Discriminator

Enhanced Definition

A discriminator, in the context of mainframe data processing, is a specific field or value within a data record, message, or control block that serves to identify its type, format, or the specific interpretation required for the rest of the data structure. It acts as a key to differentiate between various possible layouts or meanings that might share the same physical storage. A discriminator, in the context of mainframe data processing, is a specific field or value within a data record, segment, or structure that indicates the type, format, or specific variant of the data contained within that same entity. It allows application programs to interpret the subsequent data correctly by identifying its specific layout or purpose.

Key Characteristics

- Type Indicator: Explicitly defines the nature or category of the data structure it belongs to (e.g., transaction type, record format, segment type).
- Positional Significance: Often located at a fixed offset (e.g., at the beginning) within a record or structure for quick identification by processing programs.
- Conditional Interpretation: Its value dictates which specific data layout or processing logic should be applied to the remainder of the data structure.
- Data Structure Flexibility: Enables a single file, database segment, or storage area to contain multiple different record types or formats.
- Programmatic Usage: Heavily utilized in COBOL programs, for instance, with REDEFINES clauses, to process varying data structures efficiently.

Use Cases

- Processing Variable Record Types: In COBOL, reading a sequential file where different records have distinct layouts (e.g., a customer master file containing both CUSTOMER-HEADER and CUSTOMER-DETAIL records, distinguished by a RECORD-TYPE field).
- Interpreting System Logs (SMF/SYSLOG): Analyzing System Management Facilities (SMF) records where a record type field (e.g., SMF30RTY) identifies the specific event or resource usage being reported.
- Handling Polymorphic Data Structures: In IMS, a field within a segment might indicate its specific subtype or format, guiding application logic on how to interpret the rest of the segment's data.
- Dynamic Array Sizing (COBOL ODO): A field whose value determines the number of occurrences in a COBOL OCCURS DEPENDING ON clause, effectively discriminating the array's actual size.
- Message Queue Processing: A message header field in a CICS or MQ message indicating the message format or the specific