Record-oriented I/O: Record Management Services
Record Management Services is the structured I/O layer of the VMS operating system. RMS provides comprehensive program support for managing structured files, such as record-based and indexed database files. The VMS filesystem, in conjunction with RMS, extends files access past simple byte-streams and allows OS-level support for a variety of rich files types. Each file in the VMS filesystem may be thought of as a database, containing a series of records, each of which has one of more individual fields. A text file, for example, is a list of records (lines) separated by a newline character. RMS is an example of a record-oriented filesystem.
There are four record formats defined by RMS:
- Fixed length - all records in the file have the same length.
- Variable length - records vary in length, and every record is prefixed by a count byte giving its length.
- Variable record length with fixed-length control - records vary in length, but are preceded by a fixed-length control block.
- Stream - record vary in length, and every record is separated from the next one by a termination character. A text file is an example of a stream-format file using line feed or carriage return to separate records.
There are four record access methods, or methods to retrieve extant records from files:
- Sequential Access - starting with a particular records, subsequent records are retrieved in order until the end of the file.
- Relative Record Number Access - records are retrieved via a record number relative to the beginning of the file.
- Record File Address Access - records are retrieved directly by their location in the file (RFA, or Record File Address).
- Indexed Access - records are retrieved via a key, in a form of key-value mapping.
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