The key improvement in ISAM is that the indexes are small and can be searched quickly, possibly entirely in memory, thereby allowing the database to access only the records it needs.
If there are many more inserts than deletions from a table, these overflow chains can gradually become very large, and this affects the time required for retrieval of a record.
[4] Relational databases can easily be built on an ISAM framework with the addition of logic to maintain the validity of the links between the tables.
ISAM is simple to understand and implement, as it primarily consists of direct access to a database file.
To prevent this, some ISAM implementations[5][6] provide whole-file or individual record locking functionality.
The problems of locking, and deadlock are typically solved with the addition of a client–server framework which marshals client requests and maintains ordering.
[5] These are the basic concepts behind a database management system (DBMS), which is a client layer over the underlying data store.
ISAM was replaced at IBM with a methodology called VSAM (virtual storage access method).
To locate a record by key the indexes on disk are searched by a complex self-modifying channel program.