database architecture lecture

8/7/2019 Database Architecture Lecture

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Database Architecture

ANSI-SPARC Architecture

American National Standard

Institute -


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Why ?

Why we need a StandardizedArchitecture?

Three Level Architecture

At what level we interact with thesystem?

Different Users ( Nave user,

Designer/Programmer, DBA)interact at different levelsMain objective: separate the user view

from physical view, Example: DoB


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Objectives

1.Separate User View from physicalview

2.Different views of the same data

3.Consolidated representation

4.Both ways easy change


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-ANSI SPARC -Three Level Architecture

Three Schema

OR Three Models

1.External View = Level I =Virtual/Calculated Data

2.Conceptual View= Logical View/Schema Middle layer = Level - II

3.Internal / Physical View = Level III= Bottom Layer (PermanentStructure of the data)


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-Three level ANSISPARC Architecture

ExternalView 1

...ExternalView 2

ExternalView n

User 1 User 2 User n

Conceptual View

Internal View

ExternalLevel

ConceptualLevel

InternalLevel andphysical

dataorganisation

External

Schema 1

External

Schema 2

External

Schema n

ConceptualSchema

InternalSchema

External/ConceptualMapping 1 External/Conceptual

Mapping n

Conceptual/InternalMapping


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External view

The users view of database Presents only the part of database that is relevant

to a particular user / application.

Consists of a number of different external views ofthe database.

Each consists those entities, attributes andrelationships that the user is interested in.

Provides different representation of same data

(e.g. date format - dmy, mdy or ddmmyy). Derived or calculated data, e.g. date of birth, age


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Conceptual view

The only community view of the database. Describes what data is stored in the database and the

relationships among the data. Contains the logical structure of the entire database as

seen by the DBA (e.g. listing of full table). It represents: - all entities, their attributes and relationships - the constraints on the data - semantic information about the data

- security and integrity information Must not contain any storage-dependent details.


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Internal View

The physical representation of the database onthe computer.

Describes how the data is stored in the database. Physical implementation to achieve optimal run-

time performance and disk space utilization. Concern about data structure, file organization,

record placement, data compression andencryption techniques.

Some DBMS interfaces with the operating systemaccess methods.


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, ,CONCEPTUAL DESIGN SCHEMA LOGICALDESIGN


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An Example

s Note that corresponding objects could havedifferent names at each point

Example: employee number is referred as:

EMPNO in the External view EMPLOYEE_NUMBER in the

conceptual view

EMP# in the internal view

s The system is aware of the correspondencess Such correspondences are implemented through

the mappings


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Data Independences In a database system it is desirable to allow

applications to be data-independent, because: Different applications will need different

views of the same data

The DBA must have freedom to change thestorage structure or access technique in responseto changing requirements, without having tomodify existing applications

s If applications are data-dependent, changes in

data storage structure will require correspondingchanges to be made to programs.s This would tie up programmer effort to old

applications rather than creating new ones


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Data Dictionary

The result of the compilation of theDDL statements is a set of tablesstored in special files collectively

called data dictionary. The DBMS normally consults the data

dictionary before the actual data is

accessed in the database


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COMPONENTS OF DBMS

1) Query Processor:

Used to transform queries into a series oflow-level instructions directed to the

Database Manager2) Database Manager:

The DM interfaces with user-submittedapplication programs and queries.

It accepts the queries and examines theexternal and conceptual schema

Then places a call to the File Manager


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Continued Components of DBMS

3) File Manager:

It manipulates the underlying storagefiles and manages the allocation of

logical storage space on the disk. It does not directly manage the physical

input and output of data.

Rather it passes the requests on theappropriate access methods, whicheither read / write data into thesystem buffer.


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DBMS Languages

DDL: Data definition language is usedby the DBA and by the databasedesigners to define both schemas

(Conceptual and Internal). DML : Data manipulation language isused when the database schemas arecompiled and it is populated with

data. Typical manipulation s includeretrieval, insertion, deletion, andmodifications of the data.

SDL : Storage definition language is

used to specify the internal schema.


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:Example University Database

Conceptual schema:

Students(sid: string, name: string, login: string,

age: integer, gpa:real)

Courses(cid: string, cname:string,

credits:integer)

Enrolled(sid:string, cid:string, grade:string)

Physical schema:

Relations stored as unordered files.

Index on first column of Students.

External Schema (View):

Course_info(cid:string,enrollment:integer)

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