Chapter 2 Database System Concepts and Architecture Dr. Bernard Chen Ph.D. University of Central Arkansas

Outline Data Models, Schemas, and Instances Three-Schema Architecture Database language and Interfaces Centralized and client/server architecture

Categories of data models One fundamental characteristic of the database approach is that it provides some level of data abstraction High-level or Conceptual data models: Provide concept that are close to the way many users perceive data Low-level or Physical data model: Provide concepts that describe the details of how data is stored in the computer

Conceptual data models It uses concepts such as entities, attributes and relationships. Entity represents a real-world object or concept, such as employee or project Attribute represents some property of interest that further describes an entity, such as employee’s name or salary Relation represents an association among two or more entitles

Example of a Relation

Schemas and Database State In any data model, it is important to distinguish between the description of the data and database itself The description of the database is called the database schema A displayed Schema is called a schema diagram

Database V.S. File In the database approach, a single repository of data is maintained that is defined once then accessed by various users The major differences between DB and File are: 1. Self-describing of a DB 2. Insulation between programs and data 3. Support of multiple views of the data 4. Sharing of data and multiuser transaction processing

University Database

Example of a Database Schema (incomplete)

Example of a Database Schema

Schemas and Database State The data in the database at a particular moment in time is called a database state The distinction between database schema and database state is very important When we define a new database, we specify its database schema only to the DBMS At this point, the corresponding database state is the empty state with no data We get the initial state of the database when the database is first loaded From then on, every time an update operation is applied to the database, we get another database state

Schemas and Database State Valid State: a state that satisfies the structure and constrains specified in the schema. The database schema changes very infrequently. The database state changes every time the database is updated Schema is also called intension. State is also called extension.

Outline Data Models, Schemas, and Instances Three-Schema Architecture Database language and Interfaces Centralized and client/server architecture

Three-Schema Architecture Three of four important characteristics of the database approach, listed in Ch.1 are: 1. Self-describing of a DB (Schema) 2. Insulation between programs and data 3. Support of multiple views of the data Three-Schema Architecture : it was proposed to help achieve and visualize these characteristics

Three-Schema Architecture Defines DBMS schemas at three levels: Internal schema at the internal level to describe physical storage structures and access paths (e.g indexes). Conceptual schema at the conceptual level to describe the structure and constraints for the whole database for a community of users. External schemas at the external level to describe the various user views.

The three-schema architecture

Outline Data Models, Schemas, and Instances Three-Schema Architecture Database language and Interfaces Centralized and client/server architecture

DBMS Languages The first step to create a database through DBMS is to specify conceptual and internal schemas for the database Data Definition Language (DDL): is used by database designers to define schemas Data Manipulation Language (DML) View Definition Language (VDL): is to specify user views In current DBMS, the preceding types of languages are usually not considered distinct languages

DBMS Component Modules A DBMS is a complex software system The figure showed in next slide is divide into two halves. The top half refers to the various users of the database system The lower half shows the internals of the DBMS responsible for storage of data and processing of transactions

Typical DBMS Component Modules

Outline Data Models, Schemas, and Instances Three-Schema Architecture Database language and Interfaces Centralized and client/server architecture

Centralized DBMS Architecture A centralized DBMS in which all the DBMS functionality, application program execution, and user interface processing were carried out on a single machine

A Physical Centralized Architecture

Basic Client/Server Architectures The client/server architecture was developed to deal with computer environment in which a large number of PCs, workstation, file server… A client in this framework is typically a user machine that provides user interface capabilities and local processing A server is a system containing both hardware and software that can provide services to the client machines.

Logical two-tier client server architecture

Two-tier Architecture This is called two-tire architectures because the software components are distributed over two systems: client and server The emergence of the Web changed the roles of client and server, leading to the three-tier architecture

Three-tier architecture The intermediate layer or middle layer is sometimes called the application server or Web server Three-tier Architecture Can Enhance Security: 1. Database server only accessible via middle tier 2. Clients cannot directly access database server

Three-tier client-server architecture

Three-tier architecture The presentation layer displays information to the user The business logic layer handles intermediate rules and constrains before data is passed up to the user or down to the DBMS If the bottom layer is split into two layers (a web server and a database server), then it is a 4-tire architecture (possible to the n-tier)