Learning Objectives 1 Identify the major categories and
Learning Objectives 1. Identify the major categories and trends of distributed system 2. Identify the essential processes of an ecommerce 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.
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.
Chapter 1: Characterization of Distributed Systems • • • Introduction Examples of distributed systems Resource sharing and the web Challenges Summary
Ubiquitous networks • • • Internet Mobile phone networks Corporation networks Factory networks Campus networks Home networks
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.
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
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.
Chapter 1: Characterization of Distributed Systems • • • Introduction Examples of distributed systems Resource sharing and the web Challenges Summary
Familiar and widely used computer networks • The Internet • Intranet • Mobile computing
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.
The Internet intranet ISP % % backbone satellite link desktop computer: server: network link:
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
A typical Intranet
Three main issues in the design of components for the use in intranet • File services • Firewall • The cost of software installation and support
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
Mobile 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
Portable and handheld devices in a distributed system
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
Chapter 1: Characterization of Distributed Systems • • • Introduction Examples of distributed systems Resource sharing and the web Challenges Summary
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
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
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.
Three main components of the Web • Hyper. Text 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
HTML <IMG SRC = http: //www. cdk 3. net/Web. Example/Images/earth. jpg> <P> Welcome to Earth! Visitors may also be interested in taking a look at the <A HREF = “http: //www. cdk 3. net/Web. Example/moon. html>Moon</A>. <P> (etcetera) l l 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.
URL Scheme: scheme-specific-location e. g: mailto: joe@an. ISP. net ftp: //ftp. download. It. com/software/a. Prog. exe http: //net. pku. cn/ …. l l 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
Web servers and web browsers http: //e. pku. cn/cgi-bin/allsearch? word=distributed+system e. pku. cn Browsers Web servers Internet www. cdk 3. net http: //www. cdk 3. net/ www. w 3 c. org File system of www. w 3 c. org Protocols Activity. html http: //www. w 3 c. org/Protocols/Activity. html
HTTP URLs • http: //servername[: port]//path. Name. On. Server][? arguments] • e. g. http: //www. cdk 3. net/ http: //www. w 3 c. org/Protocols/Activity. html http: //e. pku. cn/cgi-bin/allsearch? word=distributed+system -------------------------------------------------- Server DNS name www. cdk 3. net www. w 3 c. org e. pku. cn Pathname on server Arguments (default) (none) Protocols/Activity. html (none) cgi-bin/allsearch word=distributed+system ---------------------------------------------------- • Publish a resource remains unwieldy
HTTP • Defines the ways in which browsers and any other types of client interact with web servers (RFC 2616) • Main features – Request-replay interaction – Content types. The strings that denote the type of content are called MIME (RFC 2045, 2046) – One resource per request. HTTP version 1. 0 – Simple access control
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 – Java. Script – Applet
Discussion of Web l l l 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
Chapter 1: Characterization of Distributed Systems • • • Introduction Examples of distributed systems Resource sharing and the web Challenges Summary
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
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
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
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
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 Date 1993, July 1995, July 1997, July 1999, July Computers 1, 776, 000 6, 642, 000 19, 540, 000 56, 218, 000 Web servers 130 23, 500 1, 203, 096 6, 598, 697 Percentage 0. 008 0. 4 6 12
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
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
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/)
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
Chapter 1: Characterization of Distributed Systems • • • Introduction Examples of distributed systems Resource sharing and the web Challenges Summary
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
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