Copy Book

Enhanced Definition

A copy book is a file containing reusable source code segments, typically used to define common data structures, record layouts, or procedural code. In the mainframe z/OS environment, it ensures data consistency and promotes code reusability across multiple COBOL, PL/I, or Assembler programs by allowing them to include identical definitions at compile time.

Key Characteristics

- Reusability: Provides a centralized definition for data structures or code snippets that can be included in numerous programs, reducing redundant coding.
- Data Consistency: Guarantees that all programs using a specific copy book will process data with the exact same layout and field definitions, preventing data misalignment issues.
- Language-Specific Syntax: Contains syntax specific to the programming language it's intended for (e.g., COBOL PIC clauses, PL/I DECLARE statements, Assembler DS/DC directives).
- Compile-Time Inclusion: The contents of a copy book are physically inserted into the source code by the compiler during the compilation process (e.g., via the COPY statement in COBOL or %INCLUDE in PL/I).
- Centralized Maintenance: Changes to a data structure or shared logic only need to be made in one place (the copy book), requiring recompilation of all dependent programs to apply the updates.
- Storage: Typically stored as members within partitioned datasets (PDS) or partitioned dataset extended (PDSE) libraries, often referenced by DD statements like SYSLIB or COPYLIB in JCL.

Use Cases

- Defining Record Layouts: Standardizing the structure of records for files (sequential, VSAM), databases (DB2, IMS), or messages exchanged between systems.
- Shared Working Storage Areas: Declaring common variables, flags, counters, or tables that need to be accessed consistently by multiple program modules.
- Linkage Section Structures: Specifying the exact format of parameters passed between calling and called programs, ensuring proper data exchange.
- Standard Constants and Error Codes: Centralizing definitions for system-wide constants, status codes, or error messages used across an application suite.
- Common Procedural Code: While less common for large procedural blocks, copy books can contain small, reusable paragraphs or sections of code, especially for common utility functions or error handling routines.

Related Concepts

Copy books are fundamental to structured programming on z/OS, directly interacting with COBOL, PL/I, and Assembler compilers which process their COPY or %INCLUDE statements. They are stored in PDS/PDSE libraries, and their location is typically specified in JCL through DD statements like SYSLIB during the compilation step. They serve as a practical implementation of a data dictionary concept, enforcing consistent data definitions across an enterprise application landscape, and are crucial for maintaining data integrity and simplifying program maintenance.

Best Practices:

Meaningful Naming Conventions: Use clear, descriptive names for copy books and the data items within them to enhance readability and understanding.
Centralized Library Management: Store copy books in well-defined, shared libraries (COPYLIB, SYSLIB) that are accessible to all relevant development and compilation environments.
Version Control: Manage copy books under a robust Source Code Management (SCM) system (e.g., CA Endevor, IBM SCM) to track changes, manage versions, and facilitate rollback if necessary.
Thorough Documentation: Document the purpose, structure, and intended usage of each copy book, including any dependencies or special considerations.
Minimize Redundancy: Strive for the "single source of truth" principle; avoid duplicating data definitions across multiple copy books or within programs.
Impact Analysis: Before modifying a copy book, perform a thorough impact analysis to identify all dependent programs that will require recompilation and testing.