CS370 Spring 2007

CS 370 Database SystemsCS 370 Database Systems

Lecture 3 Overview of Database

Systems

CS370 Spring 2007

A commonly used view of data approach is the three-level architecture suggested by ANSI/SPARC A commonly used view of data approach is the three-level architecture suggested by ANSI/SPARC

(American National Standards Institute/Standards Planning and Requirements Committee).(American National Standards Institute/Standards Planning and Requirements Committee).

View nView nView 2View 2View 1View 1

Conceptual Conceptual

SchemaSchema

Internal Internal

SchemaSchemaD

ata

base

Dat

a ba

se

External LevelExternal Level

Conceptual LevelConceptual Level

Internal LevelInternal Level

Physical data OrganizationPhysical data Organization

DBMS ArchitectureDBMS Architecture

CS370 Spring 2007

The external level is the view that the individual user of the database has. This

view is often a restricted view of the database and the same database may provide a

number of different views for different classes of users. In general, the end users

and even the applications programmers are only interested in a subset of the

database. For example, a department head may only be interested in the

departmental finances and student enrolments but not the library information. The

librarian would not be expected to have any interest in the information about

academic staff. The payroll office would have no interest in student enrolments

User’s view of the database. Describes that part of database that is relevant to a particular user.

External LevelExternal Level

CS370 Spring 2007

The conceptual view is the information model of the enterprise and contains the view of the whole enterprise without any concern for the physical implementation. This view is normally more stable than the other two views. In a database, it may be desirable to change the internal view to improve performance while there has been no change in the conceptual view of the database. The conceptual view is the overall community view of the database and it includes all the information that is going to be represented in the database. The conceptual view is defined by the conceptual schema, which includes definitions of each of the various types of data.

Community view of the database. Describe what data is stored in database and relationship.

Conceptual LevelConceptual Level

CS370 Spring 2007

Physical representation of the database on the computer. Describes how the data is stored in the database.

Internal LevelInternal Level

CS370 Spring 2007

External View (User 2)External View (User 1)1.1. EMPEMP

2.2. EMP #EMP #

3.3. SALSAL

1. EMP

2. EMP #

3. SAL

Conceptual Level

Employee

EMPLOYEE_NUMBER NUMBER(6)

DEPARTMENT_NUMBER NUMBER( 4)

SALARY NUMARIC(5)

Internal Level STORED_EMP LENGTH=18

PREFIX TYPE=BYTE(6), OFFSET=0

EMP# TYPE=BYTE(6), OFFSET=6,INDEX=EMX

DEPT# TYPE=BYTE(4), OFFSET=12

PAY TYPE=FULLWORD, OFFSET=16

CS370 Spring 2007

SNo FName LName Age Salary Staff_No L_Name BNo

Staff_No FName LName DOB Salary Branch_No

External View 1 External View 2

Conceptual Level

struct STAFF{int Staff_No;int Branch_No;char Fname [15];char Lname [15];struct date Date_of_Birth;float Salary;struct STAFF * next; /*Pointer to next Staff Record*/};index Staff_No; index Branch_No; /*Define indexes for staff*/

Internal Level

CS370 Spring 2007

UserUser UserUser

External ViewExternal View

UserUser UserUser

External ViewExternal View

UserUser UserUser

External ViewExternal View

Conceptual ViewConceptual View(Table,etc)(Table,etc)

Physical ViewPhysical View

Logical BarrierLogical Barrier

Physical BarrierPhysical Barrier

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Data IndependenceData Independence

• One big problem in application development is the separation of applications from data

• Do I have change my program when I …– replace my hard drive?– partition the data into two physical files (or merge two

physical files into one)?– store salary as floating point number instead of integer?– develop other applications that use the same set of data?– add more data fields to support other applications?– … …

CS370 Spring 2007

External/Conceptual External/Conceptual mappingmapping

Data Independence and the ANSI-SPARC Three-level Data Independence and the ANSI-SPARC Three-level ArchitectureArchitecture

External SchemaExternal Schema

Conceptual SchemaConceptual Schema

Internal SchemaInternal Schema

Conceptual/internal Conceptual/internal mappingmapping

External SchemaExternal Schema External SchemaExternal Schema

Logical data independenceLogical data independence

Physical data independencePhysical data independence

CS370 Spring 2007

Instances and SchemasInstances and Schemas

• Each level is defined by a schema, which defines the data at the corresponding level– A logical schema defines the logical structure of the

database (e.g., set of customers and accounts and the relationship between them)

– A physical schema defines the file formats and locations

• A database instance refers to the actual content of the database at a particular point in time. A database instance must conform to the corresponding schema

CS370 Spring 2007

Logical Data IndependenceLogical Data Independence

Refers to immunity of external schemas to change in Refers to immunity of external schemas to change in conceptual schema.conceptual schema.

Conceptual schema changes e.g. addition /removal of Conceptual schema changes e.g. addition /removal of entities.entities.

Should not require changes to external schema or Should not require changes to external schema or rewrites of application programs.rewrites of application programs.

Data IndependenceData Independence

CS370 Spring 2007

external

schema1

external

schema1

external

schema2

external

schema2

external

schema3

external

schema3

conceptual

schema

conceptual

schema

internal

schema

internal

schema

databasedatabase

Logical data Logical data independence is a independence is a

measure of how much measure of how much the conceptual schema the conceptual schema

can change without can change without affecting the affecting the

application programsapplication programs

• For Example, by removing For Example, by removing

record type or data item, record type or data item,

the external schemas that the external schemas that

refer only to the remaining refer only to the remaining

data should not be affected.data should not be affected.

• Only the view definitions and Only the view definitions and

mapping need to be changed mapping need to be changed

in DBMS.in DBMS.

Logical Data IndependenceLogical Data Independence

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Physical Data IndependencePhysical Data Independence

Refers to immunity of conceptual schemas to change Refers to immunity of conceptual schemas to change in internal schema.in internal schema.

Internal schema changes e.g. using different file Internal schema changes e.g. using different file organizations, storage structures/devices.organizations, storage structures/devices.

Should not require changes to Conceptual or external Should not require changes to Conceptual or external schema.schema.

Data IndependenceData Independence

CS370 Spring 2007

external

schema1

external

schema1

external

schema2

external

schema2

external

schema3

external

schema3

conceptual

schema

conceptual

schema

internal

schema

internal

schema

databasedatabase

Physical data Physical data independence is a independence is a

measure of how much measure of how much the internal schema can the internal schema can

change without change without affecting the affecting the

application programsapplication programs

• some physics files are some physics files are

reorganized.reorganized.

• For example, when create For example, when create

additional access structure to additional access structure to

improve the performance of improve the performance of

retrieval or update. If the same retrieval or update. If the same

data as before remains same in data as before remains same in

the database.the database.

• No need to change conceptual No need to change conceptual

schemaschema

Physical Data IndependencePhysical Data Independence

CS370 Spring 2007

• Here are some questions a user might ask:-– What is the name of the student whose ID = 1234?– How many students are enrolled in CS370?– Is any student with GPA less than 3.0 enrolled in CS370?

• Such questions involving the data stored in a DBMS are called queries

• A DBMS provides a specialized language, called the query language, in which queries can be imposed

• Relational algebra and relational calculus are the formal query languages based on mathematical logic and collection of operators.

• Query language is also a part of DML (which would be discussed later)

Queries in DBMSQueries in DBMS

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Transaction ManagementTransaction Management

•A transaction is a collection of operations that performs a single logical function in database application

time

Transaction 1

Transaction 2

Conflicting read/write

Transaction 1

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• Transaction-management component ensures that the database remains in a consistent (correct) state despite system failures (e.g. power failures and operating system crashes) and transaction failures.

• Concurrency-control manager controls the interaction among the concurrent transactions, to ensure the consistency of the database.

Transaction Management (cont.)Transaction Management (cont.)

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Storage ManagementStorage Management

• A storage manager is a program module that provides the interface between the low-level data stored in the database and the application programs and queries submitted to the system.

• The storage manager is responsible for the following tasks:– interaction with the file manager– efficient storing, retrieving, and updating of data.

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Overall System ArchitectureOverall System Architecture

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Two-tier architecture: E.g. client programs using ODBC/JDBC to communicate with a databaseThree-tier architecture: E.g. web-based applications, and applications built using “middleware”

Application ArchitecturesApplication Architectures

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Server hold the database and the DBMS.

Clint manage the user interface and runs applications.

LAN

Request for data Selected data returned

Databa

se

Server (With DBMS)

Client – ServerClient – Server

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Clint Server

Manages the user interface.

Accepts and checks syntax of user input.

Process application.

Generates database request and transmit to server.

Passes response back to user.

Accepts and processes database requests from clients.Checks authorizationEnsure integrity constraints not violated.Performs query/update processing and transmit response to client.Maintains system catalog.Provides concurrent database access.Provides recovery control.

Summary – Client Server functionsSummary – Client Server functions

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Two-tier ArchitectureTwo-tier Architecture

• User Interface Programs and Application Programs run on the client side

• Interface called ODBC (Open Database Connectivity) provides an Application program interface (API) allow client side programs to call the DBMS. Most DBMS vendors provide ODBC drivers.

• A client program may connect to several DBMS.• Other variations of clients are possible: e.g., in

some DBMS, more functionality is transferred to clients including data dictionary functions, optimization and recovery across multiple servers, etc. In such situations the server may be called the Data Server.

CS370 Spring 2007

Three-tier ArchitectureThree-tier Architecture

• Common for Web applications• Intermediate Layer called Application Server or

Web Server: • Stores the web connectivity software and the rules

and business logic (constraints) part of the application used to access the right amount of data from the database server

• Acts like a conduit for sending partially processed data between the database server and the client.

• Additional Features- Security: • Encrypt the data at the server before transmission• Decrypt data at the client

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Advantages include:

Wider access to existing database.

Increased performance.

Possible reduction in hardware cost.

Reduction in communication cost.

Increased consistency.

Client - ServerClient - Server

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Distributed database system consist of collecting of sites, connected together via some kind of communication network, in which.

Each site is a database system site in its own right.

The site is have agreed to work together so that a user at any

site can access data anywhere in the network exactly as

if the data were all stored at the user’s own site.

Distributed Database SystemsDistributed Database Systems

CS370 Spring 2007

Lahore

Shanghai Islamabad

Singapore

Communication network

Distributed Database SystemsDistributed Database Systems