Server side languages for the web operate as CGI
(Common Gateway Interface)
programs. CGI is not a language itself but rather is a protocol for
communicating
between the server and the client web browser. This protocol can
be implemented by many languages, of which the most common are
Perl, C, C++, various Unix scripts,
Rexx, and Python. Perl is by far the most widely used at
present.
These languages might be described as follows:
- Specific language characteristics: Perl is an interpreted language
which is especially good for "Here Documents", Regular expressions,
associative arrays, and easy access to the operating system. "Here
documents" are useful because CGI programs typically write other programs,
either in the form of HTML or embedded HTML scripts, such as JavaScript.
They let you enclose entire HTML/SCRIPT programs without any
modifications between start and end lines of the form:
print <<ANY_END_STRING
blah blah blah
. . .
ANY_END_STRING
(see this
for an example of a "Here" document
which prints out a program for a Gnuplot graph).
The only exceptions are a few special characters, such as "\n" or "\t"
often found in JavaScript which need to be doubled, e.g., "\\n" or "\\t".
Regular expressions are difficult to get used to but once learned make
it very easy to locate and/or modify text strings, and associative arrays
let
you locate and manipulate arrays by easily understood text indices,
rather than numbers, e.g., $phone_number{"John Jones"} is much more
understandable than $phone_number{273}.
Perl 5 also has some Object Oriented capabilities.
C and C++ are also widely used but are compiled languages and lack the
regular expressions and "Here documents" that are so useful in Perl.
- Characteristics in common: these languages live entirely on the
server. They can receive information from the client, usually by way of
the Submit button which transfers control and the
name-value pairs in the form to the server, and they can rewrite HTML
client side pages and send them to the client, but they cannot modify
anything on the client, except for rewriting the page.
- Invocation: you can invoke them through a URL, and can pass
parameters
to them though the URL followed by "?p1=value&p2=value&p3=value" (the GET
method: see this
for a very simple example of a CGI
program invoked by typing a URL into a browser, and this
for a more complex example).
The most usual way is by pressing the submit
button associated with a form which transfers control to the CGI program
along with the parameters in the form. You can also invoke them through
the #exec Server Side Include function in HTML scripts
whose file extension reads .shtml as opposed to just
.html (see this
for an example, and see this
for the source code)
- Advantages: access to the same environment on the server machine
that any user program has, except when specifically restricted
by the Web administrators. Thus all programs, files, graphics
(see this
for an example of a Gnuplot
CGI file),
databases,
environmental variables, and system calls, accessible thru any
normal program are available. In addition, there is information
associated with the Web interface, and a small amount of
information (such as supported MIME types and the browser name/
version) about the client browser.
- Disadvantages: these languages cannot *change* anything on the
client: they can only *re-write* web pages which they transmit
as a whole. Also, the speed and uniformity of response is lacking,
since communication is thru modem or E-net, and the load on the
server is varying (see this
for an
example as well as instructions for typing in a URL which
implements animation. Although
this method works, the animation is rather slow and jerky, and
is useless over a low speed line since the images don't appear to
be cached. When this is called through a SUBMIT button using
client-pull animation the animation is constantly interrupted by
requests to repost the data).
- Mixing client and server side scripts: you can fairly easily
combine client and server (CGI) scripts to work together. Here
is an example
of JavaScript program that writes HTML which lets you to specify the
locations of people, but which calls a
CGI Program to make those changes permanent.
|
|
 |
|
|
