Chapter 1-distribute Computing


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  • copyright (c) 2008, CAS UUM
  • An intranet is a part of the Internet that is separately administered and uses a firewall to enforce its own local security policies. Users in an intranet share data by means of file services.
  • TCP/IP: connect different type computer networks A very large distributed system WWW, email, FTP, VOD, etc The Internet is a very large distributed system that allows users throughout the world to make use of its services.
  • An intranet is a part of the Internet that is separately administered and uses a firewall to enforce its own local security policies. Users in an intranet share data by means of file services.
  • An intranet is a part of the Internet that is separately administered and uses a firewall to enforce its own local security policies. Users in an intranet share data by means of file services.
  • The World Wide Web Consortium (W3C) develops interoperable technologies (specifications, guidelines, software, and tools) to lead the Web to its full potential.
  • Chapter 1-distribute Computing

    1. 1. Universiti Utara MalaysiaDistributed Computing
    2. 2. Universiti Utara Malaysia Chapter 1: Characterization of Distributed Systems• Introduction• Examples of distributed systems• Resource sharing and the web• Challenges• Summary
    3. 3. Universiti Utara Malaysia Ubiquitous networks• Internet• Mobile phone networks• Corporation networks• Factory networks• Campus networks• Home networks
    4. 4. Universiti Utara Malaysia 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.
    5. 5. Universiti Utara Malaysia 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
    6. 6. Universiti Utara Malaysia 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.
    7. 7. Universiti Utara Malaysia Chapter 1: Characterization of Distributed Systems• Introduction• Examples of distributed systems• Resource sharing and the web• Challenges• Summary
    8. 8. Universiti Utara MalaysiaFamiliar and widely used computer networks• The Internet• Intranet• Mobile computing
    9. 9. Universiti Utara Malaysia 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.
    10. 10. Universiti Utara Malaysia The Internet intranetISP backbone satellite link desktop computer: server: network link:
    11. 11. Universiti Utara Malaysia 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
    12. 12. Universiti Utara Malaysia A typical Intranet email server Desktop computersprint and other servers Local area Web server network email server print File server other servers the rest of the Internet router/firewall
    13. 13. Universiti Utara Malaysia Three main issues in the design of components for the use in intranetFile servicesFirewallThe cost of software installation and support
    14. 14. Universiti Utara Malaysia 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
    15. 15. Universiti Utara MalaysiaMobile 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
    16. 16. Universiti Utara MalaysiaPortable and handheld devices in a distributed system InternetHost intranet WAP Wireless LAN gateway Home intranet Mobile phone Printer Laptop Camera Host site
    17. 17. Universiti Utara Malaysia 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
    18. 18. Universiti Utara Malaysia Chapter 1: Characterization of Distributed Systems• Introduction• Examples of distributed systems• Resource sharing and the web• Challenges• Summary
    19. 19. Universiti Utara Malaysia 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
    20. 20. Universiti Utara Malaysia 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 server’s response
    21. 21. Universiti Utara Malaysia 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.
    22. 22. Universiti Utara Malaysia 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
    23. 23. Universiti Utara Malaysia HTML<IMG SRC =><P>Welcome to Earth! Visitors may also be interested in taking a look at the<A HREF = “>Moon</A>.<P>(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.
    24. 24. Universiti Utara Malaysia URLScheme: scheme-specific-locatione.g: …. 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
    25. 25. Universiti Utara Malaysia Web servers and web browsers Web servers Browsers Internet www.w3c.orgFile system of Protocols Activity.html
    26. 26. Universiti Utara Malaysia HTTP URLs• http://servername[:port]//pathNameOnServer][?arguments]• e.g. DNS name Pathname on server (default) (none) Protocols/Activity.html (none) cgi-bin/allsearch word=distributed+system-------------------------------------------------------------------------------------------------------• Publish a resource remains unwieldy
    27. 27. Universiti Utara Malaysia 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
    28. 28. Universiti Utara Malaysia 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
    29. 29. Universiti Utara Malaysia 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
    30. 30. Universiti Utara Malaysia Chapter 1: Characterization of Distributed Systems• Introduction• Examples of distributed systems• Resource sharing and the web• Challenges• Summary
    31. 31. Universiti Utara Malaysia 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
    32. 32. Universiti Utara Malaysia 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
    33. 33. Universiti Utara Malaysia 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
    34. 34. Universiti Utara Malaysia 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
    35. 35. Universiti Utara Malaysia 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
    36. 36. Universiti Utara Malaysia Scalability Date Computers Web servers Percentage1993, 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
    37. 37. Universiti Utara Malaysia 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
    38. 38. Universiti Utara Malaysia 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
    39. 39. Universiti Utara Malaysia 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
    40. 40. Universiti Utara Malaysia Transparency• 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(• 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
    41. 41. Universiti Utara Malaysia Transparency … continued• 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
    42. 42. Universiti Utara Malaysia Chapter 1: Characterization of Distributed Systems• Introduction• Examples of distributed systems• Resource sharing and the web• Challenges• Summary
    43. 43. Universiti Utara Malaysia 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
    44. 44. BackupUniversiti Utara Malaysia
    45. 45. Universiti Utara MalaysiaOceanStore overview
    46. 46. Universiti Utara MalaysiaThe JXTA Search network architecture
    47. 47. Universiti Utara MalaysiaTUTORIAL QUESTION
    48. 48. Universiti Utara MalaysiaExercise 1.1Give five types of hardware resource andfive types of data or software resourcethat can usefully be shared. Giveexamples of their sharing as it occurs inpractice in distributed systems.
    49. 49. Universiti Utara MalaysiaExercise 1.2How might the clocks in two computersthat are linked by a local network besynchronized without reference to anexternal time source? What factors limitthe accuracy of the procedure you havedescribed? How could the clocks in alarge number of computers connected bythe Internet be synchronized? Discussthe accuracy of that procedure.
    50. 50. Universiti Utara MalaysiaExercise 1.3A user arrives at a railway station that shehas never visited before, carrying a PDAthat is capable of wireless networking.Suggest how the user could be providedwith information about the local servicesand amenities at that station, withoutentering the station’s name or attributes.What technical challenges must beovercome?
    51. 51. Universiti Utara MalaysiaExercise 1.4What are the advantages anddisadvantages of HTML, URLs and HTTPas core technologies for informationbrowsing? Are any of these technologiessuitable as a basis for client-servercomputing in general?
    52. 52. Universiti Utara MalaysiaExercise 1.5Use the World Wide Web as an exampleto illustrate the concept of resourcesharing, client and server. Resources in the World Wide Weband other services are named by URLs.What do the initials URL denote? Giveexamples of three different sorts of webresources that can be named by URLs.
    53. 53. Universiti Utara MalaysiaExercise 1.6Give an example of a URL. List the three main components of aURL, stating how their boundaries aredenoted and illustrating each one fromyour example. To what extent is a URL locationtransparent?
    54. 54. Universiti Utara MalaysiaExercise 1.7A server program written in one language (forexample C++) provides the implementation of aBLOB object that is intended to be accessed byclients that may be written in a differentlanguage (for example Java). The client andserver computers may have different hardware,but all of them are attached to an internet.Describe the problems due to each of the fiveaspects of heterogeneity that need to be solvedto make it possible for a client object to invoke amethod on the server object.
    55. 55. Universiti Utara MalaysiaExercise 1.8An open distributed system allows newresource sharing services such as theBLOB object in Exercise 1.7 to be addedand accessed by a variety of clientprograms. Discuss in the context of thisexample, to what extent the needs ofopenness differ from those ofheterogeneity.
    56. 56. Universiti Utara MalaysiaExercise 1.9Suppose that the operations of the BLOB objectare separated into two categories – publicoperations that are available to all users andprotected operations that are available only tocertain named users. State all of the problemsinvolved in ensuring that only the named userscan use a protected operation. Supposing thataccess to a protected operation providesinformation that should not be revealed to allusers, what further problems arise?
    57. 57. Universiti Utara MalaysiaExercise 1.10The INFO service manages a potentially verylarge set of resources, each of which can beaccessed by users throughout the Internet bymeans of a key (a string name). Discuss anapproach to the design of the names of theresources that achieves the minimum loss ofperformance as the number of resources in theservice increases. Suggest how the INFO servicecan be implemented so as to avoid performancebottlenecks when the number of users becomesvery large.
    58. 58. Universiti Utara MalaysiaExercise 1.11List the three main software componentsthat may fail when a client processinvokes a method in a server object,giving an example of a failure in eachcase. Suggest how the components canbe made to tolerate one another’sfailures.
    59. 59. Universiti Utara MalaysiaExercise 1.12A server process maintains a shared informationobject such as the BLOB object of Exercise 1.7.Give arguments for and against allowing theclient requests to be executed concurrently bythe server. In the case that they are executedconcurrently, give an example of possible‘interference’ that can occur between theoperations of different clients. Suggest howsuch interference may be prevented.
    60. 60. Universiti Utara MalaysiaExercise 1.13A service is implemented by severalservers. Explain why resources might betransferred between them. Would it besatisfactory for clients to multicast allrequests to the group of servers as a wayof achieving mobility transparency forclients?