ISAM

ISAM (an acronym for indexed sequential access method) is a method for creating, maintaining, and manipulating indexes of key-fields extracted from random data file records to achieve fast retrieval of required file records. IBM developed ISAM for mainframe computers. Today the term is used for several related concepts:

In an ISAM system, data is organized into records which are composed of fixed length fields. Records are stored sequentially, originally to speed access on a tape system. A secondary set of hash tables known as indexes contain "pointers" into the tables, allowing individual records to be retrieved without having to search the entire data set. This is a departure from the contemporaneous navigational databases, in which the pointers to other data were stored inside the records themselves. The key improvement in ISAM is that the indexes are small and can be searched quickly, thereby allowing the database to access only the records it needs. Additionally modifications to the data do not require changes to other data, only the table and indexes in question.

When an ISAM file is created, index nodes are fixed, and their pointers do not change during inserts and deletes that occur later (only content of leaf nodes change afterwards). As a consequence of this, if inserts to some leaf node exceed the node's capacity, new records are stored in overflow chains. 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.

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. Typically the field being used as the link, the foreign key, will be indexed for quick lookup. While this is slower than simply storing the pointer to the related data directly in the records, it also means that changes to the physical layout of the data do not require any updating of the pointers—the entry will still be valid.

ISAM is very simple to understand and implement, as it primarily consists of direct, sequential access to a database file. It is also very inexpensive. The tradeoff is that each client machine must manage its own connection to each file it accesses. This, in turn, leads to the possibility of conflicting inserts into those files, leading to an inconsistent database state. This is typically solved with the addition of a client-server framework which marshals client requests and maintains ordering. This is the basic concept behind a database management system (DBMS), which is a client layer over the underlying data store.

...
Wikipedia