Basic WWW Technologies Mathematic Background(Chap 2.ppt

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1、Basic WWW Technologies & Mathematic Background (Chap 2 & 1, Baldi),Wen-Hsiang Lu (盧文祥) Department of Computer Science and Information Engineering, National Cheng Kung University 2006/10/5,2,World Wide Web,The World Wide Web (Web) is a network of information resources. The Web relies on three mechani

2、sms to make these resources available: A uniform naming scheme for locating resources on the web (e.g., URIs). Protocols, for access to named resources over the web (e.g., HTTP). Hypertext, for easy navigation among resources (e.g., HTML).,3,Internet vs. Web,Internet: Internet is a more general term

3、 Includes physical aspect of underlying networks and mechanisms such as email, FTP, HTTP Web: Associated with information stored on the Internet Refers to a broader class of networks, i.e. Web of English Literature Both Internet and web are networks,4,Essential Components of WWW,Resources (HTML, Hyp

4、erText Markup Language) Conceptual mappings to concrete or abstract entities, which do not change in the short term Taggin support for structuring and laying out documents Resource identifiers (hyperlinks): Strings of characters represent generalized addresses that may contain instructions for acces

5、sing the identified resource http:/ is used to identify the Google homepage Transfer protocols (HTTP, HyperText Transmission Protocol) Conventions that regulate the communication between a browser (web user agent) and a server,5,Standard Generalized Markup Language (SGML),Based on GML (generalized m

6、arkup language), developed by IBM in the 1960s An international standard (ISO 8879:1986) defines how descriptive markup should be embedded in a document Markup: extra information characterizing structure of a document Gave birth to the extensible markup language (XML), W3C recommendation in 1998,6,S

7、GML Components,SGML documents have three parts: Declaration: specifies which characters and delimiters may appear in the application DTD (Document Type Definition)/ style sheet: defines the syntax of markup constructs Document instance: actual text (with the tag) of the documents More info could be

8、found: http:/www.W3.Org/markup/SGML,7,HTML Background,HTML was originally developed by Tim Berners-Lee while at CERN, and popularized by the Mosaic browser developed at NCSA. The Web depends on Web page authors and vendors sharing the same conventions for HTML. This has motivated joint work on speci

9、fications for HTML. HTML standards are organized by W3C : http:/www.w3.org/MarkUp/,8,HTML Functionalities,HTML gives authors the means to: Publish online documents with headings, text, tables, lists, photos, etc Include spread-sheets, video clips, sound clips, and other applications directly in thei

10、r documents Link information via hypertext links, at the click of a button Design forms for conducting transactions with remote services, for use in searching for information, making reservations, ordering products, etc,9,Sample Webpage,10,Sample Webpage: HTML Structure, The title of the webpageBody

11、 of the webpage ,11,HTML Structure,An HTML document is divided into a head section (here, between and ) and a body (here, between and ) The title of the document appears in the head (along with other information about the document) The content of the document appears in the body. The body in this ex

12、ample contains just one paragraph, marked up with ,12,HTML Hyperlink,alumni A link is a connection from one Web resource to another It has two ends, called anchors, and a direction Starts at the “source“ anchor and points to the “destination“ anchor, which may be any Web resource (e.g., an image, a

13、video clip, a sound bite, a program, an HTML document),13,Resource Identifiers,Uniform Resource Identifiers (URI): include two overlapping subsets of identifiers URL: Uniform Resource Locators URN: Uniform Resource Names,14,Introduction to URIs,Every resource available on the Web has an address that

14、 may be encoded by a URI URIs typically consist of three pieces: The naming scheme of the mechanism used to access the resource. (HTTP, FTP) The name of the machine hosting the resource The name of the resource itself, given as a path,15,URI Example,http:/www.w3.org/TR There is a document available

15、via the HTTP protocol Residing on the machines hosting www.w3.org Accessible via the path “/TR“,16,Protocols,Describe how messages are encoded and exchanged Different Layering Architectures ISO OSI 7-Layer Architecture TCP/IP 4-Layer Architecture,17,ISO OSI Layering Architecture,18,TCP/IP Layering A

16、rchitecture,19,TCP/IP Layering Architecture,A simplified model, provides the end-to-end reliable connection The network layer Hosts drop packages into this layer, layer routes towards destination Only promise “Try my best” The transport layer Reliable byte-oriented stream,20,Hypertext Transfer Proto

17、col (HTTP),A connection-oriented protocol (TCP) used to carry WWW traffic between a browser and a server One of the transport layer protocol supported by Internet HTTP communication is established via a TCP connection and server port 80,21,GET Method in HTTP,22,Form,23,Form, 1 Median Eminence (可複選):

18、 1.分泌 2.一般 3.王錫崗.垂體 其他: ,24,CGI processing,25,CGI (Common Gateway Interface),Web Browser,Web Server,Database,CGI,Service Request,Service Processing,Output,Service Response,26,HTTP Request Processing,27,GNU Wget,28,CGI: Get query search-results from Google using Wget,29,Homework (1),Meta-search engin

19、e: dispatch the user query to several engines at same time, collect and merge the results into one list to the user. Homework: Develop a meta-search engine which responds user query with combined search results from a few search engines.,30,Domain Name System,DNS (domain name service): mapping from

20、domain names to IP address IPv4: IPv4 was initially deployed January 1st. 1983 and is still the most commonly used version. 32 bit address, a string of 4 decimal numbers separated by dot, range from 0.0.0.0 to 255.255.255.255. IPv6: Revision of IPv4 with 128 bit address,31,Top Level Domains (TLD),To

21、p level domain names, .com, .edu, .gov and ISO 3166 country codes .de, .fr, .it There are three types of top-level domains: Generic domains were created for use by the Internet public Country code domains were created to be used by individual country The .arpa domain Address and Routing Parameter Ar

22、ea domain is designated to be used exclusively for Internet-infrastructure purposes,32,Server Log Files,Server Transfer Log: transactions between a browser and server are logged IP address, the time of the request Method of the request (GET, HEAD, POST) Status code, a response from the server Size i

23、n byte of the transaction Referrer Log: where the request originated Agent Log: browser software making the request (spider) Error Log: request resulted in errors (404),33,Server Log Analysis,Most and least visited web pages Entry and exit pages Referrals from other sites or search engines What are

24、the searched keywords How many clicks/page views a page received Error reports, like broken links,34,Server Log Analysis,35,Search Engines,According to Pew Internet & American Life Project Report (2002), search engines are the most popular way to locate information online About 33 million U.S. Inter

25、net users query on search engines on a typical day. More than 80% have used search engines Search Engines are measured by coverage and recency,36,Web Crawler,A crawler is a program that picks up a page and follows all the links on that page Crawler = Spider Types of crawler: Breadth First Depth Firs

26、t,37,Breadth First Crawlers,Use breadth-first search (BFS) algorithm Get all links from the starting page, and add them to a queue Pick the 1st link from the queue, get all links on the page and add to the queue Repeat above step till queue is empty,38,Breadth First Crawlers,39,Depth First Crawlers,

27、Use depth first search (DFS) algorithm Get the 1st link not visited from the start page Visit link and get 1st non-visited link Repeat above step till no non-visited links Go to next non-visited link in the previous level and repeat 2nd step,40,Depth First Crawlers,41,Coverage,Overlap analysis used

28、for estimating the size of the indexable web W: set of webpages Wa, Wb: pages crawled by two independent engines a and b P(Wa), P(Wb): probabilities that a page was crawled by a or b P(Wa)=|Wa| / |W| P(Wb)=|Wb| / |W|,42,Overlap Analysis,P(Wa Wb| Wb) = P(Wa Wb)/ P(Wb) = |Wa Wb| / |Wb| If a and b are

29、independent: P(Wa Wb) = P(Wa)*P(Wb) P(Wa Wb| Wb) = P(Wa)*P(Wb)/P(Wb) = |Wa| / |W| * (|Wb| / |W|) / (|Wb| / |W|) = |Wa| / |W| = P(Wa),43,Overlap Analysis,Using |W| = |Wa|/ P(Wa), the researchers found: Web had at least 320 million pages in 1997 60% of web was covered by six major engines Maximum cove

30、rage of a single engine was 1/3 of the web,44,How to Improve the Coverage?,Meta-search engine: dispatch the user query to several engines at same time, collect and merge the results into one list to the user. Any suggestions? Homework: Develop a meta-search engine which responds user query with comb

31、ined search results from a few search engines.,45,Probability,Model uncertainty: make inferences about events given observed data An event e: proposition or statement about the world at large “the number of Web pages in existence on 1 January 2003 was greater than five billion” A probability P(e): c

32、an be viewed as a number that reflects our uncertainty about whether e is true or false in the real world, given whatever information we have available.,46,Learning from a Bayesian Perspective,A conditional probability P(e | D): represent the degree of belief (Bayesian interpretation of probability)

33、, where D is the background information (data) on which our belief is based. Bayesian approach: probability as being a dynamic entity updated when more data arrivePrior probability: P(e) is your belief in the event e before you see any data Posterior probability: P(e | D) reflects your updated belie

34、f in event e given the observed data D Likelihood: P(D | e) is the probability of the data under the assumption that e is true How to model P(D | e)?,47,Standard Probabilistic Distribution,Discrete distributions,Continuous distributions,Geometric,Poisson,Exponential,Gamma,48,Learning from a Bayesian

35、 Perspective (cont.),Take logarithms for easier operationsObtain more data D2 (second data set),49,Parameter Estimation from Data,Maximum a posteriori (MAP) The objective of parameter estimation is to find or approximate the best set of parameters for a model, i.e., to find the set of parameters max

36、imizing the posterior P(|D), or log P(|D). This is called maximum a posteriori (MAP) estimation. To deal with positive quantities, we can minimize - log P(|D)P(D) plays the role of a normalizing constant and is thus irrelevant for the optimization, i.e.,the minimization of If the prior P() is uniform over sample space, then the problem reduces to finding the maximum of P(D|), or log P(D|). This is known as maximum likelihood (ML) estimation. Simpler ML estimation procedure, i.e., the minimization of,WMMKS Lab,Basic Formula,