Database management system: Difference between revisions

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	==Functionality provided==
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	Features commonly offered by database management systems include:
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	;Backup and replication : Copies of attributes need to be made regularly in case primary disks or other equipment fails. A periodic copy of attributes may also be created for a distant organization that cannot readily access the original. DBMS usually provide utilities to facilitate the process of extracting and disseminating attribute sets. When data is replicated between database servers, so that the information remains consistent throughout the database system and users cannot tell or even know which server in the DBMS they are using, the system is said to exhibit replication transparency.

	;Rule enforcement : Often one wants to apply rules to attributes so that the attributes are clean and reliable. For example, we may have a rule that says each car can have only one engine associated with it (identified by Engine Number). If somebody tries to associate a second engine with a given car, we want the DBMS to deny such a request and display an error message. However, with changes in the model specification such as, in this example, hybrid gas-electric cars, rules may need to change. Ideally such rules should be able to be added and removed as needed without significant data layout redesign.

	; Computation : Common computations requested on attributes are counting, summing, averaging, sorting, grouping, cross-referencing, and so on. Rather than have each computer application implement these from scratch, they can rely on the DBMS to supply such calculations.

	; Change and access logging : This describes who accessed which attributes, what was changed, and when it was changed. Logging services allow for a ] later by keeping a record of access occurrences and changes. Sometimes application-level code is used to record changes rather than leaving this to the database.

	; Development and monitoring support : A DBMS typically intends to provide convenient environment to develop and later maintain an application built around its respective database type. A DBMS either provides such tools, or allows integration with such external tools. Examples for tools relate to database design, application programming, application program maintenance, database performance analysis and monitoring, database configuration monitoring, DBMS hardware configuration (a DBMS and related database may span computers, networks, and storage units) and related database mapping (especially for a distributed DBMS), storage allocation and database layout monitoring, storage migration, etc.

	===Components===

	DBMS ] specifies its components (including descriptions of their functions) and their interfaces. DBMS architecture is distinct from database architecture. The following are major DBMS components:

	:An external interface can be either a '']'' (e.g., typically for a database administrator), or an '']'' (API) used for communication between an application program and the DBMS.
	*'''Database language engines''' (or '''processors''') - Most operations upon databases are performed through expression in Database languages (see above). Languages exist for data definition, data manipulation and queries (e.g., SQL), as well as for specifying various aspects of security, and more. Language expressions are fed into a DBMS through proper interfaces. A language engine processes the language expressions (by a compiler or language interpreter) to extract the intended database operations from the expression in a way that they can be executed by the DBMS.

	*''']''' - Performs ] on every query to choose for it the most efficient '']'' (a partial order (tree) of operations) to be executed to compute the query result.
	*''']''' - Performs the received database operations on the database objects, typically at their higher-level representation.
	*'''Storage engine''' - translates the operations to low-level operations on the storage ]s. In some references the Storage engine is viewed as part of the database engine.
	*'''DBMS management and operation component''' - Comprises many components that deal with all the DBMS management and operational aspects like performance monitoring and tuning, backup and restore, recovery from failure, security management and monitoring, change management, database storage allocation and database storage layout monitoring, etc.
	* '''Application generation subsystem''' contains facilities to help users develop transaction-intensive applications. It usually requires that the user perform a detailed series of tasks to process a transaction. It facilitates easy-to-use data entry screens, programming languages, and interfaces.

	* ]s
	* Graphics component for producing graphs and charts.

	==Database storage==
	{{Main\|Computer data storage\|Database engine}}

	Database storage is the container of the physical materialization of a database. It comprises the ''internal'' (physical) ''level'' in the database architecture. It also contains all the information needed (e.g., ], "data about the data", and internal ]s) to reconstruct the ''conceptual level'' and ''external level'' from the internal level when needed. Putting data into permanent storage is generally the responsibility of the ] a.k.a. "storage engine". Though typically accessed by a DBMS through the underlying ] (and often utilizing the operating systems' ]s as intermediates for storage layout), storage properties and configuration setting are extremely important for the efficient operation of the DBMS, and thus are closely maintained by database administrators. A DBMS, while in operation, always has its database residing in several types of storage (e.g., memory and external storage). The database data and the additional needed information, possibly in very large amounts, are coded into bits. Data typically reside in the storage in structures that look completely different from the way the data look in the conceptual and external levels, but in ways that attempt to optimize (the best possible) these levels' reconstruction when needed by users and programs, as well as for computing additional types of needed information from the data (e.g., when querying the database).

	Some DBMS support specifying which ] was used to store data, so multiple encodings can be used in the same database.

	Various low-level ] are used by the storage engine to serialize the data model so it can be written to the medium of choice. Techniques such as ] may be used to improve performance. Conventional storage is row-oriented, but there are also ] and ]s.

	====Database materialized views====
	{{Main\|Materialized view}}

	Often storage redundancy is employed to increase performance. A common example is storing '']s'', which consist of frequently needed ''external views'' or query results. Storing such views saves the expensive computing of them each time they are needed. The downsides of materialized views are the overhead incurred when updating them to keep them synchronized with their original updated database data, and the cost of storage redundancy.

	====Database and database object replication====
	{{Main\|Database replication}}

	Occasionally a database employs storage redundancy by database objects replication (with one or more copies) to increase data availability (both to improve performance of simultaneous multiple end-user accesses to a same database object, and to provide resiliency in a case of partial failure of a distributed database). Updates of a replicated object need to be synchronized across the object copies. In many cases the entire database is replicated.

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