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Indexed Sequential Access Method (ISAM) is a method for creating, maintaining, and manipulatingcomputer files of data so that records can be retrieved sequentially or randomly by one or more keys. Indexes of key fields are maintained to achieve fast retrieval of required file records inindexed files.IBM originally developed ISAM formainframe computers, but implementations are available for most computer systems.
The termISAM is used for several related concepts:
In an ISAM system, data is organized intorecords which are composed of fixed length fields, originally stored sequentially in key sequence. Secondary set(s) of records, known asindexes, contain pointers to the location of each record, allowing individual records to be retrieved without having to search the entire data set. This differs from the contemporaneousnavigational databases, in which the pointers to other records were stored inside the records themselves. 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. Additional 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.[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. Typically the field being used as the link, theforeign 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 simple to understand and implement, as it primarily consists of direct access to a database file. The trade-off 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. To prevent this, some ISAM implementations[5][6] provide whole-file or individual recordlocking functionality. Locking multiple records runs the risk ofdeadlock unless adeadlock prevention scheme is strictly followed. The problems of locking, and deadlock are typically solved with the addition of aclient–server framework which marshals client requests and maintains ordering. FullACID transaction management systems are provided by some ISAM client–server implementations.[5] These are the basic concepts behind adatabase management system (DBMS), which is a client layer over the underlying data store.
ISAM was replaced at IBM with a methodology calledVSAM (virtual storage access method). Still later, IBM developedSQL/DS and thenDb2 which IBM promotes as their primarydatabase management system. VSAM is the physical access method used in Db2.[citation needed]
TheOpenVMS operating system uses theFiles-11 file system in conjunction with RMS (Record Management Services). RMS provides an additional layer between the application and the files on disk that provides a consistent method of data organization and access across multiple 3GL and 4GL languages. RMS provides four different methods of accessing data; sequential, relative record number access, record file address access, and indexed access.
The indexed access method of reading or writing data only provides the desired outcome if in fact the file is organized as an ISAM file with the appropriate, previously defined keys. Access to data via the previously defined key(s) is extremely fast. Multiple keys, overlapping keys and key compression within the hash tables are supported. A utility to define/redefine keys in existing files is provided. Records can be deleted, although "garbage collection" is done via a separate utility.
IBM engineers designed the ISAM system to use a minimum amount ofcomputer memory. The tradeoff was that theInput/Output channel, control unit, and disk were kept busier. An ISAM file consists of a collection of data records and two or three levels of index. Thetrack index contains the highest key for eachdisk track on thecylinder it indexes. Thecylinder index stores the highest key on a cylinder, and thedisk address of the corresponding track index. An optionalmaster index, usually used only for large files, contains the highest key on a cylinder index track and the disk address of that cylinder index. Once a file is loaded data records are not moved; inserted records are placed into a separateoverflow area. To locate a record by key the indexes on disk are searched by a complexself-modifyingchannel program.[7] This increased the busy time of the channel, control unit, and disk. With increased physical and virtual memory sizes in later systems this was seen as inefficient, andVSAM was developed to alter the tradeoff between memory usage and disk activity.
ISAM's use of self-modifying channel programs later caused difficulties forCP-67 support ofOS/360, since CP-67 copied an entire channel program into fixed memory when the I/O operation was started and translated virtual addresses to real addresses.[8]