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05/04/2012 Prof. S.K.Lenka/MITSUniversity/Rajasthan-332311

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Distributed Systems:

Concepts and DesignUNIT-I

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2Prof. Dr. S.K.Lenka/ Deptartment of CSE/ MITS, Lakshmangarh, Rajsthan

CONTENTS:Distributed System covers

Most Interrelated TopicsDistributed System and its Characteristics

System Models

Distributed objects and remote invocation

Time and global state

Distributed shared memory

Distributed database systems

Processes and processor in distributed systems

Distributed file systems

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Books

Books:

Distributed systems by A.S.Tanenbaum, and Maarten VanSteen, LPE, Pearson Education Asia, 2002.

Distributed systems by G. Coulouris , J. Dollimore, T.Kindberg, IE, Pearson Education Asia.

Distributed Operating System by Sivratri, MGH.

Distributed systems by B. Dietel, Pearson Education Asia.

Usefulness of the distributed systems and applications.

Complete over view of the all the above chapters.

All the best/ Good luck

Prof. Dr. S.K.Lenak/ MITS-Deemed University, Rajasthan

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Most Interrelated Topics

Out of so many computing systems these two are most interrelated

topics.Multiprocessor Systems

Distributed Systems

What is a distributed system?

Each system of the distributed systems is completely autonomousindependent system, connected to others on the network.

There are several levels of distribution, such as loosely coupled,

closely coupled and tightly coupled.

Earliest systems used simple network programs:

FTP (File transfer programs)

Telnet (rlogin): remote login

Mail

Remote job entry: run jobs remotely

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Introduction to DSYS

What are different commonly available distributed systems?

All most all the systems used now days are distributed systems, such as:Emails

Access files over the network

Access printers over the network

Backed up over the networkShare other physical and logical resources

Cooperate with other people on the other machines

Receive audio, video, image and picture

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Why to use distributed systems?

Now days distributed systems are widely used. Economic (small computers very much cost effective)

Resource sharing Sharing and printing files at remote sites

Processing information in distributed database

Using remote specialized hardware devices

Many applications are natured distributed ATM (automatic teller machine)

Airtrip reservation System

Ticket purchasing

Shopping

Computation Speedup (Performance) using parallel programming. Using many cooperating machines and PCs can solve largest, complicated,

complex and data intensive problems.

Reliability


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A distributed system is one in which components located at networked

computers communicate and coordinate their actions only by passingmessages.

Ubiquitous Network:

Internet

Mobile phone networksCorporation networks

Factory networks

Campus networks

Home networks

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Internet:

The Internet is a very large distributed system.

The largest network of networks in the world.

Uses TCP/IP protocols and packet switching.

Runs on any communications substrate.

The implementation of the Internet and the services that it suports

has entailed the development of practical solutions to many

distributed system issues.

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A network of networks, joining many government, university and

private computers together and providing an infrastructure for the use of

E-mail, bulletin boards, file archives, hypertext documents, databases

and other computational resources

The vast collection of computer networks which form and act as a single

huge network for transport of data and messages across distances which

can be anywhere from the same office to anywhere in the world.

Internet:

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Fig. Typical Internet

Internet:

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An intranet is a portion of the Internet that is separately administered

and has a boundary that can be configured to enforce local security

policies.

The main issues arising in the design of components for use in intranetsare: file services, firewalls, and cost.

Intranets

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Intranets

Fig. Typical Intranet

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Mobile and ubiquitous computing Mobile devices

Laptop computers

Handheld devices

e.g.PDA, mobile phone, pager, video camera, digitalcamera

Wearable devices

e.g. smart watches, digital glasses

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A wristwatch that retrieves weather, messages, remindersand stock quotes from Microsoft's MSN Direct wirelessservice, which transmits over an unused portion of the FMradio band. Introduced in 2004, the Smart Watch is partof Microsoft's Smart Personal Objects Technology (SPOT)initiative. Manufacturers such as Fossil, Tissot and Swatchmake the watches, and users require an MSN Directsubscription to download data and a Web browser to set

up and customize their accounts.

Smart watches

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Network appliances

e.g. washing machines, hi-fi systems, cars and refrigerators

Mobile computing (nomadic computing)

Access resources while on the move or in an unusual

environment Location-aware computing: utilize resources that are

conveniently nearby

Ubiquitous computing (pervasive computing):

Calm technologies and the technology of virtually invisible.

The harnessing of many small, cheap computational devices

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Fig. Portable and handheld devices in a distributed system

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The portability of the devices, such as laptop computers, PDA, mobil phone,

refrigerators, togather with their ability to connect conveniently to networks in differentplaces, makes mobile computing possible.

Ubiquitous computing is the harnessing of many small cheap computational devices

that are present in users physical environments, including the home, office and

elsewhere.

Mobile and ubiquitous computing raise significant system issues presents an

architecture for mobile compuing and outlines the issues that arise from it, including

how to support the discovery of resources in a host environment; eliminating the need

for users to reconfigure their mobile devices as they move around; helping users to cope

with limited connectivity as they travel; and providing privacy and other security

guarantees to users and the environments that the visit.

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Introduction to DSYSIssues in the design of components for the use in Mobile and ubiquitous computing:

Discovery of resources

Eliminating the need for users to reconfigure their mobile devices

To cope with limited connectivity as they travel

Provide privacy and other security guarantees

Definitions of UbiComp:

Ubiquitous computing is the method of enhancing computer use by making many

computers available throughout the physical environment, but making them effectively

invisible to the user.

Generic Features:

1. Transparent Interfaces

2. Awareness of Context(s)3. Capture Experience

Example: eclass

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Significant Consequences of DS /Characteristics of Distributed System

Concurrency:

The capacity of the system to handle shared resources can be increased by adding

more resources to the network.

No global clock:

The only communication is by sending messages through a network.

Independent failures:

The programs may not be able to detect whether the network has failed or hasbecome unusually slow.

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ResourceThe term resource is a rather abstract one, but it bestcharacterizes the range of things that can usefully be shared in a

networked computer system. It extends from hardware components

such as disks and printers to software-defined entities such as files,

databases and data objects of all kinds.

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Resource sharing:

Is the primary motivation of distributed computing Resources types

Hardware, e.g. printer, scanner, camera

Data, e.g. file, database, web page

More specific functionality, e.g. search engine, file

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Important Terms of WebServicesA distinct part of a computer system that manages a collection of related resources

and presents their functionality to users and applications.

http, telnet, pop3...

Server

A running program (a process) on a networked computer that accepts requestsfrom programs running on other computers to perform a service, and responds

apppropriately.

IIS, Apache...

Client

The requesting processes.

Remote invocation

A complete interaction between clientand server, from the point when the client

sends its request to when it receives the servers response

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The World Wide Web:

The WWW is an evolving system for publishing and accessing resources and servicesacross the Internet.

The Web is an open system:it can be extended and implemented in new ways without

disturbing its existing functionality.

Its operation is based on communication standards and document standards tht are

freely published and widely implemented.

The Web is one with respect to the types of resource that can be published and shared

on.

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HTTP URLs are the most widely used

An HTTP URL has two main jobs to do: To identify which web server maintains the resource

To identify which of the resources at that server

Scheme: scheme-specific-location

e.g:mailto:[email protected]

ftp://ftp.downloadIt.com/software/aProg.exe

http://net.pku.cn/

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HTTP URLs:

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More Challenges

Following are the challenges:

Heterogeneity

OpennessSecurity

Scalability

Failure handling

Concurrency

Transparency

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05/04/2012 Prof. S.K.Lenka/MITSUniversity/Rajasthan-332311 32


Openness Openness of a computer system

-- is the characteristic that determines whether the system can be extended and

re-implemented in various way.

e.g. Unix

Openness of distributed systems

-- is determined by the degree to which new resource sharing services

can be added and be made available for use by A variety of client

programs.

e.g. Web

How to deal with openness?-- key interfaces are published, e.g. RFC

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The openness of DS 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.

Open systems are characterized by the fact that their key

interfaces are published.

Open DS are based on the provision of a uniform communication

mechanism and published interfaces for access to shared resources.

Open DS can be constrcted from heterogeneous hardware and

software.

Openness

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Confidentiality

protection against disclosure to unauthorized individuals,

e.g. ACL in Unix File System (Access Control Lists)

Integrity

protection against alteration or corruption, e.g. checksum Availability

protection against interference with the means to access the

resources,

e.g. Denial of service

Security

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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.

Two new security challenges:Denial of service attacks (DoS).

Security of mobile code.

Security

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A system is described as scalable

if will remain effective when there is a significant

increase in the number of resources and the number of users

A scalable example system: the Internet

design challenges

The cost of physical resources

The performance loss

Prevent software resources running out, e.g., IP address

Avoid performance bottlenecks, e.g., partitioning nametable of DNS, cache and replication

Scalability

i S S

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Scalability

A system is described as scalable if it remains effective when

there is a significant increase in the number of resources and

the number of users.

Challenges:

Controlling the cost of resources or money.

Controlling the performance loss.Preventing software resources from running out

Avoiding preformance bottlenecks.

I d i DSYS

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Failure handling

Failure handling:

Detecting

e.g. checksum for corrupted data

Sometimes impossible so suspect, e.g. a remote crashed server in the Internet

Masking

e.g. Retransmit message, standby server

Tolerating

e.g. a web browser cannot contact a web server

Recovery

e.g. Roll back

Redundancy e.g. IP route, replicated name table of DNS

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When faults occur in hardware or software, programs may produce

incorrect results or they may stop before they have completed the

intended computation.

Techniques for dealing with failures:Detecting failures

Masking failures

Tolerating failures

Recovering form failuresRedundancy

Failure handling

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There is a possibility that several clients will attempt to access a shared resource at

the same time.

Any object that represents a shared resource in a distributed system must be

responsible for ensuring that operates correctly in a concurrent environment.

Correctness

ensure the operations on shared resource correct in a concurrent environment

e.g. records bids for an auction

Performance

Ensure the high performance of concurrent operations

Concurrency

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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.

Eight forms of transparency:

Access transparency

Location transparency

Concurrency transparency

Replication transparency

Failure transparency

Mobility transparencyPerformance transparency

Scaling transparency

Transparency

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Transparency Access transparency

using identical operations to access local and remote resources, e.g. a

graphical user interface with folders

Location transparency

resources to be accessed without knowledge of their location, e.g. URL

Concurrency transparency several processed operate concurrently using shared resources without

interference with between them

Replication transparency

multiple instances of resources to be used to increase reliability and

performance without knowledge of the replicas by users or applicationprogrammers,

e.g. realcourse(http://vod.yf.pku.edu.cn/)

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Transparency

Failure transparency

users and applications to complete their tasks despite the failure of

hardware and software components, e.g., email

Mobility transparency

movement of resources and clients within a system without affecting theoperation of users and programs, e.g., mobile phone

Performance transparency

allows the system to be reconfigured to improve performance as loads vary

Scaling transparency

allows the system and applications to expand in scale without change to

the system structure or the application algorithms

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05/04/2012 Prof. S.K.Lenka/MITS

University/Rajasthan-332311

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Distributed systems are everywhere

Internet, intranet, wireless networks.

Resource sharing is the main motivating factor for constructing

distributed systems.The constrcution of distributed systems produces many challenges.

Summary

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