Flat File
A flat file on z/OS is a simple, non-relational data set where records are typically stored in a sequential manner without complex internal structures or explicit relationships defined within the file itself. It represents data as a single table of information, often with fixed-length or variable-length records, and lacks the inherent schema, indexing, and referential integrity features found in database management systems.
Key Characteristics
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- Simple Structure: Data is organized as a sequence of records, where each record typically has a predefined format (e.g., fields at specific byte offsets). There are no pointers or complex linkages within the file to relate records.
- Non-Relational: Unlike relational databases (like DB2), flat files do not enforce relationships between different data sets or even between records within the same data set. All relationships are managed programmatically or through external metadata.
- Sequential or Direct Access: While most commonly processed sequentially (e.g.,
PS- Physical Sequential data sets), flat files can also be organized for direct access (e.g.,DA- Direct Access data sets, though less common for general "flat files" which usually implies sequential). VSAM ESDS is also a form of sequential flat file. - External Metadata: The definition of the record layout (e.g., field names, data types, lengths) is typically external to the file itself, often defined in a COBOL
COPYBOOK, PL/IINCLUDE, or documented separately. - JCL Defined: The physical characteristics of a flat file (e.g., record format
RECFM, record lengthLRECL, block sizeBLKSIZE) are specified in JCLDDstatements usingDCBparameters.
Use Cases
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- Batch Processing Input/Output: Serving as primary input for batch programs (e.g., transaction files, master files) or as output for reports and intermediate processing results.
- Data Exchange: Transferring data between different mainframe applications, or between mainframe and distributed systems (e.g., using FTP or Connect:Direct after conversion).
- Reporting: Generating detailed or summary reports where data is extracted from databases or other files and formatted into a human-readable flat file.
- Temporary Storage: Storing intermediate results during multi-step batch jobs, which are then used as input for subsequent steps.
- Archiving and Backup: Storing historical data or backups of database tables in a simple, easily recoverable format.
Related Concepts
Flat files are fundamental to z/OS batch processing, heavily relying on JCL DD statements to define their characteristics and allocate storage. COBOL programs frequently use FILE SECTION and FD entries with COPYBOOKs to define the record layouts for reading and writing flat files. They contrast sharply with DB2 relational tables or IMS hierarchical databases, which provide built-in data integrity, indexing, and query capabilities. Utilities like SORT/MERGE are often used to reorder or combine data from multiple flat files, while IDCAMS can define and manage VSAM data sets, some of which (like ESDS) function similarly to sequential flat files.
**Best