gtpc3m1i | Concepts and Structures |
Data record attributes are necessary to describe data organization in the TPF system.
In general, the physical location of a record is transparent to the application environment. The TPF system manages the following physical forms of storage on which data records can reside:
The TPF system provides the capability for the user installation to use up to four different module device types within a database configuration.
The logical device types, called DEVA, DEVB, DEVC, and DEVD, are assigned to specific physical device types at system generation. The physical characteristics of the various types of devices are stored in the module file status table (MFST) and are used when the file reference of a data record is converted (mapped) into its physical hardware address.
Having access to these characteristics provides the ability to change the physical configuration of a user installation's database for the following reasons:
In the TPF system, the logical size of data records is predefined. The overhead of managing data is made more efficient by only allowing a limited variety of record sizes.
The logical size of a record within the TPF system is always one of three sizes:
In the TPF system, data records (on modules) can be duplicated, which means that there are two copies of a data record on the database. The copies are referred to as the primary record and the backup record, sometimes called the duplicate record or dupe.
The record allocation procedure assigns each copy of a duplicated record to different modules in order to provide an alternate copy of the record in the event of system errors (hardware or software). Duplicate records can also provide parallel paths for finding (reading) a record in a TPF system that is not loosely coupled.
The primary and backup records must be allocated to the same logical device type. Both fixed records and pool records can be duplicated.
By definition, fixed records are maintained by the TPF system forever while pool records are maintained for an interval of time, which is determined by the application environment (in terms of seconds, hours, days, weeks, or months).
The user installation actually makes the final definition between a short time and a longer time for pool records. Generally speaking, a short time is usually thought in terms of seconds, minutes, and hours. Two intervals of intended use for pool records are supported:
Pool records are classified according to the attributes of size, duplication, longevity, and residence (type of device). The basic pool types are:
where x is A, B, C, or D and represents the type of device defined for that pool type at system generation time.
Fixed file records can be classified according to the following sizes:
Do not confuse types of fixed file records with the concept of fixed file record types, which are names of sets of fixed file records.