Chapter 01: Character of Distributed

Systems

1

Distributed Computing

BOOKS

LPU2

TEXT BOOKDistributed Systems: Concepts and Design

(Fourth/Third Edition) By George Coulouris, Jean Dollimore and Tim KindbergAddison-Wesley, ©Pearson Education 2001

REFERENCE BOOKSDistributed Systems: Principles and

Paradigms, By Andrew S. Tanenbaum and van Steen

Distributed Operating System and Algorithms, By Randy Chow & Theodore Johnson.

Definition of Distributed System

LPU3

A distributed system is:Components located at networked computers

communicate and coordinate their actions by passing messages

A distributed system is:Collection of independent computers that

appear to the users of the system as a single computer [Tanenbaum]

Definition of Distributed System

LPU4

Machine B

Local OS

Machine A

Local OS

Machine C

Local OS

Network

Distributed applications

Middleware service

Definition of Distributed System

LPU5

"a collection of autonomous computers linked by a computer network with distributed system software"

"a collection of processors interconnected by a communication network in which each processor has its own local memory and other peripherals and the communication between any two processors of the system takes place by message passing over the communication network"

"one in which hardware or software components located at networked computers communicate and coordinate their actions only by passing messages"

Definition of Distributed System

LPU6

In other words, to the user, a DS looks like a single system a virtual uniprocessor

The user doesn't know (or care) - where (on what machine) files are located - where a job is executed

A Distributed System is made of several computers which have no shared memory have no shared clock communicate with each other via messages have their own operating systems

Advantages

LPU7

Why distributed systemsResource sharingCommunicationInherent distributionSpeed

10000 CPU running at speed of 50 MIPS 500000 MIPS

EconomyIncremental growthReliability

Characteristics of DS

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ConcurrencyDistributed systems are concurrent systems

Every software or hardware components is autonomous Process

Concurrent tasks A and B are concurrent if either A can happen before B

or B can happen before ASynchronization and coordination by message

passingLack of global clock

Coordination and timeIndependent failure

Failures of individual processes may remain undetected

Examples of Distributed Systems1. The internet

LPU9

Heterogeneous network of computers and applications

Implemented through Internet protocol

intranet

ISP

desktop computer:

backbone

satellite link

server:

network link:

Examples of Distributed Systems 2. Distributed multimedia

LPU 10

•Often use the Internet infrastructure

•Characteristics

•Heterogeneous data sources that need to be synchronized in real time

•Video, Audio, Text

•Often: distribution services

•multicast

•Examples•Tele teaching, Video conferencing

Examples of Distributed Systems3. Intranet

LPU11

•Locally administered network

•Usually proprietary

•Interfaces with the Internet via router

•Firewall

the rest of

email server

Web server

Desktopcomputers

File server

router/firewall

print and other servers

other servers

print

Local areanetwork

email server

the Internet

IssuesFile services need to share dataFirewall impede Legitimate access

Examples of Distributed Systems (1) 4. Mobile and Ubiquitous computing

LPU12

Laptop

Mobile

PrinterCamera

Internet

Host intranet Home intranetWAP

Wireless LAN

phone

gateway

Host site

•Resource Sharing while on the move

•Primary concerns: •resource discovery

•efficient use of limited bandwidth

•security (privacy)

Resource sharing and the Web Resource sharing pattern

LPU 13

•Search engine•CSCW

•Computer-supported cooperative work•Service

•Software that manages a collection of resources and presents them to users and applications via a well-defined interface

•Server•Running program (a process that provide one or more services)

•Remote invocation•Client’s request to server.

Challenges

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A number of challenges arises:

• Heterogeneity

• Openness

• Security

• Scalability

• Failure handling

• Concurrency of components

• Transparency

Challenges

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Heterogeneitynetworkscomputer hardwareOperating systemsProgramming languages

MiddlewareTo over come differences in network, OS

and languagesCORBA

Challenges

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OpennessEnsures extensibility and maintainability of

the systemStandard interfaces & their publicationAddition of new resources

RFC (Request for comments) specification for internet protocols. www. ietf.org

Benefits of ODSkey interfaces are publisheduniform communication mechanisms and

access to shared resourcesConstruction

Heterogeneous components

Challenges

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SecurityconfidentialityintegrityavailabilityExample

A doctor might request access to hospital patient data

In electronic commerce and banking, users send their credit card numbers across the internet

Current security challengesDenial of service attackSecurity of mobile code

Challenges

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ScalabilityControlling the cost of the physical

resourcesDoes the system remain effective given the

expected growthControlling the performance loss

www.amzon.com is more than one computerPreventing the software resources running

outIP addressees :32 bits to 128Avoiding performance bottleneck

Decentralization of data/information

Challenges

LPU19

Computers vs. Web servers in the Internet

Date Computers Web servers Percentage

1993, July 1,776,000 130 0.008

1995, July 6,642,000 23,500 0.41997, July 19,540,000 1,203,096 61999, July 56,218,000 6,598,697 12

Mid. 2002 Internet :

150 million+ computers

667 million people

2.5 billion+ web pages

Challenges

LPU20

Failure handlingDetecting failure

Checksum can be used to detect corrupted data

System crash (impossible)Masking

RetransmissionRedundant storage

Tolerating failureException handling (timeout when waiting for

web source)

Challenges

LPU21

Failure handlingRecovery from failure

Roll backRedundancy

Redundant routes in networkReplication of name tables in multiple domain

name serversDatabase replication

Availability Measure of the proportion of the time a server

is available

Challenges

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ConcurrencyConsistent scheduling of threads (so

that dependencies are preserved in concurrent transactions)

Avoidance of deadlockTransparency

Concealing the heterogeneous and distributed nature of the system so that it appears to the user like one system

Challenges :Transparency

LPU23

Access transparency: enables local and remote resources to be accessed using identical operations.

Location transparency: enables resources to be accessed without knowledge of their location.

Concurrency transparency: enables several processes to operate concurrently using shared resources without interference between them.

Replication transparency: enables multiple instances of resources to be used to increase reliability and performance without knowledge of the replicas by users or application programmers.

Failure transparency: enables the concealment of faults, allowing users and application programs to complete their tasks despite the failure of hardware or software components.

Mobility transparency: allows the movement of resources and clients within a system without affecting the operation of users or programs.

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.