Descriptor

Enhanced Definition

In the mainframe context, a descriptor is a structured piece of information that defines the attributes, characteristics, and properties of a specific resource, data element, or system component. It provides the operating system (z/OS) or an application with the necessary details to locate, access, process, or manage the described entity. In the mainframe context, a descriptor is a structured set of metadata or attributes that defines the characteristics, format, or behavior of a specific entity or "object." These objects can range from data elements and datasets to system resources like programs, transactions, or database structures, providing the system or application with the necessary information to interact with or process them correctly.

Key Characteristics

- Structured Format: Descriptors typically adhere to a predefined syntax or structure, such as JCL parameters, COBOL data description clauses, or CICS Resource Definition Online (RDO) attributes.
- Metadata: They function as metadata, providing "data about data" or "data about resources" rather than the actual content of the data or the resource itself.
- System/Application Interpretation: The operating system, subsystems (like CICS, DB2, IMS), or compilers (e.g., COBOL) interpret these descriptors to effectively manage resources or process data.
- Resource Identification and Allocation: They specify how resources (e.g., datasets, programs, terminals) are identified, allocated, and utilized within the system.
- Static or Dynamic: Descriptors can be defined statically (e.g., JCL at job submission, CICS RDO definitions) or, in some cases, dynamically generated or modified by system components during execution.

Use Cases

- JCL DD Statement: A DD (Data Definition) statement serves as a descriptor for a dataset, specifying critical attributes like the dataset name (DSN), disposition (DISP), space allocation (SPACE), and device type (UNIT) for a particular job step.
- COBOL Data Description Entry: In COBOL, PICTURE clauses, USAGE clauses, and level numbers (01, 05, 77) are descriptors that define the format, data type, and hierarchical structure of data items within records or working storage sections.
- CICS Resource Definition (RDO): CICS utilizes RDO definitions to describe various resources such as programs, transactions, files (VSAM, DB2), and terminals, enabling CICS to manage their lifecycle, security, and access.
- DB2 Catalog Tables: The DB2 catalog stores descriptors for database objects like tables, indexes, views, and stored procedures, detailing their structure, column definitions, access paths, and security authorizations.

Related Concepts

Descriptors are fundamental to how z/OS and its subsystems manage resources and data. They are intrinsically linked to control blocks, which are internal system data structures often built from or containing descriptor information. For instance, the parameters in a JCL DD statement are parsed by z/OS to construct a **Data Control Block (