Distributed Systems: Concepts and Design

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Distributed Systems Concepts and Design
Chapter 1 Pages 1 - 27
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(No Transcript)
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Topics

• Chapter 1 Characterization of Distributed Systems

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What is a distributed system

• A distributed system is one in which components
  located at networked computers communicate and
  coordinate their actions only by passing messages
• A distributed system is a collection of
  autonomous computers linked by a computer network
  that appear to the users of the system as a
  single computer.

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What is a distributed system (contd)
- Personal workstations processors not assigned
to specific users.
- For a certain command the system can look for
the best place (workstation) to execute it.
Examples of Distributed Systems
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The internet and intranets

• The Internet is a very large distributed system
  consisting of many interconnected computer
  networks.
• The implementation of the internet and the
  services that it supports, has entailed the
  development of practical solutions to many
  distributed system issues.
• An intranet is a mini-internet which uses the
  same technologies as the internet, but is
  controlled by an organization.

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Mobile computing

• Today's computing power makes it possible to
  build small mobile devices which can benefit from
  network information
• Examples
• Laptops
• PDA
• Mobile phone
• These devices can either surf the internet, or it
  is possible to exchange business information at a
  meeting

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Ubiquitous computing (Self study)

• Ubiquitous computing means always and anywhere
  computing
• All small (and large) electronic equipment, to
  wear (like watches and minidiscs), to have in the
  house (fridge, microwave, stove) and to have at
  the office (printer, fax...) can be connected to
  a network
• At factories all machines and equipment can be
  networked

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Problems with mobile and ubiquitous computing
(Self study)

• With mobile computing it is necessary that the
  systems just work, so that there is no need to
  reconfigure
• Ubiquitous computing will affect people who are
  total computer illiterate, so these systems can't
  crash and there must be no big need for user
  intervention

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Characteristics and consequences

• Concurrency
• In a distributed system it is possible that many
  clients access the same resource at the same
  time.
• No global clock
• The only communication is by sending messages
  through a network.
• Independent failures
• Components running on different computers can
  fail without hurting other programs.

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Resources and components

• The terms resources and components are used
  to describe how a distributed system is built up.
• These terms are very abstract, but the range of
  things that can be connected and shared is very
  large.

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Important terms

• Service
• A distinct part of a computer system that manages
  a collection of related resources and presents
  their functionality to users and applications
• Internet services http, telnet, pop3...
• Server
• A running program (a process) on a networked
  computer that accepts requests from programs
  running on other computers to perform a service,
  and responds appropriately
• Internet servers Apache, Sendmail, WU-ftpd,
  OpenSSH
• Client
• The requesting process

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The World Wide Web

• The WWW is an evolving system for publishing and
  accessing resources and services across the
  Internet
• The Web is an open system
• Extension of the Web is possible without
  disturbing the existing structure
• Its operation is based on communication standards
  and document standards that 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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The main standard components of the Web (Self
study)

• HyperText Markup Language (HTML)
• Uniform Resource Locators (URLs)
• HyperText Transfer Protocol (HTTP)
• HTTP is a request-reply protocol

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More Discussion of the Web (Self study)

• Dynamic pages
• CGI
• ASP, PHP, Servlets...
• Javascript
• Discussion
• Hypertext model is lacking in some respects, such
  as lost in hyperspace
• HTML is limited in exchanging structured data
• The problems of scale
• A Web page is not always a satisfactory user
  interface

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Challenges for a distributed system

• Heterogeneity
• Openness
• Security
• Scalability
• Failure handling
• Concurrency
• Transparency

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Heterogeneity

• Different networks, hardware, operating systems,
  programming languages, developers
• We set up protocols to solve these
  heterogeneities
• Middleware a software layer that provides a
  programming abstraction as well as masking the
  heterogeneity
• Mobile code code that can be sent from one
  computer to another and run at the destination

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Openness

• 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 constructed from heterogeneous
  hardware and software

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Security

• 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

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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
• Controlling loss of performance
• Preventing software resources from running out
• Avoiding performance bottlenecks

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Failure handling

• 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 from failures
• Redundancy

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Concurrency

• 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

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Transparency

• 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 transparency
• Performance transparency
• Scaling transparency

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Summary

• Distributed systems are everywhere
• Resource sharing is the main motivating factor
  for constructing distribute systems
• The construction of a distributed system produces
  many challenges