Intercept
In mainframe computing, an "intercept" refers to the programmatic capture and processing of a system event, program execution flow, or data access before it reaches its intended target or completes its standard operation. This allows for custom logic to be executed, data to be modified, or control to be diverted by an external routine.
Key Characteristics
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- Programmatic Control: Involves writing specific code (often in Assembler, COBOL, or PL/I) to gain control at a predefined point within the operating system or an application.
- Event-Driven: Triggered by specific events such as I/O operations, program calls, system calls (SVCs), abends, console messages, or data access requests.
- System Hooks: Often implemented using system exits (e.g., SMF exits, JES exits, CICS exits), SVC intercepts, or program call intercepts, which are predefined points for user customization.
- Transparency: Ideally, the intercept mechanism should be transparent to the intercepted application or system component, meaning the original logic eventually resumes or is replaced seamlessly.
- Performance Impact: Poorly designed or inefficient intercepts can introduce significant overhead, impacting system performance due to the additional processing path.
- Privilege Level: Many system-level intercepts require authorized program status (e.g.,
APF-authorized, supervisor state, or key 0-7) to function correctly and safely.
Use Cases
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- Security Monitoring & Enforcement: Intercepting data access (e.g., VSAM, DB2) to enforce granular security rules, log access attempts, or mask sensitive data before it's presented to an unauthorized user.
- Performance Monitoring & Tuning: Capturing system calls or I/O requests to gather metrics, analyze bottlenecks, or inject tracing logic without modifying the original application code.
- Data Transformation/Validation: Intercepting data as it's written to or read from a file/database to perform real-time validation, encryption/decryption, or format conversion.
- Error Handling & Recovery: Intercepting program abends or specific error conditions to provide custom recovery routines, log detailed diagnostic information, or prevent system crashes.
- System Automation: Intercepting console messages or system events (e.g., job completion, resource contention) to trigger automated responses or alerts through an automation product.
Related Concepts
Intercepts are closely related to System Exits (user exits, global exits), which are predefined points in z/OS or subsystem code where users can "hook" their own routines. They often leverage Supervisor Calls (SVCs) to gain control at a low level or modify system behavior. Intercepts are a fundamental technique for System Programming and Middleware Development, enabling customization and extension of core z/OS functions without modifying IBM-supplied code, often used in conjunction with APF Authorization for privileged operations.
- Minimize Overhead: Keep intercept routines as lean and efficient as possible to avoid degrading system performance, especially for frequently triggered events.
- Thorough Testing: Rigorously test intercepts in various scenarios, including error conditions and high-volume environments, as they operate at a low level and can destabilize the system if flawed.
- Documentation: Clearly document the purpose, implementation details, and potential side effects of all intercepts for future maintenance, troubleshooting, and audit purposes.
- Controlled Deployment: Implement intercepts in a controlled manner, often requiring IPLs or subsystem restarts, and ensure proper fallback mechanisms are in place.
- Security Considerations: Ensure intercept routines themselves are secure, properly authorized, and do not introduce new vulnerabilities, especially when handling sensitive data or system control.