Data Description - Metadata

Enhanced Definition

In the mainframe context, metadata, often referred to as data description, is data about data. It describes the characteristics, structure, format, and context of data stored or processed on z/OS systems, enabling applications and users to understand and interact with the data correctly. This includes information like data type, length, format, relationships, and access methods.

Key Characteristics

- Self-describing: Metadata makes data self-describing, meaning its structure and meaning can be understood without external documentation, often embedded within the data definition itself (e.g., COBOL DATA DIVISION).
- Machine-readable: It is typically stored in a machine-readable format, allowing compilers, database management systems (DBMS), and other software to interpret and validate data automatically.
- Persistent: Metadata is usually stored persistently alongside the data it describes, often in data dictionaries, catalogs (like the z/OS Catalog), or within the data structures themselves (e.g., VSAM clusters).
- Enforces data integrity: By defining data types, lengths, and constraints, metadata helps enforce data integrity and consistency across applications.
- Facilitates data sharing: Standardized metadata definitions are crucial for sharing data between different applications, programming languages (COBOL, PL/I, Assembler), and systems on the mainframe.
- Evolutionary: Metadata can evolve as data requirements change, necessitating careful management of schema changes and versioning.

Use Cases

- COBOL DATA DIVISION: Defining record layouts, fields, data types (PIC clauses), and group items for files and working storage within a COBOL program.
- DB2 Catalog: Storing schema definitions for tables, columns, indexes, views, and stored procedures, which DB2 uses to manage and optimize data access and query execution.
- IMS DBDs and PSBs: Describing the physical structure of IMS databases (DBD - Database Description) and the application's logical view of that data (PSB - Program Specification Block).
- VSAM Cluster Definitions: Defining the attributes of a VSAM KSDS, ESDS, or RRDS, including record size, key length, key offset, and control interval size, stored in the z/OS Catalog.
- JCL DD statements: Providing metadata about the dataset itself (e.g., DSORG, RECFM, LRECL, BLKSIZE, DISP) to the z/OS operating system for proper allocation and access.

Related Concepts

Metadata is fundamental to almost every aspect of mainframe data processing. It is the backbone of data management systems like DB2, IMS, and VSAM, providing the necessary structural information for these systems to store, retrieve, and manipulate data efficiently. Compilers, particularly for COBOL and PL/I, rely heavily on metadata defined in DATA DIVISION (often via COPYLIBS) to generate correct object code for data manipulation. The z/OS Catalog itself is a repository of metadata for datasets, volumes, and storage groups, crucial for system-wide resource management and dataset location.

Best Practices:

Standardize data definitions: Establish consistent naming conventions, data types, and lengths across applications and systems using common COPYLIBS to improve maintainability and reduce errors.
Maintain a data dictionary/catalog: Utilize tools or native system catalogs (like DB2 Catalog, z/OS Catalog) to centralize and manage metadata, ensuring a single source of truth for data structures.
Version control metadata: Treat metadata definitions (e.g., COBOL copybooks, DB2 DDL scripts) as source code and manage them under version control to track changes and facilitate rollback.
Document metadata thoroughly: Supplement technical metadata with business definitions, ownership, and usage guidelines to enhance understanding for both technical and business users.
Automate metadata generation: Where possible, use tools to generate metadata (e.g., from database schemas to COBOL copybooks) to reduce manual effort and ensure consistency across different components.