Data Structure

Enhanced Definition

In the mainframe context, a **data structure** defines the organization and format of data elements, typically within a record, file, or memory area, to facilitate efficient storage, retrieval, and processing by programs. It specifies the sequence, length, data type, and relationships of individual fields.

Key Characteristics

- Record Layouts: Often defined as a record layout or segment layout, specifying the exact byte-level organization of fields within a logical record.
- Copybooks: Frequently defined using copybooks (e.g., COBOL COPY members, PL/I INCLUDE files) to ensure consistency and reusability across multiple programs and applications.
- Fixed vs. Variable Length: Can be fixed-length, where all instances have the same size, or variable-length, where the size can change based on content (e.g., OCCURS DEPENDING ON in COBOL).
- Hierarchical Organization: Supports hierarchical organization using group items (e.g., COBOL 01 to 49 levels) to logically group related fields into a parent-child relationship.
- Data Types and Picture Clauses: Fields are defined with specific data types (e.g., character, packed decimal, binary) often using picture clauses (e.g., PIC X(10), PIC S9(5) COMP-3) to specify format and storage.
- Memory vs. Disk Representation: A data structure can describe data in memory (e.g., WORKING-STORAGE SECTION) or on disk (e.g., FILE SECTION record descriptions for datasets).

Use Cases

- File I/O: Defining the format of records read from or written to sequential files, VSAM files, or other datasets by batch COBOL or JCL programs.
- Database Schema: Specifying the structure of segments in an IMS database or the layout of rows and columns in a DB2 table.
- Program-to-Program Communication: Defining the layout of data passed between programs, such as a CICS COMMAREA, parameters in a CALL statement, or data passed via MQ messages.
- Control Blocks: Describing the internal organization of system control blocks used by z/OS and its subsystems (e.g., TCB, ASCB, DCB) for system management and resource allocation.
- Application Working Storage: Organizing temporary data, counters, flags, and intermediate results within a program's WORKING-STORAGE SECTION for processing.

Related Concepts

Data structures are fundamental to almost all mainframe programming and system operations. They are intrinsically linked to copybooks, which provide a standardized way to define and share these structures across applications. They form the basis of record layouts for files (sequential, VSAM) and define the schema for databases like DB2 and IMS. When programs interact, data structures dictate the format of program linkage parameters and CICS COMMAREAs, ensuring data integrity and interoperability.

Best Practices:

Standardize with Copybooks: Always define common data structures in copybooks and include them in all relevant programs to ensure consistency, reduce errors, and ease maintenance.
Thorough Documentation: Document the purpose, fields, data types, and any special considerations (e.g., REDEFINES, OCCURS DEPENDING ON) for each data structure to aid understanding and future modifications.
Align for Performance: Use appropriate alignment (e.g., SYNCHRONIZED in COBOL) for binary fields to optimize access and prevent performance penalties on some hardware architectures.
Plan for Extensibility: Design data structures with future growth in mind, potentially by adding reserved fields or using variable-length segments where appropriate to minimize impact