Worl Wide Web Components NYU Stern：World Wide Web部件纽约大学斯特恩

1、browsers and serverscgi processing model(common gateway interface) norman white, 2013www and client server computingforms & cgi programmingwriting a cgi programserver computers are located all around the world and respond to requests (messages) from computers running browser software (netscape,

2、ie)browser applications understand html, (and now javascript, java etc.)web server sends a header in front of each file identifying the file type (html,gif,jpeg etc.)most browsers understand html, gif and textbrowsers can be configured to call external programs to handle new types of filesthese prog

3、rams are called helper applications and dramatically extend the capabilities of the browser, since they can be developed independently of the client softwareexamples - quicktime viewers, sound players, vrml viewers etc.to see the currently configured viewers go to options on the browser title barbro

4、wser functionality can also be extended by adding plugins.plugins are not standalone applications, but executable code that is dynamically linked into the browser when necessary.html provides an easy to use form capability, which allows a wide variety of input forms to be easily generated.form data

5、types include text input - one line of texttextarea - multiple lines of textcheck boxes (on/off)radio boxes (1 of n)etc.output of form is formatted and sent to server, along with the name of a program to process the contents of the form.the web server takes information from form, and passes it on as

6、 input to a common gateway interface program (cgi)output of cgi program is sent back to client browser as an html (or other) file.cgi programs can do an almost unlimited set of activities .look up info in a database and send it to browser.take input from user and add to a file.take input and send to

7、 a standard business applicationcgi program can be in any language that runs on the server, including a shell language like sh or bash.develop form to collect information from userswrite and test cgi program to handle form informationput the name of the cgi program in the “action” statement of the f

8、orm.note: program can be on another server.two types of form processing options, get and postget - parameters sent as additions to url string. each individual parameter separated by &post - data sent in message body. this is a more general method and can handle more input data.server sends form

9、(in html document) to clientclient displays form, and user fills in fieldsclient sends form info to server (submit button)server runs the cgi program named in the action section of the formcgi program parses data as inputoutput of cgi program is sent by the server to the client (i.e. it should be ht

10、ml)advantagesvery general model, easy to do really neat things like front end existing applications, databases etc.many toolkits available to do common thingsdisadvantagesall processing is done on server. may overload serverinteraction is all through formslots of data traffic back and forthsolutionh

11、tml5 and its featurescgi program needs to parse form inputprocess the inputgenerate html outputget formatinformation is passed as a series of variable=value pairs separated by “&” to program named in action statement by adding them on to the url (after a “?”)simple example one line form with a f

12、ield named “userid” and “action=mycgiprog.cgi”user enters “nwhite”browser sends the following to the web server/nwhite/mycgiprog.cgi?userid=nwhiteweb server takes the information after the “?” and creates an environment variable named “query_string”, then executes the program

13、“mycgiprog.cgi”query_string contains userid=nwhitecgi program retrieves value of query_string, does appropriate processing, and (optionally) sends an html response backboth windows and unix support environment variables. these are user session variables which contain character strings. many are auto

14、matically created when the user logs in, like path, prompt etc. any program can create or retrieve the value of environment variables, so they are often used to pass small amounts of information from one application to another. different operating systems have different methods for setting and retri

15、eving environment variables. for example, in unix, you can retrieve an environment variables value by putting a $ in front of it i.e. $path. in windows, you put % around it. i.e. %path%try this in unixecho $pathor windowsecho %path%what if we want have more than one field?no problem query_string can

16、 contain many variable=value pairs separated by “&” i.e. userid=nwhite&password=junk&fname=normanpossible problem, how big can environment variables be (how many characters)get only useful for limited inputpost method is more general since it can handle lots of inputinput is passed as a

17、sequence of characters (stdin)variable1=value1&variable2=value2.environment variable content_length is set to the number of characters of input.environment variable request_method is set to post (instead of get)input processing logic needs to be (slightly) different for get and post methodscgi o

18、utput is passed back to the browser, hence has to be something (html) the browser can understand likecontent-type: text/html output of html from cgi script sample output what do you think of this? list the contents of your “websys” directorycreate a shell script named lister.cgi which contains#! /bi

19、n/sh#echo “content-type: text/html”echo “ “echo listingecho ls altecho list the contents of your “websys” directorywith options passed as part of url (type=xxx)create a shell script named lister.cgi which contains#! /bin/sh#eval qs=echo $query_stringeval echo $qs|sed e “s/&/ /g” echo “content-ty

20、pe: text/html”echo echo listing echo ls *.$typeecho to run it, put it in your websys directorychmod +rx lister.cgitype the following as a url/userid/websys/lister.cgi?type=htmlyour useridin the post method, form data is not passed as part of the url, instead it is passed to the “standard input” of the cgi program.advantagesnot limited by max size of environment variablesusers cant see the input fieldsdisadvantagesa little harder to handleuser