Currency

Database Position

Enhanced Definition

In the context of mainframe databases and file systems, **currency** refers to the current logical position within a dataset, database segment, or record set after a read or update operation. It dictates the starting point for subsequent operations, ensuring sequential or relative processing can continue from the last accessed point.

Key Characteristics

- Implicit Management: Often managed implicitly by the database system (e.g., IMS, VSAM) or file system, tracking the last record accessed by a program.
- Program-Specific: Currency is typically maintained on a per-program or per-thread basis, meaning different applications or concurrent instances can have their own independent current positions.
- Impacts Sequential Access: Crucial for sequential processing, where the next read operation (GET NEXT in IMS, READ NEXT in VSAM, or READ in COBOL for sequential files) retrieves the record immediately following the current position.
- Affected by I/O Operations: Any successful read, write, or update operation on a record or segment type can establish or change the current position for that specific data structure.
- Hierarchical Context (IMS): In IMS DB, currency is maintained at multiple levels of the hierarchy, with a current position for each segment type along the path to the last accessed segment.

Use Cases

- Processing Sequential Files: A COBOL program reading a VSAM KSDS or ESDS sequentially will rely on currency to move from one record to the next without explicitly specifying a key for each read.
- Iterating Through IMS Segments: An IMS application uses currency to navigate through a database, for example, reading all children segments under a specific parent segment using GET NEXT calls.
- Batch Reporting: Generating reports that require processing all records in a file or all segments of a certain type, where each record is read in sequence from the current position.
- Updating Records in Place: After locating a specific record (e.g., by key), subsequent REWRITE operations in COBOL or REPL calls in IMS implicitly operate on the record at the current position.

Related Concepts

Currency is fundamental to sequential access methods and navigational databases like IMS DB. It is closely related to file pointers or record pointers that indicate the physical or logical location of the last accessed data. While modern relational databases (like DB2) primarily use cursors for explicit position management in result sets, the underlying concept of maintaining a current position for sequential processing is analogous. In COBOL, the FILE STATUS and AT END conditions are often used in conjunction with sequential reads that rely on currency.

Best Practices:

Understand Implicit Behavior: Be aware of how different I/O operations (e.g., READ, WRITE, REWRITE, START in COBOL; GET UNIQUE, GET NEXT in IMS) affect currency for various file organizations or database types.
Reset Currency When Needed: For non-sequential access or when starting a new scan, explicitly reset currency (e.g., using a START statement in COBOL for VSAM, or a GU call in IMS to establish a new path).
Handle End-of-File/End-of-Data: Always check for end-of-file conditions (AT END in COBOL, GE status code in IMS) when processing sequentially, as currency becomes undefined or points past the last record.
Manage Concurrency Carefully: In multi-threaded or multi-user environments, ensure that currency is managed independently for each task to prevent interference and ensure data integrity.
Optimize Navigation: For IMS, strategically use GET UNIQUE (GU) to establish a new position efficiently, and GET NEXT (GN) for sequential retrieval, understanding how each impacts currency at different hierarchical levels.