World Wide Web Components

1
World Wide Web Components

• Browsers and Servers
• CGI Processing Model

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WWW is example of Client/Server Computing

• Server computers are located all around the world
  and respond to requests (messages) from Computers
  running Browsers.
• Browsers understand HTML, (and now Javascript,
  Java etc.)

3
Server Browser Interaction (simple)
http request
B
Server
HTML
4
Non-HTML files

• Server sends a header in front of each file
  identifying the file type (HTML,GIF,JPEG etc.)
• Most Browsers understand HTML, GIF and TEXT
• Browsers can be configured to call external
  programs to handle new types of files

5
Helper Apps

• These programs are called HELPER applications and
  dramatically extend the capabilities of the
  browser, since they can be developed
  independently of the client software
• Examples - Quicktime viewers, sound players, VRML
  viewers etc.
• To see the currently configured viewers go to
  options on the Netscape title bar

6
Forms and CGI Programming

• HTML provides an easy to use FORM capability,
  which allows a wide variety of input forms to be
  easily generated.
• Forms include
• Text input - One line of text
• Textarea - Multiple lines of text
• Check boxes (on/off)
• Radio boxes (1 of N)
• Etc.

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Forms Processing Logic

• Output of Form is formatted and sent to Server,
  along with the name of a program to process the
  output of the form.
• WEB Server takes information from form, and
  passes it on to a Common Gateway Interface
  Program (CGI)
• Output of CGI program is sent back to Client
  browser as an HTML (or other) file.

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CGI programming extends power of WWW

• 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 a standard business
  application
• CGI program can be in any language that runs on
  the server

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CGI Programming
http forms
B
http server
HTML
input
output
CGI Program
(Note, all processing is on server)
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What do you need to do for CGI?

• Develop form to collect information from users
• Write and test CGI program to handle form
  information
• Put the name of the CGI program in the ACTION
  statement of the form.

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CGITwo Processing options

• Two Types of FORM processing options, GET and
  POST
• GET - parameters sent additions to URL string.
  Each individual parameter separated by
• POST - Data sent in message body. This is
  slightly more general

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CGI Processing - review

• Server sends form to client
• Client displays form, and users fills in fields
• Client sends form info to server
• Server runs the CGI program named in the ACTION
  section of the FORM..
• CGI program parses data as input
• Output of CGI program is sent by the server to
  the client (i.e. it should be HTML)

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CGI Advantages and Disadvantages

• Advantages
• Very general model, easy to do really neat things
  like front end existing applications, databases
  etc.
• Many toolkits available to do common things
• Disadvantages
• All processing is done on server. May overload
  server

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Writing a CGI Program

• CGI program needs to
• Parse form input
• Process the input
• Generate html output

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GET vs. POST

• GET format
• Information is passed as a series of
  variablevalue 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 field named
  userid and ACTIONmycgiprog.cgi
• User enters nwhite
• Browser sends the following to the web server
• http//www.stern.nyu.edu/nwhite/mycgiprog.cgi?use
  ridnwhite

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GET ProcessingServer Side

• Web server takes the information after the ?
  and creates an environment variable named
  QUERY_STRING, then executes the program
  mycgiprog.cgi
• QUERY_STRING contains
• useridnwhite
• CGI program retrieves value of QUERY_STRING, does
  appropriate processing, and (optionally) sends an
  HTML response back

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GET method more than one parameter

• What if we want have more than one field?
• No problem QUERY_STRING can contain many
  variablevalue pairs separated by
• i.e.
• useridnwhitepasswordjunkfnameNorman
• Possible problem, how big can environment
  variables be (how many characters)
• GET only useful for limited input

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POST

• POST method more general
• Input is passed as a series of input lines
• Variable1value1
• Variable2value2
• .
• Environment variable CONTENT_LENGTH is set to the
  number of lines of input.
• Input processing logic needs to be different for
  GET and POST methods

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CGI Output

• CGI output is passed back to the browser, hence
  has to be something (HTML) the browser can
  understand Like
• Content-type text/html
• ltHTMLgtltHEADgt
• ltTITLEgtoutput of HTML from CGI
  scriptlt/TITLEgt
• lt/HEADgtltBODYgt
• ltH1gtSample outputlt/H1gt
• What do you think of
  ltSTRONGgtthis?lt/STRONGgt
• lt/BODYgtlt/HTMLgt

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Simple Example GET Method

• List the contents of your multim directory
• Create a Shell Script named lister.cgi which
  contains
• ! /bin/sh
• echo content-type text/html
• echo lthtmlgtltheadgtlttitlegtListinglt/titlegt
• echo lt/headgtltbodygtltpgt
• pwd
• ls alt
• echo lt/bodygtlthtmlgt

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Try it, what happens?

• To run it, put it in your multim directory
• chmod rx lister.cgi
• Type the following as a URL
• http//www.stern.nyu.edu/userid/multim/lister.sh

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CGI Example POST Method

• Adds links to a file of students in a class
• Password Protected
• Steps
• Parse data (POST form)
• Check Password
• Add info (in html format) to end of roster

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• include ltstdio.hgt
• ifndef NO_STDLIB_H
• include ltstdlib.hgt
• else
• char getenv()
• endif
• define MAX_ENTRIES 10000
• typedef struct
• char name
• char val
• entry
• char makeword(char line, char stop)
• char fmakeword(FILE f, char stop, int len)
• char x2c(char what)
• void unescape_url(char url)

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void unescape_url(char url) void
plustospace(char str) main(int argc, char
argv) entry entriesMAX_ENTRIES
register int x,m0 int cl int PASS
FILE fp printf("Content-type
text/htmlcc",10,10) if(strcmp(getenv("REQU
EST_METHOD"),"POST")) printf("This
script should be referenced with a METHOD of
POST.\n") printf("If you don't
understand this, see this ") printf("ltA
HREF\"http//www.ncsa.uiuc.edu/SDG/Software/Mosai
c/Docs/fil\ l-out-forms/overview.html\"gtforms
overviewlt/Agt.c",10) exit(1)
if(strcmp(getenv("CONTENT_TYPE"),"application/x-ww
w-form-urlencoded")) printf("This
script can only be used to decode form results.
\n")
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if(strcmp(getenv("CONTENT_TYPE"),"application/x-ww
w-form-urlencoded")) printf("This
script can only be used to decode form results.
\n") exit(1) cl
atoi(getenv("CONTENT_LENGTH")) PASS 0
for(x0cl (!feof(stdin))x) mx
entriesx.val fmakeword(stdin,'',cl)
plustospace(entriesx.val)
unescape_url(entriesx.val)
entriesx.name makeword(entriesx.val,'')
/ Check here for passwd field Z /
if (entriesx.name'p') if
(strcmp(entriesx.val,"MDSS") 0) PASS 1
if (PASS 1) if ((fp
fopen("/class/nwhite/b3146/roster.html","a"))
NULL) PASS3 printf("lth1gt
unable to open output file, tell Professor
Whitelt/h1gt")
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NULL) PASS3 printf("lth1gt unable
to open output file, tell Professor
Whitelt/h1gt")
else fprintf(fp,"ltA href\"http//www.stern
.nyu.edu/s\" gt Home Page for s \n \ lt/agt",
entries0.val,entries2.val)
fprintf(fp,"ltpgtlth3gtWhy you might want to visit
the homepage for s ... lt/h3\ gt\n ltpgt",
entries2.val) fprintf(fp,"s
ltpgt",entries3.val) fprintf(fp,"ltpgtlthrgtlthrgtlt
pgt") printf(" Link successfully added
\n") printf("lta href\"http//www.stern.nyu.
edu/nwhite/class/b3146/roster.html\ \"gt
ltbgtClick here to see the updated rosterlt/bgt
lt/agt") if (PASS ! 1)
printf("ltH1gt INVALID PASSWORD, See Professor
White lt/H1gt")
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Conclusion

• World-Wide-Web model is much more powerful than
  it appears on the surface
• Easily integrated with existing applications
• Easy to add new functionality
• CGI model can do lots of things
• Update files
• Link to corporate databases
• Specialized Applications

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Caveats

• Problems
• Overhead
• Need to start up a new program for every request
• Scalability
• All processing on server, what happens as usage
  grows?
• Reliability
• How do we replicate for redundancy?