Different Types of Access Methods

Different Types of Access Methods:

There are several ways to access information in a file. Some systems provide only one access method for files. Some systems provide different access methods. Selection of the right access method for a particular application is a major design problem.

1. Sequential Access:

Information in the file is processed in order, one record after the other. This is by far the most common mode of access of files. For example, computer editors usually access files in this fashion.

A read operation reads the next portion of the file and automatically advances the file pointer. Similarly, a write operation appends to. the end of the file and advances to the end of the newly written material i.e. the new end of file. Such a file can be reset to the beginning. On some systems, a program may be able to skip forward or backward n records for some integer n. This scheme is known as sequential access. Sequential access is based on a tape model of a file.

2 Direct Access:

Direct access is based on a disk model of a file. For direct access, the file is viewed as a numbered sequence of block or records. A direct-access file allows arbitrary blocks to be read or written. After block 18 has been read, block 57 could be next and then block 3. There are no restrictions on the order of reading and writing for a direct access file. Direct access files are of great use for intermediate access to large amounts of information.

The file operations must be modified to include the block number as a parameter. It works like "read n", where n is the block number rather than "read next". Similarly, it writes with "write n" rather that "write next".

An alternative "approach retains "read next" and "write next". It adds an operation "position file to n" where n is the block number. Then we would issue the command "position to n" and then "read next".

All operating systems support both sequential and direct access for files. Some systems allow only sequential file access. Others allow only direct access. Some systems require that a file should be defined as sequential or direct when it is created. Such a file can be accessed only in a manner defined at the time of its declaration.

3. Other Access Methods:

Other access methods can be built on top of a direct-access method. These additional methods generally involve the construction of an index for a file. The index contains pointers to the various blocks. To find an entry in the file, the index is searched first and the pointer is then used to access the file directly to find the desired entry.

With a large file, the index itself may become too large to be kept in memory. One solution is to create an index for the index file. The primary index file would contain pointers to secondary index files that would point to the actual data items.

For example, IBM's indexed sequential access method (ISAM) uses a small master index that points to disk blocks of a secondary index. The secondary index blocks point to the actual file blocks. The file is kept sorted on a defined key. We first make a binary search of the master index to find a particular item. It provides the block number of the secondary index. This block is read in. Binary search is used again to find the block containing the desired record. Finally, this block is searched sequentially. In this way, any record can be located from its key by at most direct access reads.