Tuesday, November 14, 2023

File Access Methods

File Access Methods

1. Sequential Access

A sequential access file emulates magnetic tape operation, and generally supports a few operations:

            read next - read a record and advance the tape to the next position.

            write next - write a record and advance the tape to the next position.

            rewind

            skip n records - May or may not be supported. N may be limited to positive numbers, or                                           may be limited to +/- 1.


2. Direct Access

Jump to any record and read that record. Operations supported include:

o    read n - read record number n.

o    write n - write record number n.

o    jump to record n - could be 0 or the end of file.

o    Query current record - used to return back to this record later.

o    Sequential access can be easily emulated using direct access. The inverse is complicated and inefficient.

3.      Other Access Methods

An indexed access scheme can be easily built on top of a direct access system. Very large files may require a multi-tiered indexing scheme, i.e. indexes of indexes.




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