World Wide Web Components Browsers and Servers CGI Processing Model (Common Gateway Interface) © Norman White, 2001
WWW is example of Client/Server Computing Server computers are located all around the world and respond to requests (messages) from Computers running browser software (Netscape, IE) Browser applications understand HTML, (and now Javascript, Java etc.)
Server Browser Interaction (simple) BServer http request Browser send http request to server (I.e. GET index1.html)
Index1.html file <html><head> Sample Title Sample Title </head><body> Here is some text and a picture Here is some text and a picture </body></html>
Server Response BServer http request HTML file Server retrieves file Sends file (index1.html) to Browser
BServer http request HTML index.html Browser “formats” index1.html May mean retrieving more files In order to display Browser Displays file
Browser asks for next file BServer http request HTML index1.html GET pic1.gif index.html contains reference to pic1.gif Browser then requests pic1.gif
Server sends pic1.gif BServer http request Index1.html GET pic1.gif index1.html pic1.gif Server next sends pic1.gif
BServer http request index1.html GET pic1.gif index1.html pic1.gif pic1.gif Browser displays pic1.gif
RESULT ???
Processing 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
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
Plugins Browser 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.
Forms and CGI Programming HTML provides an easy to use FORM capability, which allows a wide variety of input forms to be easily generated. Form data types include – Text input - One line of text – Textarea - Multiple lines of text – Check boxes (on/off) – Radio boxes (1 of N) – Etc.
Forms Processing Logic 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 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 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
CGI Programming B httpserver CGI Program http form content input output HTML (Note, all processing is on server)
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 URL of the CGI program in the “ACTION” statement of the form.
CGI Two 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 a more general method and can handle more input data.
CGI Processing - review Server sends form (in html document) to client Client displays form, and user fills in fields Client sends form info to server (SUBMIT button) 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)
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
Scalability At first, CGI model seems limited by server capacity, but with a few tricks we can change that. Note that the “ACTION” statement points to a URL. Since the URL can also include the server name, we can have “application” servers for specialized applications, so all the processing is not done on the web server.
Writing a CGI Program CGI program needs to – Parse form input – Process the input – Generate html output I.e the program has to dynamically generate the Appropriate HTML statements
GET vs. POST Alternative CGI methods GET format – Information 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 field named “userid” and “ACTION=mycgiprog.cgi” – User enters “nwhite” – Browser sends the following to the web server
GET Processing Server 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 – userid=nwhite CGI program retrieves value of QUERY_STRING, does appropriate processing, and (optionally) sends an HTML response back
GET method – more than one parameter What if we want have more than one field? No problem QUERY_STRING can contain many variable=value pairs separated by “&” i.e. u serid=nwhite&password=junk&fname=Norman Possible problem, how big can environment variables be (how many characters) GET only useful for limited input
POST Method POST method more general Input is passed as a series of input lines (stdin) Variable1=value1 Variable2=value2 …. Environment variable CONTENT_LENGTH is set to the number of characters of input. Input processing logic needs to be different for GET and POST methods
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 output of HTML from CGI script Sample output What do you think of this?
Simple Example – GET Method List the contents of your “websys” directory Create a Shell Script named lister.cgi which contains #! /bin/sh echo content-type: text/html echo Listing echo pwd ls –alt echo
Try it, what happens? To run it, put it in your websys directory chmod +rx lister.cgi Type the following as a URL ster.cgi
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
#include #ifndef NO_STDLIB_H #include #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); void plustospace(char *str);
void unescape_url(char *url); void plustospace(char *str); main(int argc, char *argv[]) { entry entries[MAX_ENTRIES]; register int x,m=0; int cl; int PASS; FILE *fp; printf("Content-type: text/html%c%c",10,10);
if(strcmp(getenv("REQUEST_METHOD"),"POST")) { printf("This script should be referenced with a METHOD of POST.\n"); printf("If you don't understand this, see this "); printf("<A HREF=\" l-out-forms/overview.html\">forms overview.%c",10); exit(1); } if(strcmp(getenv("CONTENT_TYPE"),"application/x-www-form- urlencoded")) { printf("This script can only be used to decode form results. \n");
if(strcmp(getenv("CONTENT_TYPE"),"application/x-www-form- urlencoded")) { printf("This script can only be used to decode form results. \n"); exit(1); } cl = atoi(getenv("CONTENT_LENGTH")); PASS = 0; for(x=0;cl && (!feof(stdin));x++) { m=x; entries[x].val = fmakeword(stdin,'&',&cl); plustospace(entries[x].val); unescape_url(entries[x].val); entries[x].name = makeword(entries[x].val,'='); /* Check here for passwd field = Z */ if (*entries[x].name=='p') {
if (strcmp(entries[x].val,"MDSS")== 0) PASS = 1;} } if (PASS == 1) {if ((fp = fopen("/class/nwhite/b3146/roster.html","a")) == NULL) {PASS=3; printf(" unable to open output file, tell Professor White ");
} else { fprintf(fp," Home Page for %s \n \ ", entries[0].val,entries[2].val); fprintf(fp," Why you might want to visit the homepage for %s... </h3\ >\n ", entries[2].val);
fprintf(fp,"%s ",entries[3].val); fprintf(fp," "); printf(" Link successfully added \n"); printf("<a href=\" \"> Click here to see the updated roster "); }} if (PASS != 1) { printf(" INVALID PASSWORD, See Professor White "); }
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
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?