The document discusses Cache-Control directives that can be used in HTTP requests and responses to control caching behavior. It describes directives like no-cache, max-age, public, and private that clients, proxies, and servers can use to force validation with origin servers, allow caching for certain periods, or restrict caching to clients only. The document also provides examples of how servers can control caching using directives like private, proxy-revalidate, and max-age in responses.

1. Dr . Majzoob Kamal Eldein Omer

2.  Provide finer cache control to both sender and recipient
 A new general Cache-Control directive that must be
obeyed!
 Cache Proxy, Client browser and server are obligated to
obey
Some of the directives in requests:
 Cache-control: no-cache Client forces up-to-date
check with origin server.

3.  Cache-control: max-age=1000 Client (user
or intermediate cache) is prepared to accept
object without up-to-date check if age is < 1000
seconds.
 Overrides expires: If document has not expired but
document age is > max-age, then cache performs an up-
to-date check.

4.  Cache-control : public The object is
cacheable by client and network
caches…basically anywhere!
 Cache-control: private The object can be
cached only by the client.
 Cache-control: max-age =1000 Server
imposes up-to-date check after 1000 seconds.
Similar to Expires header.

5. Server control:
 If origin server doesn’t want the object cached, it can
put in response cache-control:private .
 If origin server wants an up-to-date check for each
request, it can put in response cache-control: proxy-
revalidate .
 If origin server wants up-to-date check once the object
age > 1000, it can put in response: cache-control: max-
age=1000

6. Browser control:
 If client wants an up-to-date check even if the object
has not expired, it puts in request: Cache-control: no-
cache .
 In client is willing to accept the object that is less than
1000 seconds old without an up-to-date check, it can put
in request: Cache-control: max-age=1000 .

8.  CGI stands for Common Gateway Interface.
 CGI is a standard programming interface to Web
servers that gives us a way to make our sites
dynamic and interactive.
 CGI is not a programming language. It is just a
set of standards (protocols.)
 CGI can be implemented in an interpreted
language such as PERL or in a compiled language
such as C.

9.  CGI programs work as follows:
 STEP 1 (On the client side): Get Information from
the user (using HTML forms, SSI, Java Applet,
…,etc).
 STEP 2 (On the server side): Process the data,
connect to DATABASE, search for PATTERNS,
…,etc.
 STEP 3 (On the server side): Send the result of
computation back to the client.

11.  The steps involved in preparing a CGI
program for work.
1. Contact your Web host to find out where
you should put your CGI programs on the
server. Usually it will be a directory on
the server named “cgi-bin”.
2. Set up the access permission of directory
“cgi-bin”

12. 3- Set up the access permission of the CGI
program
4- Test your CGI program from the server
command line to make sure it works
perfectly before publishing it.

13.  <form action=“cgi-prog.cgi”
method=“POST”>
. . .
</form>
 Two HTTP methods are available:
 GET
 POST
 The CGI program will be invoked every
time you the button SUBMIT is clicked.

14.  GET Method the data passed within the
query string of URL : for example:
http://std.edu/info/program?querystring
the data sent after the “?”
POST Method :
The data sent via entity body

15.  CGI Script Output Format :
 In whatever language a CGI script is
programmed it MUST send information back
in the following format:
 The Output Header
 A Blank Line
 The Output Data

16.  Content-Types
 The following are common formats/content-
types :
Format Content-Type
 HTML text/html
 Gif image/gif
 JPEG image/jpeg

17.  Depending on the Content-Type defined the data that follows
the header declaration :
 • If it is HTML that follows then the CGI script must output
standard HTML syntax.
 Example: To produce aWeb page that the server sends to a
browser with a simple line of text ”HelloWorld!” . A CGI script
must output:
 Content-Type: text/html
 <html>
 <head>
 <title>Hello, world!</title>
 </head>
 <body>
 <h1>Hello, world!</h1>
 </body>
 </html>

18. #!/usr/bin/perl
print "Content-Type: text/htmlnn";
print "<html> <head>n";
print "<title>Hello, world!</title>";
print "</head>n";
print "<body>n";
print "<h1>Hello, world!</h1>n";
print "</body> </html>n";