1. Nouf Al Batati NET 422

2. Course outline Characterization of distributed systems Introduction Examples Resource sharing and the web Challenges Summary system models Introduction Architectural models Fundamentals models Summery Interprocess communication External data representation and marshalling Clint-server communication Group communication Case study: interprocess communication in UNIX summery

3. Distributed objects and remote invocation introduction Remote procedure call Events and notifications Case study : Java RMI Summery Distributed file systems Introduction File service architecture Case study: sun network file system Summery Name services Introduction Name services and the DNS Case study of the X.500 Directory Service summery

4. PEER-TO-PEER systems Introduction Peer-to-peer middleware Time and global states Introduction Clocks events and process states Synchronizing physical clocks Logical time and logical clocks Summery Coordination and agreement Introduction Distributed mutual exclusion Elections Multicast communication Consensus and related problems Summery

5. Replication Introduction System models and group communication Fault-tolerant services Summery Web services Introduction Service descriptions and IDL for web services Case study: the grid summery

6. Chapter 1: Characterization of distributed systems Introduction Examples Resource sharing and the web Challenges Summary

7. Introduction The aim of the course: Explain the characteristics of networked computers Present the main concepts and techniques that have been developed.

8. What is Distributed Systems? It is one in which hardware or software components located at networked computers communicate and coordinate their actions only by passing messages Concurrency No global clock Independent failure

9. Advantages of Distributed Computing Reliability The important advantage of distributed computing system is reliability. It is more reliable than a single system. If one machine from system crashes, the rest of the computers remain unaffected and the system can survive as a whole. Incremental Growth In distributed computing the computer power can be added in small increments i.e. new machines can be added incrementally as per requirements on processing power grow. Sharing of Resources Shared data is required to many applications such as banking, reservation system and computer-supported cooperative work. As data or resources are shared in distributed system, it is essential for various applications. Flexibility As the system is very flexible, it is very easy to install, implement and debug new services. Each service is equally accessible to every client remote or local.

10. Speed A distributed computing system can have more computing power than a mainframe. Its speed makes it different than other systems. Open system As it is open system, it can communicate with other systems at anytime. Because of an open system it has an advantage over self-contained system as well as closed system. Performance It is yet another advantage of distributed computing system. The collection of processors in the system can provide higher performance than a centralized computer. http://www.platformusers.net/distributed-computing/distributed- computing-advantages-and-disadvantages.html Advantages of Distributed Computing

11. Troubleshooting Troubleshooting and diagnosing problems are the most important disadvantages of distributed computing system. The analysis may require connecting to remote nodes or checking communication between nodes. Software Less software support is the main disadvantage of distributed computing system. Because of more software components that comprise a system there is a chance of error occurring. Networking The underlying network in distributed computing system can cause several problems such as transmission problem, overloading, loss of messages. Hence, the problems created by network infrastructure are the disadvantages of distributed computing. Security The easy distributed access in distributed computing system which increases the risk of security. The sharing of data creates the problem of data security. Disadvantages of Distributed Computing http://www.platformusers.net/distributed-computing/distributed- computing-advantages-and-disadvantages.html

12. Examples of distributed systems Internet Intranets File services Firewalls The cost issue Mobile and ubiquitous computing

13. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Figure 1.1 A typical portion of the Internet

14. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Figure 1.2 A typical intranet

15. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Figure 1.3 Portable and handheld devices in a distributed system

16. Resource sharing and the web Hardware and software sharing Service : the part of computer system that manages a collection of related resources and presents their functionality to users and applications We access shared files through file service We send document to printer through a printing service. The service provide a set of operations Resources in a distributed system are physically encapsulated within computers and can only be accessed from other computers by communication.

17. Server : is the running program on a networked computers that accepts requests from programs running on other computers to perform a service and responds appropriately Remote invocation: is a complete interaction between a client and a server Client and server refer to processes Resource sharing and the web

18. Client server example The world Wide Web (WWW) The web is open system: 1. Its operation is based on communication standards and document standards that are freely published and widely implemented 2. The web is open with respect to the types of resources that can be published and shared on it.

19. The web is based on three main standard technological components: HTML : Hyper Text Markup language URL : Uniform Resource Locator http://servername[:port][/pathName][?query][#fragment] http://www.w3.org/protocol/activity.htm#intro http://www.google.com /search?q=kindberg HTTP : Hyper Text Transfer Protocol Request replay Content types One resource per request Simple access control

20. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Figure 1.4 Web servers and web browsers Internet Browsers Web servers www.google.com www.cdk3.net www.w3c.org Protocols Activity.html http://www.w3c.org/Protocols/Activity.html http://www.google.comlsearch?q=kindberg http://www.cdk3.net/ File system of www.w3c.org

21. Challenges Heterogeneity Networks Computer hardware Operating systems Programming languages Implementations by different developers

22. Openness The ability of adding new resource sharing services and making them available for use by variety of client programs. Open distributed systems are extensible : Hardware level Software level Challenges ‘Continue’

23. Security Denial of service attacks: bombarding the service with such a large number of pointless requests that the serious user are unable to use it. Security of mobile code: the possible effect of running mobile code ‘ attachment’ are unpredictable.

24. Scalability Controlling the cost of physical resources Controlling the performance loss Preventing software resources running out Avoiding performance bottlenecks Challenges ‘Continue’

25. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Figure 1.5 Computers in the Internet Date Computers Web servers 1979, Dec. 1880 1989, July130,0000 1999, July56,218,0005,560,866 2003, Jan.171,638,29735,424,956

26. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Figure 1.6 Computers vs. Web servers in the Internet DateComputersWeb serversPercentage 1993, July 1,776,0001300.008 1995, July6,642,00023,5000.4 1997, July19,540,0001,203,0966 1999, July56,218,0006,598,69712 2001, July125,888,19731,299,59225 42,298,371

27. Challenges ‘Continue’ Failure Detecting failures Masking failures By hiding it or make it less severe: 1. Re-transmission 2. Using backups Tolerating failures Recovery from failures: ‘rolled back’ Redundancy Two different routes between any two routers Every name table is replicated in at least two different servers,in DNS Database may be replicated

28. Concurrency There is a possibility that several clients will attempt to a access a shared resources at the same time. The process that manages a shared resource could take one client request at a time. But that approach limits throughput. Challenges ‘Continue’

29. 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. Transparency Challenges ‘Continue’

30. 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. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Challenges: ‘Continue’ Transparency ( continue)

31. Small quiz

32. Question 1 In a distributed system, the existence of multiple autonomous computers is transparent to the users or appears to the users as a single computer. A.True B.False http://www.proprofs.com/quiz-school/quizshow.php?title=tsn30305-distributed- systems&quesnum=10&showNextQ=no

33. Question 2 For a distributed system, users must explicitly log on to a machine, explicitly know what the machine can do, explicitly submit data to the correct location, and explicitly tell the machine how to return their results (e.g., give their own logical addresses to the machine). A.True B.False http://www.proprofs.com/quiz-school/quizshow.php?title=tsn30305-distributed- systems&quesnum=10&showNextQ=no

34. Question 3 There are four requirements in the design of a distributed system. Choose the correct combination from the list below. A.Network integrity; Quality of Software (QoS); Caching and alteration; Dependability issues B.Network dependency; Quantity of Service (QoS); Cookies and replication; Dependability issues. C.Network performance; Quality of Service (QoS); Caching and replication; Dependability issues. D.Network Accessibility; Quality of hardware (QoH); Caching and replication; Dependability issues http://www.proprofs.com/quiz-school/quizshow.php?title=tsn30305-distributed- systems&quesnum=10&showNextQ=no

35. Question 4 Peer processes model is a distributed application based on peer processes. Sometimes, some processes play similar roles, interacting cooperatively as peers to perform a task. This model is usually used because of the nature of the application, e.g., group communication. A.True B.False http://www.proprofs.com/quiz-school/quizshow.php?title=tsn30305-distributed- systems&quesnum=10&showNextQ=no

36. Chapter 2: System models Introduction Architectural model Fundamental model

37. Introduction In this chapter we bring out the common properties and design issue for DS in the form of descriptive models. Each model is intended to provide an abstract, simplified but consistent description of a relevant aspect of distributed system design

38. Problems that designers face: Widely varying modes of use Wide range of system environment Internal problems: Non-synchronized clocks Conflicting data updates Many modes of HW and SW failure involving the individual components of a system External threats: Attacks and denial services

39. System models Architectural modelFundamental model Layered structureInteraction model Main architectural modelsFailure model Variant of client-server modelSecurity model features Design requirement

40. Architectural model Major concerns: to make the system: Reliable Manageable Adaptable Cost-effective The architectural model of a distributed system simplifies and abstracts the functions of the individual components of the DS and then it considers: The replacement of the components across a network of computers The interrelationships between the components

41. System simplification: Classifying processes to identify the responsibilities which helps us to assess their workloads and determine the impact of failures in each process. Processes classes: Server processes Client processes Peer processes Architectural model

42. Dynamic systems: Moving code to reduce access delay and minimize communication traffic Adding new computers and allowing them to discover the services and offer their services to others Architectural model

43. Software layers: Software architecture: Platform ( hardware + OS) Middleware Services: Remote method invocation Communication between a group of processes Notification of events ………..etc Examples: CORBA: Common Object Request Broker Architecture Java RMI : Java Remote Method Invocation Web services DCOM: distributed component object model The ISO/ITU-T : International Organization for Standardization /International telecommunication union Limitation of middleware Architectural model

44. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Figure 2.1 Software and hardware service layers in distributed systems

45. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Figure 2.2 Clients invoke individual servers

46. System architecture 1. Client- server Web server is a client of a local file server that manages the files in which the web pages are stored Web servers and most other internet services are clients of the DNS services. Search engines What are crawlers? Programs run in the background at a search engine site using HTTP request to access web servers throughout the internet  Simple  Scale poorly: Q: The search engine is both a server and a client, explain. Q:A typical search engine would normally include many concurrent threads of execution, what do they do? and what do they need? Architectural model

47. 2. Peer-to-peer All process in a task or activity play the same roles It enables hundreds of thousand computers to provide access to data and other resources E.g :Napster application for sharing digital music files A large number of data object are shared Each computer holds only a small part of the application database and storage This architecture is more complex Replication is provided to distribute the load and provide resilience Architectural model

48. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Figure 2.3 A distributed application based on peer processes

49. 3. Variations The use of multiple servers and caches The use of mobile code and mobile agents Users need for low-cost computers with limited hardware resources that are simple to manage The requirement to add and remove mobile devices in a convenient manner Architectural model

50. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Figure 2.4 A service provided by multiple servers

51. Services provided by multiple servers Objects partitioning and distribution by the server Objects replication : Sun NIS Cluster : used for highly scalable web services such as search engines and online stores Proxy servers and caches Caches: Collocated with each client Located in a proxy server Proxy server to increase availability and performance Provides a shared cache Used to access remote web servers through a ………………. Architectural model

52. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Figure 2.5 Web proxy server

53. Mobile code Applets are example of mobile code The user running browser select a link to an applet whose code is stored on a web server then the code is downloaded to the browser and runs there Mobile agents A moving program ( code + data) from one computer to another carrying on task on someone's behalf returning with a result Architectural model

54. Network computers: Network computer downloads its operating system and any application software needed by the user from a remote file server. Applications are run locally but the files are managed by remote file server. Thin clients: It refers to a software layer that supports window-based user interface on a computer that is local to the user while executing application programs on a remote computer. Instead of downloading the code of applications into the user’s computer, it runs them on a computer server.

55. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Figure 2.6 Web applets

56. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Figure 2.7 Thin clients and compute servers Thin Client Application Process Network computer or PC Compute server network

57. Mobile devices and spontaneous interoperation Mobile devices are HW computing components that move between physical locations carrying software component with them Connect to wireless network via: 3G and GSM WiFi (IEEE 802.11) Bluetooth Mobile servers Three main issues: Mobility transparency Variable connectivity Spontaneous interoperation What is service discovery? Architectural model

58. Interfaces and objects Modula C++ Java Architectural model

59. Design requirement for distributed architectures 1. Performance issues Responsiveness Users require fast and consistent response The speed is affected by the load, performance and delays in all software components involved To achieve a good response time, systems must be composed of relatively few SW layers and the quantity of transferred data must be small Throughput The measure of performance Affected by processing speed and data transfer rate Balancing computational loads Applet Several computers Architectural model

60. 2. Quality of service It is affected by Reliability Security Performance adaptability 3. Use of caching and replication Web-caching protocol 4. Dependability issue Fault tolerance Redundancy Retransmission security Architectural model

61. Fundamental model The system model has to address the questions : What are the main entities in the system? How do they interact? What are the characteristics that affect their individual and collective behavior? The purpose of a model: To make explicit all the relevant assumptions about the system To make generalization concerning what is possible or impossible, given those assumptions. General –purpose Guarantees

62. 1. Interaction model Multiple server processes may cooperate with one another to provide a service A set of beer processes may cooperate with one another to achieve a common goal Distribution algorithm: the steps to be taken by each of the process of which the system is composed Massages are transmitted to: Transfer information Coordinate activities It is difficult to describe the rate because of the failure Interacting processes perform all of the activity in a distributed system Each process has its own private state Interacting process is affected by: Communication performance Timing issue Fundamental model

63. Performance of communication channels Latency: is the delay between the start of a message’s transmission from one process and its receipt by another and it includes: Transmission time through the network Time needed to reach the network which increases with the increase of the network load The time taken by the operating system communication services The bandwidth of the computer network jitter Fundamental model

64. Computer clock and timing events Clock drift rate: the relative amount that a computer clock differs from a perfect reference clock Approach to correcting the times on computer clocks: Radio receivers to get time readings from the GPS

65. Two variants of the interaction model Synchronous DS The time to execute each step of a process has known lower and upper bounds Each message transmitted over a channel is received within a known bounded time Each process has a local clock whose drift rate from real time has a known bound Asynchronous DS No bounds on process execution No bounds on message transmission delay No bounds on clock drift rate Fundamental model

66. Event ordering Knowing whether an event at one process occurred before, after or concurrently with another event at another process. Logical time can be used to provide an ordering among the events at process running in different computers in a distributed system Fundamental model

67. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Figure 2.8 Real-time ordering of events

68. ItemFromSubject 23ZRe: Meeting 24XMeeting 25YRe: Meeting Fundamental model

69. 2. Failure model 1. Omission failures Process omission failure Communication omission failure Caused by lack of buffer space Network transmission error Send-omission failure Receive-omission failure Channel omission failute 2. Arbitrary failures The process may set wrong values in its data items The process my return a wrong value in response to an invocation Fundamental model

70. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Figure 2.10 Omission and arbitrary failures Class of failureAffectsDescription Fail-stopProcessProcess halts and remains halted. Other processes may detect this state. CrashProcessProcess halts and remains halted. Other processes may not be able to detect this state. OmissionChannelA message inserted in an outgoing message buffer never arrives at the other end’s incoming message buffer. Send-omissionProcessA process completes asend, but the message is not put in its outgoing message buffer. Receive-omissionProcessA message is put in a process’s incoming message buffer, but that process does not receive it. Arbitrary (Byzantine) Process or channel Process/channel exhibits arbitrary behaviour: it may send/transmit arbitrary messages at arbitrary times, commit omissions; a process may stop or take an incorrect step.

71. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Figure 2.9 Processes and channels

72. 3. Timing failures Figure 2.11:Timing failures Class of FailureAffectsDescription ClockProcessProcess’s local clock exceeds the bounds on its rate of drift from real time. PerformanceProcessProcess exceeds the bounds on the interval between two steps. PerformanceChannelA message’s transmission takes longer than the stated bound. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Fundamental model

73. Masking failures Multiple servers Knowledge of the failure characteristics of component a service masks a failure by: Hiding it Converting it into a more acceptable type Reliability of one-to-one communication Reliable communication is defined in terms of validity and integrity as follows: Validity: any message is delivered Integrity : received message = sent message with no duplication Fundamental model

74. The threats com from: Any protocol does not reject a message that arrives twice Malicious users Fundamental model

75. 3. Security model The security of a DS can be achieved by securing the processes and the channel used and by protecting the objects that they encapsulate against unauthorized access. Protecting objects Using access rights Securing processes and their interactions The enemy Threats to processes Threats to communication channels Fundamental model

76. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Figure 2.12 Objects and principals

77. Defeating security threats 1. Cryptography and shared secrets Cryptography is the science of keeping messages secure Encryption is the process of scrambling a message to hide its contents. 2. Authentication It is to include in a message an encrypted portion that contains enough of the contents of the message to guarantee its authenticity, for example: o a representation of the requesting principal’s identity oThe identity of the file oThe date and time of the request ……. Fundamental model

78. 3.Secure channels Secure channel has the following properties: Each process knows the identity of the principal on whose behalf the other process is executing A secure channel ensures the privacy and integrity of the transmitted data Each message include a physical or logical time stamp

79. Other possible threats from an enemy Denial of service Mobile code The uses of security models

80. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Figure 2.13 The enemy Communication channel Copy of m Process p q m The enemy m’

81. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Figure 2.14 Secure channels Principal A Secure channel Process p q Principal B