Tài liệu WebChat part 2

As with the other CGI programs, the HTTP header is printed to let the
Web server know that we are about to send HTML data to the Web
browser. However, unlike most other CGI programs, Web Chat also prints
a special code telling the Web browser not to cache the HTML during an
active chat session. During a real-time chat, the user may reload the page
every minute to look at the latest messages. It would be a waste of disk
space to cache data that is constantly out of date. Thus, the Pragma: nocache
message is delivered along with the normal Content-type:
text/html message.
The “no-cache” message is given only if a session form variable is set.
This is because we want to cache the initial chat room entrance screen even
if we do not cache the individual chat sessions. Because the messages are
constantly changing, it is inefficient for the Web browser to constantly cache
those pages. When the user first starts the script, no session variable has yet
been set, and the script can use this fact to determine its course of action.
print Content-type: text/html
;
if ($in{'session'} ne ) {
print Pragma: no-cache
;
} else {
print
;
}