1. Learning Objectives 1.Identify the major categories and trends of distributed system 2.Identify the essential processes of an e- commerce system, and give examples of how they are implemented in e-commerce applications. 3.Identify and give examples of several key factors and Web store requirements needed to succeed in e-commerce.

2. Learning Objectives 4.Identify and explain the business value of several types of e-commerce marketplaces. 5.Discuss the benefits and trade-offs of several e-commerce clicks and bricks alternatives.

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

4. Ubiquitous networks Internet Mobile phone networks Corporation networks Factory networks Campus networks Home networks

5. 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.

6. 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

7. 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.

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

9. Familiar and widely used computer networks The Internet Intranet Mobile computing

10. 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. Internet

11. The Internet

12. 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 Intranet

13. A typical Intranet

14. File services Firewall The cost of software installation and support Three main issues in the design of components for the use in intranet

15. 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 Mobile and ubiquitous computing

16. 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 Mobile and ubiquitous computing … continued

17. Portable and handheld devices in a distributed system

18. 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 Issues in the design of components for the use in Mobile and ubiquitous computing

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

20. 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 Resource sharing

21. 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 Some definitions

22. 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.

23. 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

24. HTML 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. http://www.cdk3.net/WebExample/Images/earth.jpg Welcome to Earth! Visitors may also be interested in taking a look at the Moon. (etcetera)

25. URL 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 Scheme: scheme-specific-location e.g: mailto:joe@anISP.net ftp://ftp.downloadIt.com/software/aProg.exe http://net.pku.cn/ ….

26. Web servers and web browsers Internet Browsers Web servers e.pku.cn www.cdk3.net www.w3c.org Protocols Activity.html http://www.w3c.org/Protocols/Activity.html http://e.pku.cn/cgi-bin/allsearch?word=distributed+system http://www.cdk3.net/ File system of www.w3c.org

27. HTTP URLs http://servername[:port]//pathNameOnServer][?arguments] e.g. http://www.cdk3.net/ http://www.w3c.org/Protocols/Activity.html http://e.pku.cn/cgi-bin/allsearch?word=distributed+system ---------------------------------------------------------------------------------------------------- Server DNS namePathname on server Arguments www.cdk3.netwww.cdk3.net(default) (none) www.w3c.orgwww.w3c.orgProtocols/Activity.html (none) e.pku.cncgi-bin/allsearch word=distributed+system ------------------------------------------------------------------------------------------------------- Publish a resource remains unwieldy

28. 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

29. 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

30. 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

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

32. 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

33. 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 Heterogeneity… continued

34. 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

35. 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

36. 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 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

37. 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. 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. 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/ )

40. 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

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

42. 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