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Systematic approach to identifying underlying issues in processes systems and operations

Interactive Debug - Online debugging

Enhanced Definition

Interactive debugging, often referred to as online debugging in the mainframe context, is a method of executing a program under the control of a specialized debugging tool, allowing a developer to observe and manipulate its execution in real-time. This enables the identification and resolution of logic errors, data issues, and performance bottlenecks within applications running on z/OS, such as COBOL, PL/I, or C/C++ programs.

Key Characteristics

- Real-time Execution Control: Provides the ability to start, stop, pause, and resume program execution at specific points, offering immediate feedback on program behavior.
- Breakpoints and Watchpoints: Allows setting breakpoints to halt execution at specific lines of code or watchpoints to pause when a variable's value changes, without modifying the source code.
- Variable Inspection and Modification: Enables developers to view the current values of program variables, registers, and memory areas, and often to modify them on the fly to test different scenarios or correct data.
- Step-by-Step Execution: Supports stepping through code line by line, statement by statement, or instruction by instruction, including stepping into or over called subroutines.
- Source Code Integration: Typically displays the executing source code, synchronized with the current execution point, making it easier to follow program flow and understand context.
- Session Management: Debugging sessions can often be saved, replayed, or shared, and can be initiated for various program types, including batch jobs, CICS transactions, or IMS transactions.

Use Cases

- Troubleshooting CICS Transactions: Debugging online CICS applications to diagnose transaction abends, incorrect data updates, or performance issues in real-time within the CICS region.
- Identifying Logic Errors in Batch Programs: Interactively stepping through COBOL or PL/I batch jobs to pinpoint where calculations go wrong, data is processed incorrectly, or loops occur.
- Analyzing Program Abends: Using the debugger to examine the program state, variable values, and execution path immediately prior to an abend (abnormal end) to understand its root cause.
- Validating New Code Functionality: Testing new features or bug fixes in a controlled environment by observing their execution flow and data manipulation before deployment.
- Performance Tuning: Identifying inefficient code sections or excessive resource consumption by monitoring execution times and resource usage during interactive sessions.

Related Concepts

Interactive debugging is a crucial component of the Software Development Life Cycle (SDLC) on z/OS, complementing compiler options like TEST that generate debug information within the load module. It contrasts with batch debugging, which typically relies on dumps or extensive SYSOUT logging for post-mortem analysis. Debugging tools often integrate with transaction managers like CICS or database systems like DB2 and IMS, allowing developers to debug programs interacting with these complex environments. Effective debugging also relies on well-structured JCL to set up the execution environment and allocate necessary resources for the program being debugged.

Best Practices:

Use Appropriate Compiler Options: Always compile programs with the necessary TEST or DEBUG options (e.g., TEST(ALL,SYM,NOEJ,SEP)) to generate the symbolic information required by the interactive debugger.
Isolate Test Environments: Conduct interactive debugging in dedicated test LPARs, CICS regions, or IMS control regions to prevent interference with production systems or other development activities.
Understand Debugger Features: Familiarize yourself with the full capabilities of your chosen debugging tool (e.g., IBM Debug Tool, Xpediter, Abend -AID) to maximize efficiency, including conditional breakpoints and data monitoring.
Set Strategic Breakpoints: