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Indexed Sequential Access Method (ISAM) is a method for creating, maintaining, and manipulating
computer file A computer file is a System resource, resource for recording Data (computing), data on a Computer data storage, computer storage device, primarily identified by its filename. Just as words can be written on paper, so too can data be written to a ...
s 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 in
indexed file An indexed file is a computer file with an index that allows easy random access to any record given its file key. The key must be such that it uniquely identifies a record. If more than one index is present the other ones are called ''alternate ...
s.
IBM International Business Machines Corporation (using the trademark IBM), nicknamed Big Blue, is an American Multinational corporation, multinational technology company headquartered in Armonk, New York, and present in over 175 countries. It is ...
originally developed ISAM for
mainframe computer A mainframe computer, informally called a mainframe or big iron, is a computer used primarily by large organizations for critical applications like bulk data processing for tasks such as censuses, industry and consumer statistics, enterprise ...
s, but implementations are available for most computer systems. The term ''ISAM'' is used for several related concepts: *The IBM ISAM product and the
algorithm In mathematics and computer science, an algorithm () is a finite sequence of Rigour#Mathematics, mathematically rigorous instructions, typically used to solve a class of specific Computational problem, problems or to perform a computation. Algo ...
it employs. *A
database In computing, a database is an organized collection of data or a type of data store based on the use of a database management system (DBMS), the software that interacts with end users, applications, and the database itself to capture and a ...
system where an application developer directly uses an
application programming interface An application programming interface (API) is a connection between computers or between computer programs. It is a type of software Interface (computing), interface, offering a service to other pieces of software. A document or standard that des ...
to search indexes in order to locate records in data files. In contrast, a
relational database A relational database (RDB) is a database based on the relational model of data, as proposed by E. F. Codd in 1970. A Relational Database Management System (RDBMS) is a type of database management system that stores data in a structured for ...
uses a
query optimizer Query optimization is a feature of many relational database management systems and other databases such as NoSQL and graph databases. The query optimizer attempts to determine the most efficient way to execute a given query by considering the pos ...
which automatically selects indexes. *An indexing algorithm that allows both sequential and keyed access to data. Most databases use some variation of the
B-tree In computer science, a B-tree is a self-balancing tree data structure that maintains sorted data and allows searches, sequential access, insertions, and deletions in logarithmic time. The B-tree generalizes the binary search tree, allowing fo ...
for this purpose, although the original IBM ISAM and VSAM implementations did not do so. *Most generally, any index for a database. Indexes are used by almost all databases.


Organization

In an ISAM system, data is organized into records which are composed of fixed length fields, originally stored sequentially in key sequence. Secondary set(s) of records, known as ''indexes'', 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 contemporaneous navigational 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.Ramakrishnan Raghu, Gehrke Johannes - Database Management Systems, McGraw-Hill Higher Education (2000), 2nd edition (en) page 252
Relational database A relational database (RDB) is a database based on the relational model of data, as proposed by E. F. Codd in 1970. A Relational Database Management System (RDBMS) is a type of database management system that stores data in a structured for ...
s 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 A foreign key is a set of attributes in a table that refers to the primary key of another table, linking these two tables. In the context of relational databases, a foreign key is subject to an inclusion dependency constraint that the tuples ...
'', 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 provide whole-file or individual record locking functionality. Locking multiple records runs the risk of deadlock unless a deadlock prevention scheme is strictly followed. The problems of locking, and deadlock are typically solved with the addition of a client–server framework which marshals client requests and maintains ordering. Full
ACID An acid is a molecule or ion capable of either donating a proton (i.e. Hydron, hydrogen cation, H+), known as a Brønsted–Lowry acid–base theory, Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis ...
transaction management systems are provided by some ISAM client–server implementations. These are the basic concepts behind a
database management system In computing, a database is an organized collection of data or a type of data store based on the use of a database management system (DBMS), the software that interacts with end users, applications, and the database itself to capture and an ...
(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). Still later, IBM developed SQL/DS and then Db2 which IBM promotes as their primary
database management system In computing, a database is an organized collection of data or a type of data store based on the use of a database management system (DBMS), the software that interacts with end users, applications, and the database itself to capture and an ...
. VSAM is the physical access method used in Db2.


OpenVMS

The
OpenVMS OpenVMS, often referred to as just VMS, is a multi-user, multiprocessing and virtual memory-based operating system. It is designed to support time-sharing, batch processing, transaction processing and workstation applications. Customers using Op ...
operating system uses the
Files-11 Files-11 is the file system used in the RSX-11 and OpenVMS operating systems from Digital Equipment Corporation. It supports record-oriented I/O, remote network access, and file versioning. The original ODS-1 layer is a flat file system; th ...
file system in conjunction with RMS (
Record Management Services Record Management Services (RMS) are procedures in the VMS, RSTS/E, RT-11 and RSX-11M operating systems that programs may call to process files and records within files. Its file formats and procedures are similar to of those in some IBM acc ...
). 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.


Design considerations

IBM engineers designed the ISAM system to use a minimum amount of
computer memory Computer memory stores information, such as data and programs, for immediate use in the computer. The term ''memory'' is often synonymous with the terms ''RAM,'' ''main memory,'' or ''primary storage.'' Archaic synonyms for main memory include ...
. The tradeoff was that the Input/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. The ''track index'' contains the highest key for each disk track on the
cylinder A cylinder () has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base. A cylinder may also be defined as an infinite ...
it indexes. The ''cylinder index'' stores the highest key on a cylinder, and the disk address of the corresponding track index. An optional ''master 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 separate ''overflow area''. To locate a record by key the indexes on disk are searched by a complex self-modifying channel program. 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, and VSAM was developed to alter the tradeoff between memory usage and disk activity. ISAM's use of self-modifying channel programs later caused difficulties for
CP-67 CP-67 is a hypervisor, or Virtual Machine Monitor, from IBM for its System/360 Model 67 computer. CP-67 is the control program portion of CP/CMS, a virtual machine operating system developed by IBM's Cambridge Scientific Center in Cambridge, ...
support of
OS/360 OS/360, officially known as IBM System/360 Operating System, is a discontinued batch processing operating system developed by IBM for their then-new System/360 mainframe computer, announced in 1964; it was influenced by the earlier IBSYS/IBJOB a ...
, 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.


ISAM-style implementations

*
Advantage Database Server Sap is a fluid transported in the xylem cells (vessel elements or tracheids) or phloem sieve tube elements of a plant. These cells transport water and nutrients throughout the plant. Sap is distinct from latex, resin, or cell sap; it is a sep ...
database manager *
Berkeley DB Berkeley DB (BDB) is an embedded database software library for key/value data, historically significant in open-source software. Berkeley DB is written in C with API bindings for many other programming languages. BDB stores arbitrary key/data ...
* Btrieve *FairCom DB *C-ISAM * DataFlex proprietary database *
dBase dBase (also stylized dBASE) was one of the first database management systems for microcomputers and the most successful in its day. The dBase system included the core database engine, a query system, a Form (programming), forms engine, and a pr ...
and related products
Clipper A clipper was a type of mid-19th-century merchant sailing vessel, designed for speed. The term was also retrospectively applied to the Baltimore clipper, which originated in the late 18th century. Clippers were generally narrow for their len ...
and
Foxpro FoxPro is a text-based (computing), text-based Procedural programming, procedurally oriented programming language and database management system (DBMS), and it is also an object-oriented programming language, originally published by Fox Softwar ...
*
Digital Equipment Corporation Digital Equipment Corporation (DEC ), using the trademark Digital, was a major American company in the computer industry from the 1960s to the 1990s. The company was co-founded by Ken Olsen and Harlan Anderson in 1957. Olsen was president until ...
Record Management Services Record Management Services (RMS) are procedures in the VMS, RSTS/E, RT-11 and RSX-11M operating systems that programs may call to process files and records within files. Its file formats and procedures are similar to of those in some IBM acc ...
*
Enscribe Enscribe is the native hierarchical database in the commercial HP NonStop (Tandem) servers. It is designed for fault tolerance and scalability and is currently offered by Hewlett Packard Enterprise. The product was originally developed by Ta ...
is the HP Tandem structured file access method *
Extensible Storage Engine Extensible Storage Engine (ESE), also known as JET Blue, is an ISAM (indexed sequential access method) data storage technology from Microsoft. ESE is the core of Microsoft Exchange Server, Active Directory, and Windows Search. It is also used b ...
* Access Database Engine (ACE and formerly JET) used by
Microsoft Access Microsoft Access is a database management system (DBMS) from Microsoft that combines the relational database, relational Access Database Engine (ACE) with a graphical user interface and software-development tools. It is a member of the Microsof ...
*
MySQL MySQL () is an Open-source software, open-source relational database management system (RDBMS). Its name is a combination of "My", the name of co-founder Michael Widenius's daughter My, and "SQL", the acronym for Structured Query Language. A rel ...
implements and extends ISAM as MyISAM *
Paradox A paradox is a logically self-contradictory statement or a statement that runs contrary to one's expectation. It is a statement that, despite apparently valid reasoning from true or apparently true premises, leads to a seemingly self-contradictor ...
*pblIsam GPL implementation written in C * Superbase database family * dbm DBM and Flat File databases working in tandem


See also

* Sequential access memory (SAM) *
Virtual storage access method Virtual Storage Access Method (VSAM) is an IBM direct-access storage device (DASD) file storage access method, first used in the OS/VS1, OS/VS2 Release 1 (SVS) and Release 2 (MVS) operating systems, later used throughout the Multiple Virtual S ...
(VSAM) *
Flat file A flat-file database is a database stored in a file called a flat file. Records follow a uniform format, and there are no structures for indexing or recognizing relationships between records. The file is simple. A flat file can be a plain t ...
*
NoSQL NoSQL (originally meaning "Not only SQL" or "non-relational") refers to a type of database design that stores and retrieves data differently from the traditional table-based structure of relational databases. Unlike relational databases, which ...
* dbm


References

{{DEFAULTSORT:Isam Computer file systems Computer file formats IBM software