A distributed system is a collection of independent computers that appears as a single coherent system to its users. It allows sharing of resources and workload across networked computers. Key characteristics include multiple autonomous components, lack of shared memory, and message-based communication. The World Wide Web is a large-scale distributed system that allows sharing of documents, files, and other resources across the internet through web servers and browsers. It faces challenges like heterogeneity, security, scalability, and fault tolerance.

1. Distributed Systems

2. UNIT-1
INTRODUCTION

3. Outline
• Definition of a Distributed System
• Needs of a Distributed System
• Examples of distributed system
• Trends in distributed system
• Focus on resource sharing
• Challenges
• Case study: WWW

4. What Is A Distributed System?
• A collection of independent computers that
appears to its users as a single coherent system.
• A distributed system is a set of networked
computers communicate and coordinate their
actions by passing messages.
• Features:
– No shared memory – message-based
communication
– Each runs its own local OS
– Heterogeneity

5. Distributed System
A distributed system organized as middleware.
Note that the middleware layer extends over multiple machines.
1.1

6. Need of distributed systems
• Resource sharing
• Fault tolerance
• Security
• Reliability
• Concurrent transactions

7. 1.2 Distributed System Characteristics
• Multiple autonomous components
• Components are not shared by all users
• Resources may not be accessible
• Software runs in concurrent processes on
different processors
• Multiple Points of control
• Multiple Points of failure

8. Advantages
• Economy- which increases computational
speed
• Speed- more number of system can be added
to increase computational speed.
• Reliability - works even in failure mode.
• Extensibility- computers and software can be
added incrementally.

9. Centralized system Distributed system
CS have non –autonomous components DS have autonomous system
Multiple users share the resources
centralized at all times
DS have components in concurrent
process
CS have a Single point of failure DS have multiple point of failure.

10. Advantages of Distributed vs
Standalone systems
• Data sharing- multiple users can access
common database, data files
• Extensibility- add resources and software's as
Needed.
Resource sharing- resources can be shared
across all the devices in the network

11. Examples of distributed systems
• Web search
• Massively multiplayer online games
• Financial trading

12. Web search
• searching is the most important activity in
web.
• Web search stores all formats of web pages.
• search engine is a computer program that
searches for particular keywords and returns a
list of documents.
– search by keywords
– Search by categories
– Specialize in language searching

13. Massively multiplayer online games
• A massively multi player online games which is
capable of supporting large number of players
simultaneously.
• These games can be able to co-operate and
compete each other in a large scale.
• Challenges:
– Fast response needed

14. Financial trading
• Finance industry used the wide application of
distributed system for real time access and
automation process.
• Share market which includes economic and
political development. Where in
communication and data process is a
distributed approach.

15. Applications
• Local Area Network and Intranet
• Database Management System
• Automatic Teller Machine Network
• Internet/World-Wide Web
• Mobile and ubiquitous computing
• Air traffic control

16. 2.1 Local Area Network
Name Server:
Machine -> IP address
User name -> user id
Group name -> group id
the rest of
email server
Web server
Desktop
computers
File serv er
router/firewall
print and other serv ers
other servers
print
Local area
network
email server
the Internet

17. 2.2 Database Management System

18. 2.3 Automatic Teller Machine Network

19. 2.4 Internet
intranet
ISP
desktop computer:
backbone
satellite link
server:
%
network link:
%
%
%

20. 2.4.1 World-Wide-Web

21. 2.4.2 Web Servers and Web Browsers
Internet
Browsers
Web servers
www.google.com
www.cse.cuhk.edu.hk
www.w3c.org
Protocols
Activity.html
http://www.w3c.org/Protocols/Activity.html
http://www.google.comlsearch?q=lyu
http://www.cse.cuhk.edu.hk/
File system of
www.w3c.org

22. Disadvantages
• Implementing Distributed systems is much
difficult
• security issues
• saturation

23. Trends in distributed system
• Stand-alone workstations delivering several
tens of millions of operations per second are
commonplace, and continuing increases in
power are predicted.
• When these computer systems are
interconnected by an appropriate high-speed
network, their combined computational
power can be applied to solve a variety of
computationally intensive applications

24. • pervasive networking and modern internet
• Mobile and ubiquitous computing
• distributed multimedia systems
• Distributed computing as a utility

25. pervasive networking and modern
internet
• Internet
– The modern Internet is a vast interconnected
collection of computer networks of many different
types, with the range of types increasing all the
time and now including
– for example, a wide range of wireless
communication technologies such as WiFi,
WiMAX, Bluetooth and third-generation mobile
phone networks.

27. Intranet

28. Mobile and ubiquitous computing
• Technological advances in device
miniaturization and wireless networking have
led increasingly to the integration of small and
portable computing devices into distributed
systems.

29. • Laptop computers.
• Handheld devices, including mobile phones,
smart phones, GPS-enabled devices, pagers,
personal digital assistants (PDAs), video cameras
and digital cameras.
• Wearable devices, such as smart watches with
functionality similar to a PDA.
• Devices embedded in appliances such as washing
machines, hi-fi systems, cars and refrigerators.

30. 2.5 Mobile and Ubiquitous Computing
Laptop
Mobile
Printer
Camera
Internet
Host intranet Home intranet
GSM/GPRS
Wireless LAN
phone
gateway
Host site

31. Distributed multimedia system
• Computer controlled integration of text,
graphics, still images, moving pictures,
animation, sound.
• it able to store and locate audio or video files
to transmit across the network to support the
presentation of the media types to the user
and optionally also to share media. Across a
group of users.

32. RESOURCE SHARING
• Is the primary motivation of distributed
computing.
• Resource sharing in a distributed system are
encapsulated within one computer and can
only be accessed from other computers by
communication.

33. • Ability to use any hardware, software or data
anywhere in the system.
• Resource manager controls access, provides
naming scheme and controls concurrency.
• Resource sharing model (e.g. client/server or
object-based) describing how
– resources are provided they are used
– provider and user interact with each other

34. • Resources types
• Hardware, e.g. printer, scanner, camera
• Data, e.g. file, database, web page
• More specific functionality, e.g. search engine,
file
• Service
• manage a collection of related resources and
present their functionalities to users and
applications

35. • Server
– process on networked computer that accepts
requests from processes on other computers to
perform a service and responds appropriately.
– Able to split the resources based on request.

36. • Hardware resources
– CPU
– Memory unit
– Disk
– Screen
– printer
• Software resources.
– Web pages
– Files
– Database
– Video/audio streams

37. THE CHALLENGES IN DISTRIBUTED
SYSTEM
• Heterogeneity
• Openness
• Security
• Scalability
• Failure handling
• Concurrency
• Transparency
• Quality of service

38. Heterogeneity
• The Internet enables users to access services and
run applications over a heterogeneous(mixed)
• collection of computers and networks.
Heterogeneity (that is, variety and difference)
applies to all of the following:
• networks;
• computer hardware;
• operating systems;
• programming languages;
• implementations by different developers

39. • Middleware services:
– COBRA(Common Object request Broker
Architecture)
– DCOM(Distributed Component Object Model)

40. Openness
• The openness of a computer system is the
characteristic that determines whether the
system can be extended and reimplemented
in various ways.
• The openness of distributed systems is
determined primarily by the degree to which
new resource-sharing services can be added
and be made available for use by a variety of
client programs.

41. Security
• Many of the information resources that are made available
and maintained in distributed systems have a high intrinsic
value to their users. Their security is therefore of
considerable importance.
• Security for information resources has three components:
confidentiality (protection against disclosure to
unauthorized individuals),
• integrity (protection against alteration or corruption),
• availability (protection against interference with the means
to access the resources).

42. Scalability
• A system is said to be scalable of it handle the
addition of users and resources without
suffering loss of performance or increase in
complexity.
– In size
– In location
– In administration (regions and ownership)

43. Fault Handling
• Computer systems sometimes fail. When
faults occur in hardware or software,
programs may produce incorrect results or
may stop before they have completed the
intended computation.
– Denial of service

44. Concurrency
• Components executed in distributed system in
concurrent process.
• It should be able to handle several clients will
attempt to access a shared resources at same
time.
• Multiple resources make request to read,
write, and update operation.

45. Transparency
• Transparency is defined as the concealment
from the user and the application
programmer of the separation of components
in a distributed system, so that the system is
perceived as a whole rather than as a
collection of independent components.
• The implications of transparency are a major
influence on the design of the system
software.

46. • Access transparency
– enables local and remote resources to be
accessed using identical operations.
• Location transparency
– enables resources to be accessed without
knowledge of their physical or network location

47. CASE STUDY:WWW
• www is an largest application of distributed
systems for publishing and accessing
resources and services across the internet
• www is a collection of millions of file stored
on thousands of servers over the world. These
files contain documents, files, pictures, video
and audio files.

48. • A web server is a computer connected to
internet that runs a program takes the
responsibility for storing, retrieving and
distributing contents on the web.
• HTML is the universal language used in web.
• Java scripts, VB script are some of the
language used in web.

49. • A web page is an HTML document that stored
on web server, a web site is a collection of
web pages that are interlinked to each other.
• A web browser is a program which is used to
communicate with the web servers on
internet.
• Web crawlers (Spider)is program used by
Google.
• Internet explorer, Netscape navigator, Mozilla
Firefox, are some of the example of web
browser.