What is ISAM?

[Anuj Dhawan]

ISAM (pronounced "eye sam") stands for Indexed Sequential Access Method. It is a technique for storing and accessing data in a file on secondary storage, for example a disk. ISAM is not a stand-alone program; it is a library of functions that implement an algorithm.

ISAM is a file management system developed at IBM that allows records to be accessed either sequentially (in the order they were entered) or randomly (with an index). Each index defines a different ordering of the records. An employee database may have several indexes, based on the information being sought. For example, a name index may order employees alphabetically by last name, while a department index may order employees by their department. A key is specified in each index. For an alphabetical index of employee names, the last name field would be the key.

ISAM was developed prior to VSAM (Virtual Storage Access Method) and relational databases. In the past several years, the term ISAM has grown from its original meaning (an access method dependent on specific hardware) to a more general usage. ISAM now refers to almost any way of indexing a file of data records.