Header

Enhanced Definition

A header, in the mainframe and z/OS context, refers to a block of data or metadata that precedes the main body of information (e.g., a file, message, record, or program module). It serves as "leading information," providing essential descriptive, control, or contextual details necessary for the proper processing, interpretation, or routing of the subsequent data.

Key Characteristics

- Positional: Always located at the beginning of the data unit it describes or controls.
- Informative: Contains metadata such as length fields, timestamps, identifiers, flags, addresses, or control codes.
- Contextual: Provides the necessary context for the system or application to understand and process the following data.
- Structured: Often follows a predefined format, which might be fixed-length or variable, and is frequently defined using data structures like DSECTs in assembler.
- Varied Purpose: Its specific content and purpose vary widely depending on what it precedes (e.g., a file header differs from a message header).

Use Cases

- File Processing: A sequential or VSAM file might have a header record containing dataset attributes, creation date, record format, or block size information.
- Message Queuing (e.g., IBM MQ): Messages typically include a header (like the MQMD - Message Descriptor) that specifies message type, priority, reply queue, correlation ID, and other routing or processing instructions.
- Network Communication (e.g., VTAM, TCP/IP): Data packets or segments exchanged over a network protocol always include headers with source/destination addresses, sequence numbers, control flags, and protocol-specific information.
- Program Modules: Load modules or object modules often begin with a header containing information about the module's entry points, size, external references, attributes, and linkage editor control information.
- Report Generation: Printed reports commonly feature headers on each page or at the beginning of the report, providing titles, dates, page numbers, and column headings.

Related Concepts

Headers are a fundamental form of metadata, providing "data about data" that enables systems to manage and interpret information efficiently. They are closely related to data structures and control blocks, as their layout is often precisely defined (e.g., using an assembler DSECT) to allow programs to access specific fields. In communication protocols (like SNA or TCP/IP), headers are critical for framing messages, routing, and ensuring reliable data transfer. Many z/OS system components, such as JCL (JOB statement parameters) or control blocks (e.g., TCB, ASCB), can be considered forms of leading information or headers for tasks, resources, or system states.

Best Practices:

Clear Documentation: Thoroughly document the format and purpose of all fields within custom application headers to ensure maintainability and correct interpretation.
Validation: Implement robust validation logic to check header content upon receipt or processing, ensuring data integrity and preventing errors from malformed or unexpected values.
Efficiency: Design headers to be as concise as possible, especially in high-volume transaction or network environments, to minimize overhead and improve performance.
Standardization: Utilize industry-standard header formats (e.g., MQMD, IP headers) whenever possible to promote interoperability and reduce development effort.
Version Control: Include a version number within custom headers to allow for future format changes without breaking compatibility with older applications.