Database Management Systems-15CS33T 2020-21

DATABASE SYSTEM CONCEPTS AND

ARCHITECTURE

1.1 Introduction

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 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_std_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

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Provide concepts that fall between the above two, used by many commercial
DBMS implementations (e.g. relational data models used in many commercial
systems).
1.2 Classification of Data Models
Data Models

Physical Logical Based

Heaps Object Based Recorded Based

ISAM ER Models FILE
VSAM HIERARCHICAL
NETWORK
HASH
RELATIONAL
BTREE

There are basically two types of data model

1. Record based data model

2. Object based data model

Record based data model

When the database is organized in some fixed format of records of several than the
model is called record-based data model. A fixed number of fields, or attributes in
each record type and each field is usually of a fixed length. The three most popular
record-based data models are,

• File Management System (FMS)

File oriented approach of file File management system was the first method
used to store data in a computerized database, The data items are stored
sequentially in one large file, In order to lacate a particular data item or record
the search is used to begin sequentially from starting or first record till the
desired item is found.

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➢ Hierarchical Approach
• It represents relationship between two entities by means of links.
• The data is represented by a simple tree structure
• Data storage is in the form of a parent child relationship
• The origin of a data tree is root
• A tree can grow in size like a generations i.e father becomes
grandfather and so on..
• The last node in the tree is called root node
• This model supports one to many relationships
Problems with hierarchical model
• Redundancy, Complexity, many to many Relationship cannot be
represented
• Network Approach
o In the network data model, data model data are represented by
collections of records.
o Relationships among data are represented by links.
o In this data model, graph data structure is used. It permits a record to
have more than one parent. i.e many to many relationships can be
established.
o This approach uses pointer to locate the record.
Problems with network model
o Redundancy can be eliminated, complexity increases as the database
grows up

➢ Relational Approach

o The relational data model uses tables to represent the data and the
relationships among these data.
o Each table has multiple columns(attributes) and each column is
identified by a unique name.

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o A table is simply a collection of records. each data item or data field is

considered as column and each record is considered as a row (tuple) of
a table.

➢ Object based data model

o In the object-based data model, the database is organized in real-world
objects of several types. A number of fields or attributes are defined in
each object. The two most popular object-based data models are

Object oriented model

The object-oriented model is based on a collection of objects. An object

contains values stored in instances variable within the object. An object
contains bodies of code that operate on the object.

Advantages of object oriented data model

• It represents relationships explicitly supporting both navigated and

associative access to information.
• Object-oriented database systems are not suited for all applications.
• It is difficult to maintain when organizational information changes.

ER (Entity Relational model)

The entity relational data model based on the perception of the real world that
consist of a collection of basics objects and relationships between them. It is
an object-based logical model. It is also high-level data model.

1.3 Schemas versus Instances

• Database Schema
➢ The description of a database.

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➢ 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.
• 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.
• 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.

Schema Diagram
Student
regno name dob Age class

Instance of schema construct

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Student
12345 Asha 11/06/1978 42 V Sem CSE

1.4 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

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). 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 thevarious user views.
Usually uses the same data model as the conceptual schema.
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)

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Fig 1: Three Schema Architecture

1.4 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.
➢ 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.
1.5 DBMS Languages
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• Data Definition Language (DDL)

• Data Manipulation Language (DML)
➢ High-Level or Non-procedural Languages These include the relational
language SQL may be used in stand alone
o 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
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.
o SDL is typically realized via DBMS commands provided to the DBA
and database designers
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.

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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 ,
o 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
o (Point and Click, Drag and Drop, etc.)
▪ Natural language: requests in written English
▪ Combinations of the above:
o 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.

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▪ Interfaces for the DBA

o Creating user accounts, granting authorizations
o Setting system parameters

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.

▪ Application Development Environments and CASE (computer-aided

software engineering) tools:
▪ Examples:
o PowerBuilder (Sybase)
o JBuilder (Borland)
o JDeveloper 10G (Oracle)

Typical DBMS Component Modules

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Fig 2: DBMS components and their interactions

1.6 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

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Fig 3: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

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Logical two-tier client server architecture

Fig 4: Logical two tier 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.
o (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:
o ODBC: Open Database Connectivity standard
o JDBC: for Java programming access
▪ Client and server must install appropriate client module
and server module software for ODBC or JDBC

Two Tier Client-Server Architecture

▪ A client program may connect to several DBMSs, sometimes called the data
sources.

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▪ 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

Fig 5: Logical three tier architecture

1.7 Classification of DBMSs

▪ Based on the data model used
o Traditional: Relational, Network, Hierarchical.
o Emerging: Object-oriented, Object-relational.
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▪ Other classifications
o Single-user (typically used with personal computers) vs. multi-user
(most DBMSs).
o 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:
o 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
o These offer additional specialized functionality when purchased
separately
o Sometimes called cartridges (e.g., in Oracle) or blades
▪ Different licensing options: site license, maximum number of concurrent users
(seat license), single user, etc.
1.8 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).

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▪ 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- ackard)),
VAX -DBMS (Digital Equipment Corp., next COMPAQ, now H.P.).
Example of Network Model Schema

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.
o Programmers can do optimal navigation through the database.
Disadvantages:
▪ Navigational and procedural nature of processing
▪ Database contains a complex array of pointers that thread through a set of
records.
o Little scope for automated “query optimization”

Hierarchical Data Model

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▪ 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.)
Advantages
▪ Simple to construct and operate
▪ Corresponds to a number of natural hierarchically organized domains, e.g.,
organization (“org”) chart
▪ Language is simple:
o 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,Postgre SQL
Currently most dominant for developing database applications.
▪ SQL relational standards: SQL-89 (SQL1), SQL-92 (SQL2), SQL-99, SQL3,
…
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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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.

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
History of Data Models

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