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    DBMS Architecture

    DBMS architecture allows in evolution, execution, structure, and support of a database that save and make records for agencies, businesses, and establishments. It is the establishment of any database executive framework, which award it to implement the tasks completely and proficiently. The whole idea of DBMS relies upon its structure. It can be composed as centralized, decentralized, or hierarchical.

    Types of DBMS Architecture

    Database Architecture is divided into three types:

    Three Schema DBMS Architecture

    Database architecture can be view as a single-tier or multi-tier. But logically, DBMS architecture is of two types like Two-tier architecture and Three-tier architecture.

    One Tier Architecture

    In One-tier Architecture all software are present on a single machine (i.e., server system), and can be accessed by other terminals as shown in fig:

    Three Schema DBMS Architecture
    Two-Tier Architecture

    The 2-tier architecture of DBMS is based on a client-server machine. In this type of architecture besides client device, that invokes database system working through a query language, standards like
    Open Database Connectivity (ODBC) and Java Database Connectivity (JDBC).

    Examples of Two-tier Architecture are Oracle, Sybase, Microsoft SQL Server, etc.
    The client invokes database framework functionally at the server device through a query language statement, as shown in fig:

    Three Schema DBMS Architecture
    Three Tier Architecture

    Because of the addition of middleware, it provides a way for the clients of one DBMS to access data from another DBMS on some other machine. Three Tier Architecture is mostly suitable for huge applications, as shown in fig:

    Three Schema DBMS Architecture
    Three Schema Database Architecture

    The three-schema architecture defines how the data is represented or viewed by the customer in the database. This architecture is also called three-level architecture and is sometimes referred to as ANSI/SPARC architecture.

    An early idea for a standard language and common structure for database framework was developed in 1971 by means of DBTG (Data Base Task Group) selected by using the Conference on Data Systems and Languages (CODASYL, 1971). The DBTG identified the needed for a two-level method with a framework view called the schema and user views called sub-schemas.

    This architecture includes three layers or levels of the database management system:

    • External level
    • Conceptual level
    • Internal level

    Here is the figure displaying the ANSI_SPARC Architecture of the database framework:

    Three Schema DBMS Architecture

    External Level

    The external level is the user’s view of the database. This level is the highest level of information abstraction, where only those part of the database of concern to a user or application software is included. In other words, this level defines that a part of the database this is applicable to the user. Any number of user views, even identical, might also exist for a given conceptual or global view of the database. Each user has a view of the “real world” described in a structure that is familiar for that user.

    In the external level, the different views may have a different representation of the same data. For example, one user may view data in the form as day, month, year while another may view as a year, month, day. Some views might include derived or evaluated data, that is; data is not stored in the database but are created when needed. The individual age of all employees stored in the database.

    An external schema describes each external view. The external schema consists of the definitions of the logical records and the relationships in the external view. It also includes the method of deriving the objects like entities, attributes and relationships in the external view from the object in the conceptual view.

    Conceptual Level

    The conceptual level is the middle level in the three-tier architecture. At this level of the database abstraction, all the database entities and relationships among them are contained. Conceptual level provides the community view of the database and describes that data is stored in the database and the relationships among the data. It includes the logical framework of the entire database as view by the DBA. One conceptual view describes the whole database of an organization. It is a complete view of the data requirements of the company that is independent of any storage considerations.

    The conceptual schema represents a conceptual view. It is also called the logical schema. This schema contains the method of deriving the objects in the conceptual view from the object in the internal view.

    Conceptual Level is concerned with the following activities:

    • All entities, their attributes and their relationships.
    • Constraint on the data.
    • Semantic information about the data.
    • Check to retain data consistency and integrity.
    • Security Information
    Internal Level/Physical Level

    Internal level is the physical representation of the database on the computer, and this view is found at the lowest level of abstraction of the database. This level denoted how the record will be stored in the database and describes the data structures, file structures, and access methods to be utilized by the database.

    The internal schema specifies the internal level. The internal schema contains the description of the stored records, the technique of representing the information fields (or attributes), indexing and hashing schemes and the access techniques used.

    Internal Schema compile how the relations defined in the conceptual schema are absolutely saved on the secondary storage devices, which include disks and tapes. It interfaces with the operating function access methods (also known as file management techniques for saving and retrieving data records) to place the data on the storage devices, develop the indexes, retrieve the data and so on.

    Internal level is involved with the following activities:

    • Storage space allocation for data and storage.
    • Record descriptions for storage with saved sizes for data elements.
    • Record Placement.
    • Data compression and data encryption techniques.

    The process of changing request and results among three levels it is known as mapping.

    There are two types of mappings:

    1. External/Conceptual Mapping
    2. Conceptual/Internal Mapping
    External/Conceptual Mapping

    An External/Conceptual Mapping defines the correspondence among the specific external view and the conceptual view. The user sees and manipulates a record corresponding to the external view. There is a mapping from a specific logical record in the external view to one or more conceptual records in the conceptual view. A number of differences could exist between the two views.

    Conceptual/Internal Mapping

    The Conceptual/Internal Mapping determines the correspondence among the conceptual data and the save database. It prescribes how conceptual records and fields are defined at the internal level. The difference could exist between these two views and are resolved in the mapping.

    Example (Various Views of Data)

    An example of the three levels is shown in fig:

    Three Schema DBMS Architecture

    Advantages of Three-Tier Architecture

    The main objectives of the three-tier database architecture are to isolate each user’s view of the database from the way the database is physically stored or represented.

    Following are the advantages of a three-tier database architecture:

    • Each user is able to access the same data but has a different customized view of the data as per their own needs.
    • A user can change his view, and this change does not affect other users.
    • The user’s interactions with the database are independent of physical data storage organization.
    • The DBA is able to change the conceptual architecture of the database without affecting all clients.
    • The DBA can change an existing storage device with the new storage device without affecting other users.

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