基于Web的应用系统开发.ppt

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1、基于Web的应用系统开发4.Web browsers,servers and HTTP2OverviewlClient-server paradigm lWeb browserslWeb serverslURL lMIMElHTTP 3The client server paradigmlA widely used form of communicationlServer application waits passively for contact from clientslA server provides a specific servicelClient application act

2、ively initiates contact with the serverlInformation can flow in both directionslTypical situation is many clients interacting with each server4Web BrowserslBrowsers are clients always initiate,servers react lAllow user to browse resources available on server either existing or dynamically built docu

3、mentsl Mosaic-NCSA(Univ.of Illinois),in early 1993First to use a GUI,led to explosion of Web useInitially for X-Windows,under UNIX,but was ported to other platforms by late 1993lCurrent common browsersInternet Explorer,Netscape,Opera,Mozilla Firefox,Safari,Google Chrome(as of 9/3/08)5Web Servers lPr

4、ovide responses to browser requestslAll communications between browsers and servers use Hypertext Transfer Protocol(HTTP)lWeb servers run as background processes in the operating systemMonitor a communications port on the host,accepting HTTP messages when they appearlCommon serversApache,Internet In

5、formation Server(IS)6Uniform Resource Locators(URLs)lStandard way of specifying entities on networkslFirst part-protocolterminated by colon(:)common protocols are http,ftp,mailto,telnet,i.e.:http:ftp:mailto:telnet:lSecond part-varies according to protocolmailto-e-mail address e.g.:resource-oriented

6、protocols(http,ftp etc)Host name+domain names(preceded by/)may optionally include username,password and portPathname(usually related to the path of a file on the server)i.e./fully-qualified-domain-name/path-to-documentlOptional third partsQuery string(preceded by?)Fragment identifier(preceded by#)7E

7、xample URLslhttp:/:80/lhttp:/:8799/lhttp:/ Server CharacteristicslWeb servers have two main directories:1.Server root(server system software)2.Document root(servable documents)This will map to the URL of the full domain name,e.g.:User document root directoryDirectories of a standard name in the user

8、s home directoryUsually this is called public_htmlThe URL is then mapped as username e.g.:http:/ Server CharacteristicslDocument root is accessed indirectly by clientsIts actual location is set by the server configuration fileRequests are mapped to the actual locationE.g.doc root is topdocs and stor

9、ed in/admin/webSite isWhen there is a request for Server searches for file with address/admin/web/topdocs/bulbs/tulips.html 10Additional Server Features lVirtual document treesPart of servable document collection stored outside the document root lVirtual hosting Support for more than one site on a c

10、omputerlProxy serversServe documents that are in the document root of other machines11Multipurpose Internet Mail Extensions(MIME)lOriginally developed for emaillUsed to specify document types transmitted over the WebMIME type attached by the server to the beginning of the documentl Type specificatio

11、nsForm:type/subtypeExamples:text/plain,text/html,image/gif,image/jpeg12MIME lServer gets type from the requested file names suffix(.html implies text/html)lBrowser gets the type explicitly from the serverlExperimental typesSubtype begins with x-e.g.video/x-msvideoExperimental types require the serve

12、r to send a helper application or plug-in so the browser can deal with the file13World Wide Web Overview HTTP Server Apache MS IISClientNetscape Navigator MS Internet ExplorerHTTP request(URL)HTTP response(HTML data)14Design Paradigm of the WWWlWWW is a global hypertext systemlThe page is the basic

13、unit of the WWWlEach page has a unique identifier the URLlPages may contain links to data of any typelSome data(e.g.images)may be interpreted by the browser and displayed“inline”lPages may contain links to other URLs15The HTTP Protocol lInvented by Tim Berners-Lee in 1990lRFC 1945(1996)-HTTP/1.0lRFC

14、 2068(1997)-HTTP/1.1lRFC 2616(1999)-HTTP/1.1(update to 2068)16Features of HTTPlApplication level,client-server protocolPrimarily for distributed hypermedia systemsFlexible-thus has many other uses-e.g.:NameserversDistributed&collaborative document management systemslHTTP is small and fastMinimal per

15、formance overheadEasy to implementlHTTP is a stateless protocolEach request is an independent transaction-unrelated to any previous requests(unlike session-based protocols,e.g.FTP)AdvantageSimplifies server design-information about previous transactions does not need to be storedDisadvantageMore inf

16、ormation must be included in each request17HTTP OperationlOn the Internet HTTP usually uses TCP/IP connectionslTCP Port 80 is the default(though others can be specified)lHTTP uses a Request/Response paradigmClient establishes a connection to the server,and sends it a requestServer responds to the re

17、quest by generating a response(which may or may not contain content)18HTTP RequestlDelivered from a client to a server containing instructions for the serverlContainsthe method to be applied to the data resourcethe identifier of the resourcethe protocol version in uselMost commonly used methods:GET-

18、Fetch a document HEAD-Fetch just the header of the documentPOST-Execute the document,using the data in bodyPUT-Store a new document on the serverDELETE-Remove a document from the server19Request messageMETHOD /path-to-resource HTTP/version-numberHeader-Name-1:valueHeader-Name-2:valueoptional request

19、 body General request message structure GET/index.html HTTP/1.1 Host: Accept:text/*User-Agent:Mozilla/2.02Gold(WinNT;I)Example 20telnet HTTP requestlA browsers is not necessary to communicate with a web server telnet http GET/respond.html HTTP/1.121HTTP Response lMessage generated by a server after

20、receiving and interpreting a requestlResponses contain:Status line with the protocol version,a status code,and a“reason phrase”Response-Header(containing information about the server)Entity Header(meta-information)Entity Body(data)22Response messageHTTP/version-number status-code message Response-He

21、ader-Name-1:valueResponse-Header-Name-2:valueEntity-Header-Name-1:valueEntity-Header-Name-2:valueoptional entity body General response message structure HTTP/1.1 200 OKServer:Apache(Red-Hat/Linux)Content-Type:text/htmlContent-Length:9934 School of Management Example 23Some HTTP Status Codes l200:OKl

22、201:Createdl202:Acceptedl204:No Contentl301:Moved Permanentlyl302:Moved Temporarilyl400:Bad Requestl401:Unauthorizedl403:Forbiddenl404:Not Foundl500:Internal Server Errorl503:Service Unavailable24SummarylClient-server paradigm lWeb browserslWeb serverslURLs lMIME typeslHTTP protocolRequests and responses