1. CGI
CS422
Dick Steflik

2. Common Gateway Interface
"Old School"
Basis for all of the current ways of making Web Apps
Php
ASP
JSP
Java Servlets

3. CGI
Commonly used languages
PERL
Python
Ruby

4. CGI
Provides a mechanism for a browser to request a web server to run a local program that will provide the body of an HTTP response packet
The request is in the form of an HTTP request packet and can be a GET or POST

5. Environment Variables
CGI specifies a set of Environment Variables that are created from a combination of:
the servers runtime environment
information from the header of the browser's request packet

6. COMSPEC="C:\Windows\system32\cmd.exe"
DOCUMENT_ROOT="C:/Program Files (x86)/Apache Software Foundation/Apache2.2/htdocs" GATEWAY_INTERFACE="CGI/1.1"
HOME="/home/SYSTEM"
HTTP_ACCEPT="text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8" HTTP_ACCEPT_CHARSET="ISO ,utf-8;q=0.7,*;q=0.7"
HTTP_ACCEPT_ENCODING="gzip, deflate"
HTTP_ACCEPT_LANGUAGE="en-us,en;q=0.5"
HTTP_CONNECTION="keep-alive"
HTTP_HOST="example.com"
HTTP_USER_AGENT="Mozilla/5.0 (Windows NT 6.1; WOW64; rv:5.0) Gecko/ Firefox/5.0" PATH="/home/SYSTEM/bin:/bin:/cygdrive/c/progra~2/php:/cygdrive/c/windows/system32:..." PATHEXT=".COM;.EXE;.BAT;.CMD;.VBS;.VBE;.JS;.JSE;.WSF;.WSH;.MSC"
PATH_INFO="/foo/bar"
PATH_TRANSLATED="C:\Program Files (x86)\Apache Software Foundation\Apache2.2\htdocs\foo\bar" QUERY_STRING="var1=value1&var2=with%20percent%20encoding"
REMOTE_ADDR=" "
REMOTE_PORT="63555"
REQUEST_METHOD="GET"
REQUEST_URI="/cgi-bin/printenv.pl/foo/bar?var1=value1&var2=with%20percent%20encoding" SCRIPT_FILENAME="C:/Program Files (x86)/Apache Software Foundation/Apache2.2/cgi-bin/printenv.pl" SCRIPT_NAME="/cgi-bin/printenv.pl"
SERVER_ADDR=" "
SERVER_ADMIN="(server admin's address)"
SERVER_NAME=" "
SERVER_PORT="80"
SERVER_PROTOCOL="HTTP/1.1"
SERVER_SIGNATURE=""
SERVER_SOFTWARE="Apache/ (Win32) PHP/5.2.17"
SYSTEMROOT="C:\Windows"
TERM="cygwin"
WINDIR="C:\Windows"

7. timeline user clicks "Submit" on web page
Browser builds an HTTP request packet
HTTP method is from <form> tag method attribute
if method is GET data is added on to URL as Query_String
if method is POST data (name value pairs) are put into the request packet body as URL encoded data
CGI is from <form> action attribute)
HTTP_User_Agent is set by the browser
TCP connection is made to port 80 on server from a random high numbered port on the client and request packet is sent to server
Server blocks port 80 and transfers to a random high numbered port to process the request packet then unblocks port 80
The server processes the request

8. timeline (cont.) server processing packet is received
body goes to SYSIN
info is taken from the request header and moved into environment variables
if HTTP method is GET
is the requested resource a CGI
then run the CGI
get data from QUERY_STRING
build the response packet (set Content-type to text/html)
else retrieve the resource
if HTTP method is POST
run the CGI
read the data from SYSIN
send the response packet
close the TCP connection

9. name/value data
depending on the CGI language the name/value data is handeled differently
python name/values are consumed from SYSIN or QUERY_STRING and moved to a Python dictionary called FieldStorage() and can be retrieved via getvalue()
perl usually moves the name/value pairs from SYSIN or QUERY_STRING into a buffer and then splits them into an associative array
java usually has library support to put it into a hash table

10. State in a Stateless Environment
program state in the HTTP stateless environment must be maintained by coordinating the browser and the CGI
one way is by hiding state variables in the HTML form by using hidden fields modified by the CGI to reflect state
<input name= something type=hidden />
another way is by using cookies in the response header
or a combination of the two