Key Characteristics

- User-Written Routines: Installation exits are typically assembler, COBOL, PL/I, or C programs developed by the customer's system programmers.
- Predefined Invocation Points: IBM software is designed with specific, documented points where control is passed to a user-supplied routine, often with parameters containing system or application context.
- Customization and Extension: They enable sites to implement custom security checks, logging, data manipulation, resource management, or modify default processing logic.
- System Integration: Exits run within the context of the IBM product, often with full access to system resources and control blocks, making them powerful but also potentially disruptive if not carefully coded.
- Managed by SMP/E: Installation exits and their associated modules are typically managed and installed using SMP/E (System Modification Program/Extended), ensuring proper linkage and version control within the z/OS environment.
- Impact on Upgrades: Exits must be reviewed and potentially updated with each new release or maintenance level of the IBM product they customize, as internal interfaces might change.

Use Cases

- Enhanced Security: Implementing additional security checks beyond standard RACF, ACF2, or Top Secret rules, such as restricting access to certain datasets or commands based on time of day or specific user attributes.
- Auditing and Logging: Capturing detailed information about specific system events, such as dataset allocations, program executions, or resource accesses, and writing them to a custom log for compliance or analysis.
- Data Transformation: Modifying data streams or records before they are written to disk or sent over a network, or after they are read, for purposes like encryption, compression, or format conversion.
- Resource Management: Controlling the allocation or deallocation of system resources (e.g., JES spool space, CICS transactions, DB2 connections) based on site-defined policies.
- Custom Message Handling: Intercepting and modifying system messages, suppressing unwanted messages, or routing specific messages to different consoles or alerting systems.

Related Concepts

Installation exits are a fundamental aspect of z/OS system programming and site customization, allowing organizations to tailor IBM products. They interact closely with IBM software products (z/OS, CICS, DB2, IMS, JES) by extending their functionality. Unlike JCL or REXX scripts which operate at a job or command level, exits provide deeper, programmatic control within the core logic of the operating system or subsystem. Their management is often intertwined with SMP/E for maintenance and system libraries like SYS1.LINKLIB for execution.

Best Practices:

Thorough Testing: Always develop and rigorously test installation exits in a non-production environment before deploying them to production, as errors can lead to system instability or outages.
Clear Documentation: Maintain comprehensive documentation for each exit, detailing its purpose, logic, invocation point, parameters, and any dependencies or known issues.
Minimalistic Design: Code exits to do only what is absolutely necessary. Avoid complex logic or unnecessary processing within an exit to minimize performance impact and potential for errors.
Robust Error Handling: Include extensive error checking and recovery mechanisms within the exit code to prevent abends or loops that could affect the entire system or subsystem.
Performance Considerations: Be mindful of the performance implications, especially for exits that are frequently invoked. Optimize code for speed and efficiency to avoid introducing system bottlenecks.
Version Control: Manage exit source code and load modules under a strict version control system to track changes, facilitate rollbacks, and ensure consistency across environments.