Web

What is server push, and what is client pull?
A: Server push and client pull are each methods of initiating delivery of content
from a server computer to a client on a network. Most often, the terms are used to
describe the delivery methods of online content, such as World Wide Web page
code over the nternet, but it could also apply to the movement of messages or
other content across a local or wide area network.
!he current customary method to access World Wide Web content is to browse the
World Wide Web using a client computer and browser software. !he user usually
starts at a home page and moves to other World Wide Web pages by selecting
hyperlinks within the home page or by typing the "#$ of a destination page. n
either event, the client computer sends an %!!& re'uest for a particular file over
the nternet to a server computer at a particular nternet &rotocol ()&)* address.
!he server computer responds by sending the re'uested content over the nternet to
the & address of the re'uesting client computer. !his method is called client pull
because it is necessary for the client to send a re'uest in order for the client to
receive any content.
!he server push method uses server computers that send content independent of
re'uests from client computers. A server push network may use very broad
delivery such as that of the broadcast television system, in which the television
broadcast )server) sends content to everyone, but the messages are received only
by those who have their )client) television sets turned on and tuned in. A server
push network may also deliver content very narrowly, such as that of the ".S. Mail
system, in which a )server) letter writer sends a letter over the information footpath
to a )client) mailbo+ at a particular address. n each case, the transfer of the
content is initiated by the server rather than the client.
Most current content delivery methods described as server push are actually a
hybrid of client pull and server push delivery methods. #ather than send content
independent of any re'uest from a client computer, most servers using today,s
server push methods send content to a client computer in response to a client,s
automatic, software-driven re'uest. n most cases, the client software of a server
push client software automatically re'uests a server to send certain content, such as
a web page, an entire web site or speciali.ed content designed for the particular
delivery system. !he client software may then cause the client computer to store
the content on its hard drive for later use. t is contemplated that server push
delivery combined with additional programming may permit a server to send and
install content such as updates of software programs to all subscribers of the update
delivery service.
Extra Path Information
you can pass information to your /0 program by adding e+tra information to the
standard "#$. f your /0 program is used for searching your site, for e+ample,
you can pass some information to specify which directory to search. !he server
gauges where the /0 program name ends1 anything following is deemed )e+tra)
and is stored in the environment variable &A!%2345. !he following %!M$
hyperlink will invoke a search script and tell it to search the 6root6documents
directory.
7A %#849)cgi-bin6search.pl6root6documents):
Search the ;ocument ;irectory
76A:
!his extra path information can be accessed through the &A!%2345 8nvironment
<ariable.
8verything after search.pl is the e+tra path. !his extra path information can be
accessed through the &A!%2345 environment variable.
!he &A!%2!#A3S$A!8; variable is also set, mapping the&A!%2345 to the
document root (;5/"M83!2#55!* directory (e.g., /root/documents*.
Server Side Includes
Server Side Includes (SSI) is a simple interpreted server-side scripting language used almost
exclusively for the Web.
The most frequent use of SSI is to include the contents of one or more files into a eb page on a eb
server. !or example" a eb page containing a daily quotation could include the quotation by placing
the folloing code into the file of the eb page#
<!--#include virtual="../quote.txt" -->
With one change of the quote.txt file" all pages including the file ill display the latest daily
quotation. The inclusion is not limited to files" and may also be the text output from a program" or the
value of a system variable such as the current time.
Server Side Includes are useful for including a common piece of code throughout a site" such as a
page header" a page footer and a navigation menu. $onditional navigation menus can be conditionally
included using control directives.
In order for a eb server to recogni%e an SSI-enabled &T'( file and therefore carry out these
instructions" either the filename should end ith a special extension" by
default .shtml" .stm".shtm" or" if the server is configured to allo this" set the execution bit of the
file.
ExecCommand
the SSI exec command. It inserts the output from a CGI script or a
shell command in the document. Note that in the Jigsaw architecture
CGI scripts are just another resource class, so that no distinction is
made between executing a CGI script or including a fle.
Parameters:
• cmd = the command to execute
Example:
<!--#exec cmd="ls -lsa" -->
display the result of the ls command