database management system (DBMS)

A Database is an integrated collection of logically related records or files consolidated into a common pool that provides data for one or more multiple uses. One way of classifying databases involves the type of content, for example: bibliographic, full-text, numeric, image. Other classification methods start from examining database models or database architectures: see below. Software organizes the data in a database according to a database model. As of 2009 the relational model occurs most commonly. Other models such as the hierarchical model and the network model use a more explicit representation of relationships.

A database management system (DBMS) consists of software that organizes the storage of data. A DBMS controls the creation, maintenance, and use of the database storage structures of social organizations and of their users. It allows organizations to place control of organization wide database development in the hands of Database Administrators (DBAs) and other specialists. In large systems, a DBMS allows users and other software to store and retrieve data in a structured way.

Database management systems are usually categorized according to the database model that they support, such as the network, relational or object model. The model tends to determine the query languages that are available to access the database. One commonly used query language for the relational database is SQL, although SQL syntax and function can vary from one DBMS to another. A common query language for the object database is OQL, although not all vendors of object databases implement this, majority of them do implement this method. A great deal of the internal engineering of a DBMS is independent of the data model, and is concerned with managing factors such as performance, concurrency, integrity, and recovery from hardware failures. In these areas there are large differences between the products.

A relational database management system (RDBMS) implements features of the relational model. In this context, Date's "Information Principle" states: "the entire information content of the database is represented in one and only one way. Namely as explicit values in column positions (attributes) and rows in relations (tuples). Therefore, there are no explicit pointers between related tables." This contrasts with the object database management system (ODBMS), which does store explicit pointers between related types.

Components of DBMS

According to the wikibooks open-content textbooks, "Design of Main Memory Database System/Overview of DBMS", most DBMS as of 2009 implement a relational model. Other less-used DBMS systems, such as the object DBMS, generally operate in areas of application-specific data management where performance and scalability take higher priority than the flexibility of ad hoc query capabilities provided via the relational-algebra execution algorithms of a relational DBMS.

RDBMS components
Interface drivers - A user or application program initiates either schema modification or content modification. These drivers[which?] are built on top of SQL. They provide methods to prepare statements, execute statements, fetch results, etc. Examples include DDL, DCL, DML, ODBC, and JDBC. Some vendors provide language-specific proprietary interfaces. For example MySQL provides drivers for PHP, Python, etc.
SQL engine - This component interprets and executes the SQL query. It comprises three major components (compiler, optimizer, and execution engine).
Transaction engine - Transactions are sequences of operations that read or write database elements, which are grouped together.
Relational engine - Relational objects such as Table, Index, and Referential integrity constraints are implemented in this component.
Storage engine - This component stores and retrieves data records. It also provides a mechanism to store metadata and control information such as undo logs, redo logs, lock tables, etc.

ODBMS components
Language drivers - A user or application program initiates either schema modification or content modification via the chosen programming language. The drivers then provide the mechanism to manage object lifecycle coupling of the application memory space with the underlying persistent storage. Examples include C++, Java, .NET, and Ruby.
Query engine - This component interprets and executes language-specific query commands in the form of OQL, LINQ, JDOQL, JPAQL, others. The query engine returns language specific collections of objects which satisfy a query predicate expressed as logical operators e.g. >, <, >=, <=, AND, OR, NOT, GroupBY, etc. Transaction engine - Transactions are sequences of operations that read or write database elements, which are grouped together. The transaction engine is concerned with such things as data isolation and consistency in the driver cache and data volumes by coordinating with the storage engine. Storage engine - This component stores and retrieves objects in an arbitrarily complex model. It also provides a mechanism to manage and store metadata and control information such as undo logs, redo logs, lock graphs, Primary tasks of DBMS packages
Database Development: used to define and organize the content, relationships, and structure of the data needed to build a database.
Database Interrogation: can access the data in a database for information retrieval and report generation. End users can selectively retrieve and display information and produce printed reports and documents.
Database Maintenance: used to add, delete, update, correct, and protect the data in a database.
Application Development: used to develop prototypes of data entry screens, queries, forms, reports, tables, and labels for a prototyped application. Or use 4GL or 4th Generation Language or application generator to develop program codes.

Operational database

These databases store detailed data needed to support the operations of an entire organization. They are also called subject-area databases (SADB), transaction databases, and production databases.

For example:
customer databases
personal databases
inventory databases

Transactions and concurrency

In addition to their data model, most practical databases ("transactional databases") attempt to enforce database transactions. Ideally, the database software should enforce the ACID rules, summarized here:
Atomicity: Either all the tasks in a transaction must happen, or none of them. The transaction must be completed, or else it must be undone (rolled back).
Consistency: Every transaction must preserve the integrity constraints — the declared consistency rules — of the database. It cannot leave the data in a contradictory state.
Isolation: Two simultaneous transactions cannot interfere with one another. Intermediate results within a transaction must remain invisible to other transactions.
Durability: Completed transactions cannot be aborted later or their results discarded. They must persist through (for instance) restarts of the DBMS after crashes.

In practice, many DBMSs allow the selective relaxation of most of these rules — for better performance.

Concurrency control ensures that transactions execute in a safe manner and follow the ACID rules. The DBMS must be able to ensure that only serializable, recoverable schedules are allowed, and that no actions of committed transactions are lost while undoing aborted transactions.

Replication of databases often relates closely to transactions. If a database can log its individual actions, one can create a duplicate of the data in real time. DBAs can use the duplicate to improve performance and/or the availability of the whole database system.

Common replication concepts include:
master/slave Replication: All write-requests are performed on the master and then replicated to the slave(s)
quorum: The result of Read and Write requests are calculated by querying a "majority" of replicas
multimaster: Two or more replicas sync each other via a transaction identifier

Parallel synchronous replication of databases enables the replication of transactions on multiple servers simultaneously, which provides a method for backup and security as well as data availability. This is commonly referred to as "database clustering"

Database security denotes the system, processes, and procedures that protect a database from unintended activity. Enforcing security is one of the major tasks of the DBA.

DBMSs usually enforce security through access control, auditing, and encryption:
Access control ensures and restricts who can connect and what they can do to the database.
Auditing logs what action or change has been performed, when and by whom.
Encryption: many commercial databases include built-in encryption mechanisms to encode data natively into tables and to decipher information "on the fly" when a query comes in. DBAs can also secure and encrypt connections if required using DSA, MD5, SSL or legacy encryption standards.

In the United Kingdom, legislation protecting the public from unauthorized disclosure of personal information held on databases falls under the Office of the Information Commissioner. Organizations based in the United Kingdom and holding personal data in electronic format (databases for example) must register with the Data Commissioner