Chapter 1: Characterization of Distributed Systems

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Chapter 1 Characterization of Distributed Systems

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

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Ubiquitous networks

• Internet
• Mobile phone networks
• Corporation networks
• Factory networks
• Campus networks
• Home networks

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Distributed System Definition

• A distributed system is one in which hardware or
  software components located at networked
  computers communicate and coordinate their
  actions only by passing messages.

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Characteristics of Distributed System

• Concurrency
• concurrent programs execution share resource
• No global clock
• programs coordinate actions by exchanging
  messages
• Independent failures
• when some systems fail, others may not know

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Share resources

• It characterizes the range of the things that can
  usefully be shared in a networked computer
• It extends from hardware components to
  software-defined entities.
• It includes the stream of video frames and the
  audio connection.

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Chapter 1 Characterization of Distributed Systems

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

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Familiar and widely used computer networks

• The Internet
• Intranet
• Mobile computing

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Internet

• It is a very large distributed system that allows
  users throughout the world to make use of its
  services.
• Internet protocols is a major technical
  achievement.

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The Internet
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Intranet

• 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

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A typical Intranet
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Three main issues in the design of components for
the use in intranet

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

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Mobile and ubiquitous computing

• Mobile devices
• Laptop computers
• Handheld devices
• PDA, mobile phone, pager, video camera, digital
  camera
• Wearable devices
• e.g. smart watches, digital glasses
• Network appliances
• e.g. washing machines, hi-fi systems, cars and
  refrigerators

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Mobile and ubiquitous computing continued

• 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

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Portable and handheld devices in a distributed
system
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Issues 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

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Chapter 1 Characterization of Distributed Systems

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

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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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Some definitions

• 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 servers response

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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 without disturbing 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.

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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 fetch
  documents and other resources from web servers

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HTML
ltIMG SRC http//www.cdk3.net/WebExample/Images/e
arth.jpggt ltPgt Welcome to Earth! Visitors may also
be interested in taking a look at the ltA HREF
http//www.cdk3.net/WebExample/moon.htmlgtMoonlt/Agt
. ltPgt (etcetera)

• HTML text is stored in a file of a web server.
• A browser retrieves the contents of this file
  from a web server.
• -The browser interprets the HTML text
• -The server can infer the content type from the
  filename extension.

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URL
Scheme scheme-specific-location e.g mailtojoe_at_
anISP.net ftp//ftp.downloadIt.com/software/aProg
.exe http//net.pku.cn/ .

• 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

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Web servers and web browsers
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HTTP URLs

• http//servernameport//pathNameOnServer?argum
  ents
• e.g.
• http//www.cdk3.net/
• http//www.w3c.org/Protocols/Activity.html
• http//e.pku.cn/cgi-bin/allsearch?worddistributed
  system
• --------------------------------------------------
  --------------------------------------------------
  
• Server DNS name Pathname on server Arguments
• www.cdk3.net (default) (none)
• www.w3c.org Protocols/Activity.html (none)
• e.pku.cn cgi-bin/allsearch
  worddistributedsystem
• --------------------------------------------------
  --------------------------------------------------
  ---
• Publish a resource remains unwieldy

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HTTP

• Defines the ways in which browsers and any other
  types of client interact with web servers
  (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

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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

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Discussion of Web

• Dangling a resource is deleted or moved, but
  links to it may still remain
• Find information easily e.g. Resource
  Description Framework which standardize the
  format of metadata about web resources
• Exchange information easily e.g. XML a self
  describing language
• Scalability heavy load on popular web servers
• More applets or many images in pages increase in
  the download time

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Chapter 1 Characterization of Distributed Systems

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

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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

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Heterogeneity continued

• Middleware
• applies to a software layer that provides a
  programming abstraction as well as masking the
  heterogeneity of the underlying networks,
  hardware, OSs and programming languages
• Mobile code
• is used to refer to code that can be sent from
  one computer to another and run at the
  destination

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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
  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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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

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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(logn)
• Prevent software resources running out, e.g., IP
  address
• Avoid performance bottlenecks, e.g., partitioning
  name table of DNS, cache and replication

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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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Concurrency

• 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

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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 application
  programmers,
• e.g. realcourse(http//vod.yf.pku.edu.cn/)

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Transparency continued

• 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 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
  scale without change to the system structure or
  the application algorithms

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Chapter 1 Characterization of Distributed Systems

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

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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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Backup
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OceanStore overview
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The JXTA Search network architecture