Figure 15.1 A distributed multimedia system - PowerPoint PPT Presentation

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Figure 15.1 A distributed multimedia system

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Chapter 1: Distributed Systems Overview – PowerPoint PPT presentation

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Title: Figure 15.1 A distributed multimedia system


1
Chapter 1 Distributed Systems Overview
2
Objectives
  • To be aware of the characteristics of
    concurrency, independent failure of components
    and lack of a global clock, which necessarily
    arise in a distributed systems.
  • To place distributed systems in a realistic
    context through examples the Internet, an
    intranet and mobile computing.
  • To motivate the benefits of resource sharing and
    to introduce the Web as an example.
  • To gain a good understanding of the challenges
    which apply to distributed systems.

3
Introduction
4
What is Distributed System?
  • Distributed system (DS)
  • Components at networked computers communicate
    coordinate their actions only by
    passing messages.
  • Allows for concurrent execution of programs, but
    prevents the possibility of a global clock -
    means that components can fail independently of
    one another.

5
Introduction (cont.)
  • Computer that are connected by a network maybe
    spatially separated by any distance.
  • DS definition has
  • significant consequence
  • - Concurrency
  • - No global clock
  • - Independent failure

6
Introduction (cont.)
  • Concurrency
  • In a network of computer, concurrent program
    execution of the norm.
  • I can do my work on my computer while you do
    your work on yours
  • Sharing resources such as web pages, file etc.
  • The capacity of the system to handle shared
    resources can be increased by adding more
    resources (example computers) to the network.

7
Introduction (cont.)
  • No global clock
  • there are limits to the accuracy with which the
    computer in a network can synchronize their clock
    there is no single global nation of the correct
    time.

8
Introduction (cont.)
  • Independent failure
  • All computer system can fail and it is the
    responsibility of system designers to plan for
    the consequences of possible failure.
  • Each component of the system can fail
    independently, leaving that other still running.
    This leaves a computer running in isolation.
  • The programs running on the isolated computer
    may not detect immediately that network has
    failed or become slower.

9
Examples of DS
  • 3 examples of DS
  • internet
  • intranet - a portion of internet
    managed by organization
  • mobile and ubiquitous computing

10
Internet
  • Internet is a very large distributed system
  • It enables users to make use of services such as
    World Wide Web, email and file transfer.
  • The set of services is open-ended it can be
    extended by addition of server computers and new
    types of services.

11
Figure 1.2 A typical intranet
  • Intranet is a part of Internet
  • Separately administered
  • Uses firewall to enforce its own local security
    policies.
  • Users share data by means of file services.

12
Figure 1.3 Portable and handheld devices in DS
Internet
WAP
Host intranet
Home intranet
Wireless LAN
gateway
Mobile
phone
Printer
Laptop
Host site
Camera
  • DS applicable to
  • a) mobile computing - laptop, mobile phones,
    GPRS, webcams
  • b) ubiquitous computing - small comp. embedded in
    appliances

13
Resource sharing and the Web
  • Main motivation for
  • constructing DS
  • Sharing of resources
  • Resource may be managed
  • by servers access by clients

14
Examples of hardware sharing in DS
  • a) CPU compute server (executes
    processor-intensive applications for clients),
    remote object server (executes methods on behalf
    of clients), worm program (shares cpu capacity of
    desktop machine with local user)
  • b) memory cache server (holds recently-accessed
    web pages in its RAM, for faster access by other
    local computers)
  • c) disk file server, virtual disk server, video
    on demand server.
  • d) screen Network window systems, such as X-11,
    allow processes in remote computers to update the
    content of windows.
  • e) printer networked printers accept print jobs
    from many computers. Managing them with a queuing
    system.
  • d) network capacity packet transmission enables
    many simultaneous communication channels to be
    transmitted on the same circuits.

15
Examples of data/software sharing in DS
a) web page web servers enable multiple clients
to share read-only page content. b) file file
servers enable multiple clients to share
read-write files. c) object possibilities for
software objects are limitless. E.g. shared
whiteboard, shared diary, room booking system,
etc. d) database intended to record definitive
state of some related sets of data. They shared
ever since multi-user computers appeared. e)
newsgroup content netnews system makes read-only
copies of recently-posted news items available
to clients throughout Internet. A copy of
newsgroup content is maintained at each netnews
server. Each server makes its data available to
multiple clients.
16
Resource sharing and the Web (cont.)
  • Shared resources are managed by server
    processes.
  • A service of a computer system manages a
    collection of related resources. For example we
    access shared files through a file service, we
    send documents to printer through printing
    service, we buy goods through electronic payment
    service
  • A service is accessed via the well-defined set
    of operations. For example- a file service
    provides read, write and delete operations on
    files.

17
Resource sharing and the Web (cont.)
  • The term server refers to a running program
    (service) on a networked computer system that
    accepts request from programs running on other
    computers (client) .
  • A Request is sent in the form of a message from
    client to server. A Client sends request to
    invoke an operation. When the client recieves the
    servers response, it is called as remote
    invocation.
  • In DS written an object-oriented language,
    resources may be encapsulated as objects whose
    methods are invoked by client objects.

18
Resource sharing and the Web (cont.)
  • World Wide Web
  • -gt example of resource sharing.
  • -gt 3 main standard technological components
    a) HTML (HyperText Markup
    Language), is a language for specifying the
    contents and layout of the pages.
  • b) URLs (Uniform Resource Allocators) identify
    documents and other resources stored as part of
    web
  • , c) HTTP (HyperText Transfer Protocol) by which
    web browsers and other clients fetch documents
    and other resources from web servers.

19
Figure 1.4 Web servers and web browsers
20
Challenges
  • Challenges arising for construction DS
  • heterogeneity
  • openness
  • security
  • scalability
  • failure handling
  • concurrency
  • transparency

21
Challenges (cont.)
  • heterogeneity
  • - A DS is constructed from variety different
    network, OS, computer hardware and programming
    language, implementation by different developers.
  • - Internet communication protocols mask the
    difference in network, and middleware can deal
    with the other differences. Some middlewares are
    CORBA, JAVA RMI.
  • openness
  • - allows components to be add or replaced
    (re-implemented)
  • - DS should be extensible publish key
    interfaces of components and integrate components
    written by different programmer

22
Challenges (cont.)
  • security
  • - Security has three components
  • Confidentiality (Protection against disclosure
    to unauthorized individual)
  • Integrity (Protection against alteration or
    corruption)
  • Availability (Protection against interference
    with the means to access the resources)
  • - encryption can be used to provide
    protection of shared resources keep sensitive
    information secret when is transmitted in
    messages over a network.

23
High Security!
Dr Noraziah Ahmad Mac 2008 Email
noraziah_at_ump.edu.my
24
Challenges (cont.)
  • scalability
  • - ability to work when num. of users ?
  • - algorithms used to access shared data
    should avoid performance bottlenecks
  • - data should be structured hierarchically to
    get the best times
  • - Web illustrates some approaches to address
    it (e.g. the use of hierarchic naming,
    partitioned data, caching replication).

Figure 1.5 Computers in the Internet
25
Figure 1.6 Computers vs. Web servers in the
Internet
Date
Computers
Web servers
Percentage
1,776,000
130
0.008
1993, July
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
26
Challenges (cont.)
  • failure handling
  • - any process, computer or network may fail
    independently
  • - each component need to be aware of possible
    ways its components may fail
  • concurrency
  • - multiple users is a source of concurrent
    request to resources
  • - each resources must be designed to be safe
  • Transparency
  • - make certain aspects of distribution
    invisible to the application

27
Transparencies
  • Access transparency enables local and remote
    resources to be accessed using identical
    operations.
  • Location transparency enables resources to be
    accessed without knowledge of their physical or
    network location (for example, which building or
    IP address).
  • 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.

28
Transparencies (cont.)
  • 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.
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