01 Characterization of Distributed Systems

8/8/2019 01 Characterization of Distributed Systems

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Chapter 1: Characterization of DistributedSystems

• Introduction• Examples of distributed systems• Resource sharing and the web• Challenges

• Summary



Ubiquitous networks

• Internet• Mobile phone networks• Corporation networks

• Factory networks• Campus networks• Home networks

• ……



Distributed System Definition

• A distributed system is one in whichhardware or software componentslocated at networked computerscommunicate and coordinate their actions only by passing messages.



Characteristics of Distributed System

• Concurrency• concurrent programs execution – shareresource

• No global clock • programs coordinate actions by exchangingmessages

• Independent failures• when some systems fail, others may not know



Share resources

• It characterizes the range of the things that canusefully be shared in a networked computer

• It extends from hardware components to

software-defined entities.• It includes the stream of video frames andthe audio connection.





• Summary



Familiar and widely used computer networks

• The Internet• Intranet• Mobile computing



• It is a very large distributed system that allowsusers throughout the world to make use of itsservices.

• Internet protocols is a major technical

achievement.

Internet



The Internet

intranet

ISP

desktop computer:

backbone

satellite link

server:

¤

network link:

¤

¤

¤



• What is Intranet? – A portion of the Internet that is separately

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

– Composed of several LANs linked by backbone connections

– Be connected to the Internet via a router

Intranet



A typical Intranet

the rest of

email server

Web server

D e s k t o pc o m p u t e r s

File server

router/firewall

print and other servers

other servers

print

Local areanetwork

email server

the Internet



• File services• Firewall• The cost of software installation and

support

Three main issues in the design of components for the use in intranet



• Mobile devices – Laptop computers – Handheld devices

• PDA, mobile phone, pager, video camera, digitalcamera

– Wearable devices• e.g. smart watches, digital glasses

– Network appliances• e.g. washing machines, hi-fi systems, cars and

refrigerators

Mobile and ubiquitous computing



• 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) – The harnessing of many small, cheap

computational devices

Mobile and ubiquitous computing … continued



Portable and handheld devices in adistributed system

Laptop

Mobile

Printer Camera

Internet

Host intranet Home intranetWAP

Wireless LAN

phone

gateway

Host site



• 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 securityguarantees

Issues in the design of components for theuse in Mobile and ubiquitous computing





• Summary



• Is the primary motivation of distributedcomputing

• Resources types

– Hardware, e.g. printer, scanner, camera – Data, e.g. file, database, web page – More specific functionality, e.g. search

engine, file

Resource sharing



• Service – manage a collection of related resources and present their functionalities to

users and applications

• Server – a process on networked computer that accepts requests from processes on

other computers to perform a service and responds appropriately

• Client – the requesting process

• Remote invocation – A complete interaction between client and server , from the point when the

client sends its request to when it receives the server’s response

Some definitions



Case study: the World Wide Web

• Motivation of WWW – Documents sharing between physicists of CERN

• Web is an open system: it can be extended

and implemented in new ways withoutdisturbing its existing functionality.– Its operation is based on communication standards

and document standards – Respect to the types of ‘resource’ that can be

published and shared on it.



Three main components of the Web

• HyperText Markup Language – A language for specifying the contents and layout

of pages• Uniform Resource Locators

– Identify documents and other resources

• A client-server architecture with HTTP

– By with browsers and other clients fetchdocuments and other resources from webservers





URL

HTTP URLs are the most widely usedAn 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.downloadIt.com/software/aProg.exehttp://net.pku.cn/ ….

mailto:[email protected]

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

http://net.pku.cn/

http://net.pku.cn/

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

mailto:[email protected]



Web servers and web browsers

Internet

BrowsersWeb servers

e.pku.cn

www.cdk3.net

www.w3c.org

Protocols

Activity.html

http://www.w3c.org/Protocols/Activity.html

http://e.pku.cn/cgi-bin/allsearch?word=distributed+system

http://www.cdk3.net/

File system of www.w3c.org



HTTP URLs• http://servername[:port]//pathNameOnServer][?arguments]

• e.g.

http://www.cdk3.net/http://www.w3c.org/Protocols/Activity.htmlhttp://e.pku.cn/cgi-bin/allsearch?word=distributed+system

----------------------------------------------------------------------------------------------------Server DNS name Pathname on server Argumentswww.cdk3.net (default) (none)

www.w3c.org Protocols/Activity.html (none)e.pku.cn cgi-bin/allsearch word=distributed+system-------------------------------------------------------------------------------------------------------

• Publish a resource remains unwieldy





http://www.w3c.org/

http://www.w3c.org/







HTTP

• Defines the ways in which browsers andany other types of client interact with webservers (RFC2616)

• Main features – Request-replay interaction – Content types. The strings that denote the type of

content are called MIME (RFC2045,2046) – One resource per request . HTTP version 1.0 – Simple access control



More features-services and dynamic pages

• Dynamic content – Common Gateway Interface : a program that

web servers run to generate content for their

clients

• Downloaded code

– JavaScript – Applet



Discussion of Web

Dangling : a resource is deleted or moved, but linksto it may still remainFind information easily : e.g. Resource DescriptionFramework which standardize the format of metadata about web resourcesExchange information easily : e.g. XML – a self describing languageScalability : heavy load on popular web serversMore applets or many images in pages increase inthe download time





• Summary



Heterogeneity

• Networks – Ethernet, token ring, etc

• Computer hardware – big endian / little endian

• Operating systems – different API of Unix and Windows

• Programming languages – different representations for data structures

• Implementations from different developers – no application standards



• Middleware – applies to a software layer that provides a

programming abstraction as well as masking theheterogeneity of the underlying networks,

hardware, OSs and programming languages• Mobile code

– is used to refer to code that can be sent fromone computer to another and run at thedestination

Heterogeneity … continued



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 witch new resourcesharing 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



Security

• Confidentiality– protection against disclosure to

unauthorized individuals,e.g. ACL in Unix File System

• 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



Scalability

• 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, e.g., servers support users at most O(n)– The performance loss, e.g., DNS no worse than O( log n)– Prevent software resources running out, e.g., IP address– Avoid performance bottlenecks, e.g., partitioning name table of DNS,

cache and replication

Date Computers Web servers Percentage

1993, July 1,776,000 130 0.0081995, July 6,642,000 23,500 0.41997, July 19,540,000 1,203,096 61999, July 56,218,000 6,598,697 12



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



Concurrency

• Correctness – ensure the operations on shared resourcecorrect in a concurrent environmente.g. records bids for an auction

• Performance – Ensure the high performance of concurrent

operations

T



Transparency

• Access transparency – using identical operations to access local and remoteresources, 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 sharedresources without interference with between them

• Replication transparency – multiple instances of resources to be used to increase

reliability and performance without knowledge of thereplicas by users or application programmers,e.g. realcourse( http://vod.yf.pku.edu.cn/ )

T



Transparency … continued

• Failure transparency –

users and applications to complete their tasks despite thefailure of hardware and software components, e.g., email

• Mobility transparency – movement of resources and clients within a system

without affecting the operation 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 scalewithout change to the system structure or the application

algorithms




• Introduction• Examples of distributed systems

• Resource sharing and the web• Challenges

• Summary



Summary

• Distributed systems are pervasive• Resource sharing is the primary motivation for

constructing distributed systems• Characterization of Distributed System

– Concurrency – No global clock – Independent failures

• Challenges to construct distributed system – Heterogeneity – Openness – Security – Scalability – Failure handling – Concurrency – Transparency

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