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.
I have to disagree with your post. ISAM is not a technique. It was an access method that used a record key to index the data. It is not and has not been supported for a long time, and the last traces of it [in the form of IEBISAM] has been dropped from support.
ISAM doesn’t exist and it doesn’t mean anything in today’s environment.
ISAM did not allow for multiple indexes. That is a capability for VSAM files. Also, there is no comparison between access methods and data bases.
In any keyed sequential file, the data is always recorded sequentially and the index serves to allow for random retrievals with a minimal number of I/O operations. Even ISAM files used OVERFLOW areas to accommodate record insertions. However, ISAM was a cumbersome access method and was replaced by VSAM KSDS file organizations, even to the extent that there was a VSAM/ISAM application interface so that programs did not require changes to be able to take advantage of VSAM.
Alas – I have to agree with your post, Gerhard. In this post I’ve mixed two different concepts – one the ISAM, used n modern databases and the other ISAM from IBM. I’ll have to change the original post to avoid confusion.
thanks for keeping me on the track.