gtpc3m1j | Concepts and Structures |
Record IDs are used within the TPF system to ensure and validate database integrity. Every record in the system, whether fixed or pool, must be associated with a two-byte record ID.
A record ID is placed in each fixed record when the database is initialized, and in each pool record by application programs as these records are acquired.
This record ID is given as a parameter within a file address reference word (FARW) of the entry control block (ECB) when a record is accessed with a find or file type macro request. Database integrity is ensured and validated because the record ID within a data record is compared with the record ID that is given as a parameter in the FARW when a record is accessed with a find or file macro. If the comparison fails, the access request is not valid.
The record ID attribute table (RIAT) is a system table that holds the information about data organization that is necessary for data management by the TPF system. The RIAT is organized by and accessed by record ID. The RIAT contains information for both fixed records and pool records, such as:
Logging characteristics indicate whether or not a record is to be written to a real-time tape whenever a file type macro is processed for a record of the specified record ID. Logging is another way of ensuring database integrity by keeping alternate copies of important data records.
Exception recording characteristics indicate whether or not the record is to be written to a real-time tape if a file type macro has been processed after the record is captured by the TPF system utility file capture and restore (sometimes referred to as capture/restore), which is used to back up the online module (database).
User exit characteristics permit an application to dynamically modify the data record attributes that are assigned to a record ID at system generation time.
VFA candidacy characteristics specify whether a record is a VFA candidate. If the record is a VFA synchronization candidate, the data is also specified whether it is delay file or immediate file. (VFA is discussed in more detail in Virtual File Access (VFA).)
Module record caching candidacy characteristics specify whether or not a record is a candidate for module record caching. If it is a candidate, the data specifies whether a file type macro request for a record of the specified record ID results in an I/O operation that is a fast write or retentive write. Record caching is discussed in more detail in Retain Module Records in Module Cache Memory.
For a record that is a VFA candidate, lock maintenance characteristics specify whether the lock is held in the record hold table or the hold table associated with an external lock facility (XLF). See Record Hold Table and XLF Lock Table for more information about XLFs.
A record ID must be specified for each pool record type (such as SSTx or LDPx) incorporated in a TPF system. This RIAT item is used to identify the appropriate pool record type when a get file storage macro request is invoked to obtain a pool record. In addition, the RIAT contains the following additional information about pool records: