ch02-Database System Concepts and Architecture.pdf

Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe
Chapter 2
Database System Concepts and
Architecture

Outline
 Data Models and Their Categories
 History of Data Models
 Schemas, Instances, and States
 Three-Schema Architecture
 Data Independence
 DBMS Languages and Interfaces
 Database System Utilities and Tools
 Centralized and Client-Server Architectures
 Classification of DBMSs

Data Models
 Data Model:
 A set of concepts to describe the structure of a database,
the operations for manipulating these structures, and
certain constraints that the database should obey.
 Data Model Structure and Constraints:
 Constructs are used to define the database structure
 Constructs typically include elements (and their data
types) as well as groups of elements (e.g. entity, record,
table), and relationships among such groups
 Constraints specify some restrictions on valid data; these
constraints must be enforced at all times

Data Models (continued)
 Data Model Operations:
 These operations are used for specifying database
retrievals and updates by referring to the
constructs of the data model.
 Operations on the data model may include basic
model operations (e.g. generic insert, delete,
update) and user-defined operations (e.g.
compute_student_gpa, update_inventory)

Categories of Data Models
 Conceptual (high-level, semantic) data models:
 Provide concepts that are close to the way many users
perceive data.
 (Also called entity-based or object-based data models.)
 Physical (low-level, internal) data models:
 Provide concepts that describe details of how data is stored
in the computer. These are usually specified in an ad-hoc
manner through DBMS design and administration manuals
 Implementation (representational) data models:
 Provide concepts that fall between the above two, used by
many commercial DBMS implementations (e.g. relational
data models used in many commercial systems).

Schemas versus Instances
 Database Schema:
 The description of a database.
 Includes descriptions of the database structure,
data types, and the constraints on the database.
 Schema Diagram:
 An illustrative display of (most aspects of) a
database schema.
 Schema Construct:
 A component of the schema or an object within
the schema, e.g., STUDENT, COURSE.

Schemas versus Instances
 Database State:
 The actual data stored in a database at a
particular moment in time. This includes the
collection of all the data in the database.
 Also called database instance (or occurrence or
snapshot).
 The term instance is also applied to individual
database components, e.g. record instance, table
instance, entity instance

Database Schema
vs. Database State
 Database State:
 Refers to the content of a database at a moment
in time.
 Initial Database State:
 Refers to the database state when it is initially
loaded into the system.
 Valid State:
 A state that satisfies the structure and constraints
of the database.

Database Schema
vs. Database State (continued)
 Distinction
 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.

Example of a Database Schema

Example of a database state

Three-Schema Architecture
 Proposed to support DBMS characteristics of:
 Program-data independence.
 Support of multiple views of the data.
 Not explicitly used in commercial DBMS products,
but has been useful in explaining database
system organization

 Defines DBMS schemas at three levels:
 Internal schema at the internal level to describe physical
storage structures and access paths (e.g indexes).
 Typically uses a physical data model.
 Conceptual schema at the conceptual level to describe the
structure and constraints for the whole database for a
community of users.
 Uses a conceptual or an implementation data model.
 External schemas at the external level to describe the
various user views.
 Usually uses the same data model as the conceptual schema.

The three-schema architecture

 Mappings among schema levels are needed to
transform requests and data.
 Programs refer to an external schema, and are
mapped by the DBMS to the internal schema for
execution.
 Data extracted from the internal DBMS level is
reformatted to match the user’s external view (e.g.
formatting the results of an SQL query for display
in a Web page)

Data Independence
 Logical Data Independence:
 The capacity to change the conceptual schema
without having to change the external schemas
and their associated application programs.
 Physical Data Independence:
 The capacity to change the internal schema
without having to change the conceptual schema.
 For example, the internal schema may be changed
when certain file structures are reorganized or new
indexes are created to improve database
performance

Data Independence (continued)
 When a schema at a lower level is changed, only
the mappings between this schema and higher-
level schemas need to be changed in a DBMS
that fully supports data independence.
 The higher-level schemas themselves are
unchanged.
 Hence, the application programs need not be
changed since they refer to the external schemas.

DBMS Languages
 Data Definition Language (DDL)
 Data Manipulation Language (DML)
 High-Level or Non-procedural Languages: These
include the relational language SQL
 May be used in a standalone way or may be
embedded in a programming language
 Low Level or Procedural Languages:
 These must be embedded in a programming
language

DBMS Languages
 Data Definition Language (DDL):
 Used by the DBA and database designers to
specify the conceptual schema of a database.
 In many DBMSs, the DDL is also used to define
internal and external schemas (views).
 In some DBMSs, separate storage definition
language (SDL) and view definition language
(VDL) are used to define internal and external
schemas.
 SDL is typically realized via DBMS commands
provided to the DBA and database designers

DBMS Languages
 Data Manipulation Language (DML):
 Used to specify database retrievals and updates
 DML commands (data sublanguage) can be
embedded in a general-purpose programming
language (host language), such as COBOL, C,
C++, or Java.
 A library of functions can also be provided to access
the DBMS from a programming language
 Alternatively, stand-alone DML commands can be
applied directly (called a query language).

Types of DML
 High Level or Non-procedural Language:
 For example, the SQL relational language
 Are “set”-oriented and specify what data to retrieve
rather than how to retrieve it.
 Also called declarative languages.
 Low Level or Procedural Language:
 Retrieve data one record-at-a-time;
 Constructs such as looping are needed to retrieve
multiple records, along with positioning pointers.

DBMS Interfaces
 Stand-alone query language interfaces
 Example: Entering SQL queries at the DBMS
interactive SQL interface (e.g. SQL*Plus in
ORACLE)
 Programmer interfaces for embedding DML in
programming languages
 User-friendly interfaces
 Menu-based, forms-based, graphics-based, etc.

DBMS Programming Language Interfaces
 Programmer interfaces for embedding DML in a
programming languages:
 Embedded Approach: e.g embedded SQL (for C,
C++, etc.), SQLJ (for Java)
 Procedure Call Approach: e.g. JDBC for Java,
ODBC for other programming languages
 Database Programming Language Approach:
e.g. ORACLE has PL/SQL, a programming
language based on SQL; language incorporates
SQL and its data types as integral components

User-Friendly DBMS Interfaces
 Menu-based, popular for browsing on the web
 Forms-based, designed for naïve users
 Graphics-based
 (Point and Click, Drag and Drop, etc.)
 Natural language: requests in written English
 Combinations of the above:
 For example, both menus and forms used
extensively in Web database interfaces

Other DBMS Interfaces
 Speech as Input and Output
 Web Browser as an interface
 Parametric interfaces, e.g., bank tellers using
function keys.
 Interfaces for the DBA:
 Creating user accounts, granting authorizations
 Setting system parameters
 Changing schemas or access paths

Database System Utilities
 To perform certain functions such as:
 Loading data stored in files into a database.
Includes data conversion tools.
 Backing up the database periodically on tape.
 Reorganizing database file structures.
 Report generation utilities.
 Performance monitoring utilities.
 Other functions, such as sorting, user monitoring,
data compression, etc.

Other Tools
 Data dictionary / repository:
 Used to store schema descriptions and other
information such as design decisions, application
program descriptions, user information, usage
standards, etc.
 Active data dictionary is accessed by DBMS
software and users/DBA.
 Passive data dictionary is accessed by
users/DBA only.

Other Tools
 Application Development Environments and
CASE (computer-aided software engineering)
tools:
 Examples:
 PowerBuilder (Sybase)
 JBuilder (Borland)
 JDeveloper 10G (Oracle)

Typical DBMS Component Modules

Centralized and
Client-Server 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.

A Physical Centralized Architecture

Basic 2-tier 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

Logical two-tier client server architecture

Clients
 Provide appropriate interfaces through a client
software module to access and utilize the various
server resources.
 Clients may be diskless machines or PCs or
Workstations with disks with only the client
software installed.
 Connected to the servers via some form of a
network.
 (LAN: local area network, wireless network, etc.)

DBMS Server
 Provides database query and transaction services to the
clients
 Relational DBMS servers are often called SQL servers,
query servers, or transaction servers
 Applications running on clients utilize an Application
Program Interface (API) to access server databases via
standard interface such as:
 ODBC: Open Database Connectivity standard
 JDBC: for Java programming access
 Client and server must install appropriate client module
and server module software for ODBC or JDBC
 See Chapter 9

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

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 the corresponding
data from the 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

Three-tier client-server architecture

Classification of DBMSs
 Based on the data model used
 Traditional: Relational, Network, Hierarchical.
 Emerging: Object-oriented, Object-relational.
 Other classifications
 Single-user (typically used with personal
computers)
vs. multi-user (most DBMSs).
 Centralized (uses a single computer with one
database)
vs. distributed (uses multiple computers, multiple
databases)

Variations of Distributed DBMSs
(DDBMSs)
 Homogeneous DDBMS
 Heterogeneous DDBMS
 Federated or Multidatabase Systems
 Distributed Database Systems have now come to
be known as client-server based database
systems because:
 They do not support a totally distributed
environment, but rather a set of database servers
supporting a set of clients.

Cost considerations for DBMSs
 Cost Range: from free open-source systems to
configurations costing millions of dollars
 Examples of free relational DBMSs: MySQL, PostgreSQL,
others
 Commercial DBMS offer additional specialized modules,
e.g. time-series module, spatial data module, document
module, XML module
 These offer additional specialized functionality when
purchased separately
 Sometimes called cartridges (e.g., in Oracle) or blades
 Different licensing options: site license, maximum number
of concurrent users (seat license), single user, etc.

History of Data Models
 Network Model
 Hierarchical Model
 Relational Model
 Object-oriented Data Models
 Object-Relational Models

 Network Model:
 The first network DBMS was implemented by
Honeywell in 1964-65 (IDS System).
 Adopted heavily due to the support by CODASYL
(Conference on Data Systems Languages)
(CODASYL - DBTG report of 1971).
 Later implemented in a large variety of systems -
IDMS (Cullinet - now Computer Associates), DMS
1100 (Unisys), IMAGE (H.P. (Hewlett-Packard)),
VAX -DBMS (Digital Equipment Corp., next
COMPAQ, now H.P.).

Example of Network Model Schema

Network Model
 Advantages:
 Network Model is able to model complex
relationships and represents semantics of
add/delete on the relationships.
 Can handle most situations for modeling using
record types and relationship types.
 Language is navigational; uses constructs like
FIND, FIND member, FIND owner, FIND NEXT
within set, GET, etc.
 Programmers can do optimal navigation through the
database.

Network Model
 Disadvantages:
 Navigational and procedural nature of processing
 Database contains a complex array of pointers
that thread through a set of records.
 Little scope for automated “query optimization”

 Hierarchical Data Model:
 Initially implemented in a joint effort by IBM and
North American Rockwell around 1965. Resulted
in the IMS family of systems.
 IBM’s IMS product had (and still has) a very large
customer base worldwide
 Hierarchical model was formalized based on the
IMS system
 Other systems based on this model: System 2k
(SAS inc.)

Hierarchical Model
 Advantages:
 Simple to construct and operate
 Corresponds to a number of natural hierarchically organized
domains, e.g., organization (“org”) chart
 Language is simple:
 Uses constructs like GET, GET UNIQUE, GET NEXT, GET
NEXT WITHIN PARENT, etc.
 Disadvantages:
 Navigational and procedural nature of processing
 Database is visualized as a linear arrangement of records
 Little scope for "query optimization"

 Relational Model:
 Proposed in 1970 by E.F. Codd (IBM), first commercial
system in 1981-82.
 Now in several commercial products (e.g. DB2, ORACLE,
MS SQL Server, SYBASE, INFORMIX).
 Several free open source implementations, e.g. MySQL,
PostgreSQL
 Currently most dominant for developing database
applications.
 SQL relational standards: SQL-89 (SQL1), SQL-92 (SQL2),
SQL-99, SQL3, …
 Chapters 5 through 11 describe this model in detail

 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.
 Chapters 20 and 21 describe this model.

 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.
 Chapter 22 describes this model.

Summary
 Data Models and Their Categories
 History of Data Models
 Schemas, Instances, and States
 Three-Schema Architecture
 Data Independence
 DBMS Languages and Interfaces
 Database System Utilities and Tools
 Centralized and Client-Server Architectures
 Classification of DBMSs

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