Definition - Specification or description

Enhanced Definition

In the context of IBM mainframe systems and z/OS, a "definition" refers to the precise, structured description or specification of a resource, data structure, process, or system component. These definitions provide the operating system, compilers, subsystems (like CICS, DB2, IMS), or applications with the necessary metadata and parameters to correctly allocate, manage, or process the described entity. It's how components are formally specified for system interpretation and execution.

Key Characteristics

- Structured Syntax: Definitions typically adhere to a specific, often rigid, syntax (e.g., JCL statements, COBOL data descriptions, RDO commands) that must be strictly followed for correct interpretation.
- Resource Allocation and Description: They are fundamental for describing and allocating system resources such as datasets, memory, I/O devices, programs, and transaction types.
- System Interpretation: Definitions are parsed and acted upon by various system components—the z/OS Job Entry Subsystem (JES), compilers, link-editors, or subsystem managers—to configure the environment or execute tasks.
- Metadata Provision: They provide critical metadata, detailing attributes like data type, length, access methods, storage location, security profiles, and relationships to other entities.
- Persistence: Many definitions (e.g., dataset definitions in catalogs, CICS RDO, DB2 DDL) are stored persistently within system catalogs, libraries, or control blocks for ongoing use.

Use Cases

- Job Control Language (JCL): DD (Data Definition) statements define datasets, their attributes (e.g., DSN, DISP, DCB), and their relationship to a program's I/O operations.
- COBOL Data Division: The DATA DIVISION defines the structure and attributes of data used by a COBOL program, including FILE SECTION (for files) and WORKING-STORAGE SECTION (for program variables).
- CICS Resource Definition Online (RDO): Used to define CICS resources such as programs, transactions, files, terminals, and connections, specifying their behavior and attributes within the CICS region.
- DB2 Data Definition Language (DDL): SQL CREATE statements (e.g., CREATE TABLE, CREATE INDEX, CREATE VIEW) define the structure and properties of database objects within a DB2 subsystem.
- IMS Database and Program Specification Blocks (DBD/PSB): These define the physical structure of IMS databases (DBD) and the logical view of data that application programs can access (PSB).

Related Concepts

Definitions are foundational to almost every aspect of mainframe computing. They are inextricably linked to JCL for job execution, providing the operating system with the blueprint for resource allocation. In COBOL, data definitions are crucial for compiling and executing programs, ensuring data integrity and correct processing. For CICS, DB2, and IMS, robust resource definitions are essential for the subsystems to manage transactions, access data, and interact with applications effectively. They underpin system programming by configuring the z/OS environment, its components, and its interaction with hardware.

Best Practices:

Standardization and Naming Conventions: Implement clear, consistent naming conventions for all defined resources (datasets, programs, transactions) to improve readability and maintainability.
Modularity and Reusability: Design definitions to be modular and reusable where possible, especially for common resources, often leveraging symbolic parameters in JCL or shared copybooks in COBOL.
Version Control: Manage all definition source code (JCL, COBOL, DDL, RDO scripts) under a robust source code management system to track changes, facilitate rollbacks, and support team collaboration.
Thorough Documentation and Comments: Include comprehensive comments within definition source code and external documentation to explain purpose, dependencies, and critical parameters.
Validation and Testing: Rigorously validate and test all new or modified definitions in development and test environments before deployment to production to prevent system outages or application errors.