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Fundamentals of Database Systems

Chapter 2

Database System Concepts andArchitecture

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Data Models Definition

A set of concepts to describe the structure of a database, and certain constraints that the database should obey

Most data models also include a set of basic operations for specifying retrievals and updates on the database

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Data Models (cont.)

Categories of data models Conceptual (high-level, semantic) data

models» Provide concepts that are close to the way many

users perceive data» e.g., Entity-relationship model, object data model

Physical (low-level, internal) data models» Provide concepts that describe details of how

data is stored in the computer

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Data Models (cont.)

Implementation (record-oriented) data models » Provide concepts that fall between the above two,

balancing user views with some computer storage details

» e.g., relational model, hierarchical model, network model

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Data Models (cont.)

History of data models Relational Model

» Proposed in 1970 by E.F. Codd (IBM) » First commercial system in 1981-82 » e.g., ORACLE, SYBASE, SQL Server

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Data Models (cont.)

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Data Models (cont.)

Network Model » The first one to be implemented by Honeywell in

1964-65 (IDS System) » Adopted heavily due to the support by CODASY

L (CODASYL - DBTG report of 1971) » Later implemented in a large variety of systems -

IDMS (Cullinet - now CA), DMS 1100 (Unisys), IMAGE (H.P.), VAX -DBMS (Digital)

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Data Models (cont.)

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Data Models (cont.)

Hierarchical Data Model » Implemented in a joint effort by IBM and North

American Rockwell around 1965 » Resulted in the IMS family of systems » The most popular model

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Data Models (cont.)

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Data Models (cont.)

Object-oriented Data Models» Several models have been proposed for implementing in

a database system

» One set comprises models of persistent O-O Programming Languages such as C++ (e.g., in OBJECTSTORE or VERSANT), and Smalltalk (e.g., in GEMSTONE)

» Additionally, systems like O2, ORION (at MCC - then ITASCA), IRIS (at H.P.- used in Open OODB)

» Object Database Standard: ODMG-93, ODMG-version 2.0, ODMG-version 3.0

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Data Models (cont.)

Object-Relational Models» Most recent trend, started with Informix

Universal Server » Relational systems incorporate concepts from

object databases leading to object-relational » Exemplified in the latest versions of Oracle-10i,

DB2, and SQL Server and other DBMSs» Standards included in SQL-99 and expected to be

enhanced in future SQL standards

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Schemas vs Instances

Database Schema The description of a database Includes descriptions of the database

structure and the constraints that should hold on the database

Can be displayed as a diagram (called schema diagram)

Database schema changes very infrequently

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Schemas vs Instances (cont.) Example schema diagram

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Schemas vs Instances (cont.)

Database Instance The actual data stored in a database at a

particular moment in time Also called database state (or occurrence) The database state changes every time the

database is updated» e.g., insert or delete a record

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Schemas vs Instances (cont.)

Example database state

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Schema Architecture

Three-Schema Architecture Internal schema

» Describe data storage structures and access paths at the internal level

» Typically uses a physical data model

Conceptual schema » Describe the structure and constraints for the

database at the conceptual level» Uses a conceptual or an implementation data

model

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Schema Architecture (cont.)

External schemas » Describe the various user views at the external

level to » Usually uses the same data model as the

conceptual level

Purposes » Support program-data independence» Support of multiple views of the data

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DBMS Architecture (cont.)

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Data Independence

Logical Data Independence The capacity to change the conceptual

schema without having to change the external schemas and their application programs

Only view definition and the mapping need be change

e.g., Changing the Grade_Report

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DBMS Architecture (cont.)

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Data Independence (cont.)

Physical Data Independence The capacity to change the internal schema

without having to change the conceptual schema

e.g., providing an access path by Semester and Year

should not change query “list all sections offered in fall 1998”

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DBMS Languages

Data Definition Language (DDL) Used by the DBA and database designers to s

pecify the conceptual schema In many DBMSs, DDL is also used to define

conceptual and external schemas (views) In some DBMSs,

» Internal: storage definition language (SDL)» External: view definition language (VDL)

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DBMS Languages (cont.)

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DBMS Languages (cont.)

Data Manipulation Language (DML) Used to specify database retrievals and

updates DML commands (data sublanguage) can be

embedded in a general-purpose programming language, such as COBOL, PL/1, C/C++

Alternatively, stand-alone DML commands can be applied directly (query language)

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DBMS Languages (cont.)

Example in SQLSELECT BDATE, ADDRESS

FROM EMPLOYEE

WHERE FNAME='John' AND MINIT='B' AND LNAME='Smith'

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DBMS Languages (cont.)

Example of embedded SQLEXEC SQL

declare c cursor for select customer-name, customer-cityfrom depositor, customer, accountwhere depositor.customer-name = customer.customer-name and depositor account-number = account.account-number

and account.balance > :amountEND-EXEC

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Database System Environment

DBMS component modules Database system utilities Tools

CASE tools, data dictionary

Application environment e.g., PowerBuilder

Communication facilities

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DBMS Component Modules

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DBMS Architectures

Centralized DBMS Combines everything into single system

including- DBMS software, hardware, application programs, and user interface processing software

User can still connect through a remote terminal – however, all processing is done at centralized site

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DBMS Architectures (cont.)

A physical centralized architecture

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DBMS Architectures (cont.)

Basic Client-Server Architectures Specialized Servers with Specialized

functions» Print server» File server» DBMS server» Web server» Email server

Clients can access the specialized servers as needed

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DBMS Architectures (cont.)

Logical client-server architecture

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DBMS Architectures (cont.)

Two Tier Client-Server Architecture A client program may connect to several

DBMSs, sometimes called the data sources In general, data sources can be files or other

non-DBMS software that manages data Other variations of clients are possible

» in some object DBMSs, more functionality is transferred to clients including data dictionary functions, optimization and recovery across multiple servers, etc.

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DBMS Architectures (cont.) Three Tier Client-Server Architecture

Common for Web applications Intermediate Layer called Application Server or

Web Server: » Stores the web connectivity software and the business

logic part of the application used to access data from database server

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

Three-tier Architecture Can Enhance Security » Database server only accessible via middle tier» Clients cannot directly access database server

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DBMS Architectures (cont.)

Logical three tier client-server architecture

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Classification of DBMSs

Based on the data model used Traditional: Relational, Network, Hierarchica

l Emerging: Object-oriented, Object-relational

Based on the number of users Single-user (typically used with micro- comp

uters) multi-user (most DBMSs)

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Classification of DBMSs (cont.)

Based on the number of sites Centralized (uses a single computer with one

database) Distributed (uses multiple computers,

multiple databases)» Homogeneous» Heterogeneous (Federated DBMS)