part 1 introduction to distributed computing

8/14/2019 Part 1 Introduction to Distributed Computing

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Chapter 1:Part 1

Introduction to Distributed

Computing


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Contents

Definition

Examples of DS

Advantages and Disadvantages

Key characteristics

Challenges in designing DS


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Definition

Coulouris - A system

in which hardware orsoftware components

located at networked

computers

communicate and

coordinate their actions

only by message

passing


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Tanenbaum - A

distributed system is acollection of

independent

computers that appear

to the users of the

system as a single

computer


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A distributed system is a collection of autonomous

computers linked by a network with softwaredesigned to produce an integrated computingfacility

Sharing of resources is a main motivation forconstructing DS

Resources may be managed by servers and accessedby clients or they may be encapsulated as objects andaccessed by other client objects

The term resource extends from hardware such as

disks, printers, to software defined entities such asfiles, databases and data objects of all kinds.

Also includes video streams and audio streams


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Reasons for DS

DS consisting of collections of microcomputers may

have processing powers that no supercomputer willever achieve

10,000 CPUs, each running at 50 MIPS* yields500000 MIPS, then instructions to be executed in

0.002 nsec, equivalent to light distance of 0.6 mm -any processor chip of that size would meltimmediately.

Collection of microprocessors offer a better price/

performance ratio than main frames- main frames: 10times faster, 1000 times as expensive

(*) MIPS million instructions per second


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Why DS and not

isolated

hardware?

Enhance person to

personcommunication

Need to share data

and resourcesamongst users

Flexibility: different computers with different

capabilities can be shared among users


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Problems with distributed, connected systems

Software - how

to design and

manage it in a

DS

Dependency

on the

underlying

infrastructure

Easy access to

shared data

raises security

concerns


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Example of DS - Internet

intranet

ISP

desktop computer:

backbone

satellite link

server:

network link:

Figure 1.1: A typical portion of the Internet - Coulouris


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Vast collection of interconnected computer networks

of many different types programs on various computers interact by passing

messages as a means of communication

the design and construction of the internetcommunication mechanisms (the internet protocols)

is a major technical achievement enabling a program

running anywhere to address messages programs

anywhere else.


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As shown in Figure 1.1, a collection of intranets,

subnets are operated by organizations. ISPs arecompanies that provide modem links and otherconnection to individual users and smallorganizations enabling the users to access internetservices.

Intranets are linked together by backbones. abackbone is a new link with a high transmissioncapacity, employing satellite connections, fiber opticcables and other high bandwidth circuits

Multimedia services are available in the internet, butquite limited as the internet does not have facilities toreserve network capacities for individual streams ofdata


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Example of DS - Intranet

the rest of

email server

Web server

Desktopcomputers

File server

router/firewall

print and other servers

other servers

print

Local area

network

email server

the Internet

Figure 1.2: A typical intranet - Coulouris


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Example of DS - Intranet

Portion of the internet is intranet, but separately

administered and has a boundary that can beconfigured to enforce local security policies.

Composed of several LANs linked by backboneconnections

intranet is connected to internet via routers. Manyorganizations need to protect their own services

Firewall is to protect an intranet by preventingunauthorized messages leaving or entering


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Example of DS - DMS

Often use internet infrastructure

characteristics: Heterogeneous data sources andsinks that need to be synchronized in real time

(Video, Audio, Text)

Often distribution services- Multicast

Examples:

1. Tele-teaching

2. Video-conferencing

3. Video and audio on demand


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Example of DS - Mobile and Ubiquitous systems

Laptop

Mobile

Printer

Camera

Internet

Host intranet Home intranetWAP

Wireless LAN

phone

gateway

Host site

Figure 1.3: Portable and handheld devices in a distributed system -

Coulouris


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Integration of small and portable devices such as

Laptop computers

Handheld devices including PDAs, mobile phones,

pagers, cameras

wearable devices such as smart watches withfunctionality similar to a PDA

Devices embedded in appliances such as washing

machines, cars, and refrigerators are integrated into

DS


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In mobile computing, users who are away from their

home intranet are still provided with access toresources via the devices they carry with them. They

can continue to access the Internet.

Ubiquitous computing:

It would be convenient for users to control their washing

machine etc from a universal remote control device in the

home.

Equally the washing machine could page the user via a

smart badge when the washing is done.


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As shown in Figure 1.3, the user has access to three

forms of wireless connection1. their laptop has a means of connecting to the hosts

wireless LAN. This provides coverage for few hundred

meters

2. It connects to the rest of intranet through gateway

3. Mobile users are connected to the internet through WAP

via a gateway

users carries a digital camera, which can

communicate over a infrared link when pointed at

corresponding device such as printer.


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Other examples of DS

MMU campus IT network, where each faculty has a pool of computers linked together in

a LAN , sharing common resources, such as email server, web server and ftp server.


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Advantages of DS

1. Extensibility: can be

extended as the demand for

service grows with out replacingany of the existing components

2. Resource sharing: Extensive

hardware can be shared among users.

Data files can be managed andmaintained centrally, but remains

accessible to all users

3. Replication: High reliability

achieved by maintaining several

copies of data in different

computers

4. Continued availability: the

failure of a single workstation does

not affect the service to all users


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Disadvantages of DS

1. Loss of flexibility in allocation of memory and processing resources: In a centralized

system, all processor and memory resources are available for allocation by the OS in any

manner required by the current workload. In DS, the processor and memory capacity of theworkstation determine the largest task that can be performed.

2. Dependence on network performance and reliability: failure of the local network can

cause the service to users to be interrupted. Overloading of the network can degrade theperformance and responsiveness to the users.

3. Security weakness: To achieve openness, many of the software interfaces are made readily

available to clients. This leads to security problems


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Challenges

1. HETEROGENEITY: Internet enables users to access services

and run applications over a heterogeneous collection ofcomputers and networks.

Heterogeneity applies to the following:

computer networks

computer hardware

OS

Programming languages

Implementation by different developers

Middleware: It applies to a software layer that provides a

programming abstraction as well as masking the heterogeneity of

the underlying networks, hardware, OS and PLs.

Example: CORBA, Java RMI


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2. OPENNESS: The openness is determined by the degree to

which the new resource sharing services can be added andbe made available for use by a variety of client programs.

Openness cannot be achieved unless the specification and

documentation of the key software interfaces are

published. Open DS constructed from heterogeneous hardware and

software possibly from different vendors.


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3. SECURITY: There are 3 components:

i. confidentiality (protection against disclosure to unauthorizedindividuals)

ii. Integrity (protection against alteration or corruption)

iii. Availability (protection against interference with the means to accessthe resources).

Two security challenges that is yet to be met:

1. Denial of service attacks: Here the users may disrupt a service forsome reason. Achieved by bombarding the service with such a largenumber of pointless requests that the serious users are unable to useit. This is denial of attack

2. Security of mobile code: Assume some one who receives the EXEfrom email as attachment: it may access local resources, or denial ofservice attack.


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4. SCALABILITY:

DS is scalable if it remains effective when there is asignificant increase in the number of resources and the

number of users.

Date Computers Web servers

1979, Dec. 188 0

1989, July 130,000 0

1999, July 56,218,000 5,560,866

2003, Jan. 171,638,297 35,424,956

Figure 1.5: Computers in the Internet - Coulouris


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Design of Scalable DS presents challenges given

below1. Controlling the cost of physical resources: as a demand

of the resource grows, it should be possible to extend the

system, at reasonable cost. In general, for a system with

n users to be scalable, then the quantity of the physicalresources required to support them should be at most

O(n)- that is proportional to n. Ex: 20 users- 1 server,

then 40 users- 2 server and so on.

2. Controlling the performance Loss: In managing thedata the time taken to access data is O(log n), where n is

the size of the data. For a system to be scalable, the

maximum performance loss should be no worse than this.


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3. Preventing software resources running out: An

example of lack of scalability is shown by the numbersused in the Internet. In late 1970s, it was decided to use

32-bit. It run out in early 2000s. New version use 128 bit

Internet address. The demand is difficult to predict. But

too large Internet address occupy extra space in messages

and in computer storage.

4. Avoiding bottleneck: Algorithms to be decentralized to

avoid having performance bottleneck. Example : Domain

Name System can be kept in a single master file and

downloaded to any computers when needed. But this isefficient only when the number of computers are less. If

more, then it is to be decentralized.


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Date Computers Web servers Percentage

1993, July 1,776,000 130 0.008

1995, July 6,642,000 23,500 0.4

1997, July 19,540,000 1,203,096 6

1999, July 56,218,000 6,598,697 12

2001, July 125,888,197 31,299,592 25

42,298,371

Figure 1.6: Computers vs. Web servers in the Internet - Coulouris


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5. FAILURE HANDLING: Failures in DS is partial - that is

some components fail while others continue to functions.Therefore the handling of failures is difficult.

Detecting failures: Some failures may be detected e.g. check sums to

be used to find the error in a message. But difficult to detect the

failures such as remote crashed server in the Internet. The challenge

of DS is to manage in the presence of failures that cannot be detected,

but may be suspected

Masking failures: some failures may be detected but can be hidden

or made less severe. Examples.

1. Messages can be retransmitted

2. File data can be written to a pair of disks so that if one is corrupted, the

other may still be correct


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Tolerating failures: example: when a web browser cannot

contact the web server, it does not make the user wait for ever

while it keeps on trying- it informs the user about the problem

Recovery from failures: Recovery involves the design of the

software so that the state of the permanent data can be recovered

or rolled back after the server has crashed.

Redundancy: services may be made to tolerate failures by the

use of redundant components. Examples:

1. Always two different routes between any two routers in Internet

2. In the DNS, every name table is replicated in at least two different

servers

3. A database may be replicated in several servers to ensure that data

remains accessible after the failure of the single server


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Concurrency: since 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 DS must be responsible for ensuring that it operatescorrectly in a concurrent environment.

For an object to be safe in a concurrent environment, its operations must

be synchronized in such a way that its data remains consistent.

This can be achieved by standard techniques such as semaphores.

Example: IF TWO CONCURRENT BIDS ARE A: 15 and B: 56 AND

THE CORRESPONDING OPERATIONS ARE INTERLEAVED

WITHOUT ANY CONRTOL, THEN THEY MIGHT GET STORED AS

A:56 and B:15.

It is possible that several threads may be executing concurrently within

an object and in which case their operations on the object may conflict

with one another and produce inconsistent results as above.

T i


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Transparencies Access transparency: enables local and remote resources to be accessed

using identical operations.

Location transparency: enables resources to be accessed withoutknowledge of their physical or network location (for example, whichbuilding or IP address).

Concurrency transparency: enables several processes to operateconcurrently 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 thereplicas by users or application programmers. Failure transparency: enables the concealment of faults, allowing users

and application programs to complete their tasks despite the failure ofhardware 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 toimprove performance as loads vary.

Scaling transparency: allows the system and applications to expand inscale without change to the system structure or the application algorithms.

C


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Consequences

1. Concurrency:

In a network of computers, concurrent program executionis the norm. (ex. Sharing of files from common server).

The coordination of concurrently executing programs thatshare the resources is an important one

1. No Global clock:

When programs need to cooperate, they coordinate theiractions by exchanging messages.

Close coordination often depends on a shared idea of thetime at which programs actions occur.

But it turns out that there are limits to the accuracy with

which the computers in a network can synchronize theirclocks.

There is no single global notion of the correct time.


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3. Independent Failures:

DS can fail in new ways. Faults in the network result inthe isolation of the computers that are connected to it, but

does not mean that they stop running.

In fact, the programs on them may not be able to detect

whether the network has failed or has become unusuallyslow

Each component can fail independently leaving the

others still running

Cl k h i ti


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Clock synchronization

With a single computer and a single clock, it does not matter

much if this clock is off by a small amount.

As soon as multiple CPUs are introduced, each with its ownclock, the situation changes. Although the frequency at whicha crystal oscillator runs is usually fairly stable, it is impossible

to guarantee that the crystals in different computers all run atexactly the same frequency. This leads to the clocks to get outsynchronization and different values when read out. It is calledclock skew.

As a consequence of this clock skew, programs that expect thetime associated with a file, object process or message to becorrect and independent of the machine on which it wasguaranteed can fail.


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1. Computer on which compiler runs

2. Computer on which editor runs

Output.ocreated

Output.ccreated

2144 2145 2146 2147

2142 2143 2144 2145

Figure 1.7: when each machine has its own clock, an event that

occurred after another event may nevertheless be assigned an earlier

time


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Let output.o has time 2144 and shortly thereafter output.c ismodified but is assigned a time 2143 because the clock of its

machine is slightly behind. Make will not call the compiler. The resulting exe binary

program will then contain a mixture of object files from theold sources and the new sources.

It will probably crash and the programmer will go crazy trying

to understand what is wrong with the code. Another case: programmer has finished changing all the

source files, he starts make, which examines the times atwhich the source and object files were last modified.

If the source file input.c has time 2151 and the corresponding

object file input.o has time 2150, make knows that input.c hasbeen changed since input.o was created, and thus input.c mustbe recompiled.


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For ordinary clocks based on quartz crystal, this drift is about 10 -6

seconds/ second- giving a difference of 1 second in every 11.6days.

High precision quartz clock: 10-7 to 10-8

More accurate physical clocks use atomic oscillators whose

drift rate is about one part in 1013


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2. Resource sharing

1. Openness3. Concurrency

4. Scalability

5. Fault tolerance

6. Transparency

Key

characteristics

C t N t k d DS


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Computer Networks and DS

Computer Network: The autonomous computers are

explicitly visible (have to be explicitly addressed) Distributed system: Existence of multiple

autonomous computers is transparent

However

many problems in common

in some sense networks are also DS

Normally every DS relies on services provided by acomputer network

part 1 introduction to distributed computing

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