DBMS and its Architecture

Transcription

1 DBMS and its Architecture DCS COMSATS Institute of Information Technology Rab Nawaz Jadoon Assistant Professor COMSATS IIT, Abbottabad Pakistan Management Information Systems (MIS)

2 Lecture Agenda DBMS Architecture The Three-Level ANSI-SPARC Architecture External Level Conceptual Level Internal Level Physical Data Organization Differences between Three Levels of ANSI-SPARC Architecture Objectives of Three-Level Architecture Database Schema Data Independence Database Languages Data Models and Conceptual Modeling Object Based Data Model Record-based Data Models Physical Data Models

3 DBMS Architecture A major aim of a database system is to provide users with an abstract view of data, hiding certain details of how data is stored and manipulated. The starting point for the design of a database must be an abstract and general description of the information requirements of the organization that is to be represented in the database.

4 Cont A database is a shared resource, each user may require a different view of the data held in the database. To satisfy these needs, the architecture of most commercial DBMSs available today is based to some extent on the so-called ANSI-SPARC architecture

5 Three Level ANSI-SPARC Architecture The American National Standards Institute (ANSI) Standards Planning and Requirements Committee (SPARC) produced this architecture in 1975 (ANSI, 1975). It comprising an external, a conceptual and an internal level External level: The way users perceive the data Internal level: The way the DBMS and the operating system perceive the data, where the data is actually stored using the data structures and file

6 Three Level ANSI-SPARC Architecture Conceptual level: Provides both the mapping and the desired independence between the external and internal levels. The objective of the three-level architecture is to separate each user s view of the database from the way the database is physically represented.

7 External Level Describes that part of the database that is relevant to each user. Users view of the database. Describes that part of database that is relevant to a particular user. A user may require few attributes and may not need rest of the attributes.

8 Conceptual Level Describes what data is stored in database and relationships among the data. The community view of the database. It contains logical structure of the database It represents: All entities, attributes and their relationships Constraints on data Semantic information about the data Security and integrity of data It does not concern any storage considerations, such as how many bytes occupied

9 Physical representation of the database on the computer Describes how the data is stored in the database. It covers following things: Storage space allocation for data and indexes Record descriptions for storage (size) Record placement Internet Level Data compression and encryption techniques

10 Physical Data Organization Below the internal level is the Physical Level. This level is maintained by the Operating System under the direction of the DBMS. Examples : Fields are stored contiguously on disk or not.

11 Differences between Three Levels of ANSI- SPARC Architecture

12 Objectives of Three-Level Architecture Same data accessible to all users. A user s view is immune to changes made in other views. Hide physical database storage details from users. DBA should be able to change database storage structures without affecting the users views. Internal structure of database should be unaffected by changes to physical aspects of storage. DBA should be able to change conceptual structure of database without affecting all users.

13 Database Schema The description of overall database is called database schema. External Schema Corresponds to different views of data Conceptual Schema Describes entities, attributes, relationships and constraints. Internal Schema It is a complete description of the internal model, including definitions of records, indexes and hashing functions.

14 Data Independence Major objective of three level architecture is to provide data independence. Upper levels are unaffected when changes are made in the low levels. There are two kinds of data independences Logical Data Independence Refers to immunity of external schemas to changes in conceptual schema. Conceptual schema changes (e.g. addition/removal of entities). Should not require changes to external schema or rewrites of application programs.

15 Physical Data Independence Data Independence Refers to immunity of conceptual schema to changes in the internal schema. Internal schema changes (e.g. using different file organizations, storage structures/devices). Should not require change to conceptual or external schemas

16 Data Independence

17 Consists of two parts a Data Definition Language (DDL) and a Data Manipulation Language (DML). Both do not include constructs for all computing needs such as conditional or iterative statements DDL Used to specify the database schema A language that allows the DBA or user to describe and name the entities, attributes, and relationships required for the application, together with any associated integrity and security constraints. The result of the compilation of the DDL statements is a set of tables stored in special files collectively called the system catalog. The system catalog integrates the metadata, that is data that describes objects in the database and makes it easier for those objects to be accessed or manipulated. Database Languages

18 DML Database Languages A language that provides a set of operations to support the basic data manipulation operations on the data held in the database. Data manipulation operations include: Retrieval/Selection Insertion Modification Deletion Procedural DML allows user to tell system what data is needed and exactly how to retrieve data. Non-Procedural DML allows user to state what data is needed rather than how it is to be retrieved.

19 Data Models and Conceptual Data Model Modeling Integrated collection of concepts for describing data, relationships between data, and constraints on the data in an organization. Categories of data models include: Object-based Record-based Physical.

20 Object Based Data Model Object based models use concepts like entities, attributes and relationships Some of the more common types of object-based data model are: Entity Relationship Semantic Functional Object-Oriented. Entity It is a distinct object such as a person, thing, place etc in the organization that is to be represented. Attribute Is the property that describe some aspects of the entity. Relationship Association between entities.

21 Record Based Data Model In this model, database consists of a number of fixed format records and each record has a fixed number of fields. There are three principal types of record-based logical data model The relational data model, the network data model, and the hierarchical data model. The hierarchical and network data models were developed almost a decade before the relational data model, so their links to traditional file processing concepts are more evident.

22 Physical Data Model Describe how data is stored in the computer, representing information such as record structures, record orderings, and access paths. There

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