ch17_03.htm

用perl编写CGI的好书。本书从解释CGI和底层HTTP协议如何工作开始

<p>If we do not load a module at startup, but use it in our
applications, then that module will have to be loaded once for each
child process. The same applies for our CGI applications running
under Apache::Registry. Each child process compiles and caches the
CGI application once, so the first request that is handled by that
child will be relatively slow, but all subsequent requests <a name="INDEX-3295" /> <a name="INDEX-3296" />will be much faster.</p>
</div>




<a name="ch17-13-fm2xml" /><div class="sect2">
<h3 class="sect2">17.3.2. mod_perl Considerations</h3>


<p>In general, Apache::Registry, does provide <a name="INDEX-3297" /> <a name="INDEX-3298" />a good emulation of a standard CGI
environment. However, there are some differences you need to keep in
mind:</p>


<ul><li><p>The same precautions that apply to FastCGI apply to
<em class="emphasis">mod_perl</em>, namely, always use
<a name="INDEX-3299" />
<a name="INDEX-3300" />strict mode and it helps to enable
warnings. You should also always initialize your
<a name="INDEX-3301" />variables and not assume they are empty
when your script starts; the warning flag will tell you when you are
using undefined values. Your environment is not cleaned up with you
when your script ends, so variables that do not go out of scope and
global variables remain defined the next time your script is called.</p></li><li><p>Due to the fact that your code is only compiled once and then
<a name="INDEX-3302" />cached, <a name="INDEX-3303" />lexical variables in the body of your
scripts that you access within your subroutines create closures. For
example, it is possible to do this in a standard CGI script:
<blockquote><pre class="code">
my $q = new CGI;

check_input(  );
.
.

sub check_input {
    unless ( $q-&gt;param( "email" ) ) {
        error( $q, "You didn't supply an email address." );
    }
    .
    .</pre></blockquote></p>


<p>Note that we do not pass our CGI object to
<tt class="function">check_input</tt>
<a name="INDEX-3304" />. However, the
variable is still visible to us from within that subroutine. This
works fine in CGI. It will create very subtle, confusing errors in
<em class="emphasis">mod_perl</em>. The problem is that the first time the
script is run on a particular Apache child process, the value of the
CGI object becomes trapped in the cached copy of
<tt class="function">check_input</tt>. All future calls to that same
Apache child process will reuse the original value of the CGI object
within <tt class="function">check_input</tt>. The solution is to pass
<tt class="literal">$q</tt> to <tt class="function">check_input</tt> as a
parameter or else change <tt class="literal">$q</tt> from a lexical to a
global <tt class="function">local</tt> variable.</p>

<p>If you are
not familiar with closures (they are not commonly used in Perl),
refer to the <em class="citetitle">perlsub</em> manpage or
<em class="citetitle">Programming Perl</em>.</p></li>

<li><p>The
<em class="emphasis">constant</em>
<a name="INDEX-3305" />
<a name="INDEX-3306" />
<a name="INDEX-3307" /> module creates
constants by defining them internally as subroutines. Since
Apache::Registry creates a persistent environment, using constants in
this manner can produce the following warnings in the error log when
these scripts are recompiled:
<blockquote><pre class="code">Constant subroutine FILENAME redefined at ...</pre></blockquote></p>


<p>It will not affect the output of your scripts, so you can just ignore
these warnings. Another alternative is to simply make them global
variables instead; the closure issue is not an problem for variables
whose values never change. This warning should no longer appear for
unmodified code in Perl 5.004_05 and higher.</p></li>

<li><p>
<a name="INDEX-3308" />Regular expressions that are
compiled with the <em class="emphasis">o</em> flag will remain compiled
across all requests for that script, not just for one
request.</p></li>

<li><p>
<a name="INDEX-3309" /> <a name="INDEX-3310" />File age
functions, such as <tt class="function">-M</tt>, calculate their values
relative to the time the application began, but with
<em class="emphasis">mod_perl</em>, that is typically the time the server
begins. You can get this value from <tt class="literal">$^T</tt> . Thus
adding <tt class="literal">(time - $^T)</tt> to the age of a file will
yield the true age.</p></li>

<li><p><tt class="literal">BEGIN</tt>
<a name="INDEX-3311" /> <a name="INDEX-3312" /> blocks are executed once when your script
is compiled, not at the beginning of each request. However,
<tt class="literal">END</tt> blocks are executed at the end of each
request, so you can use these as you normally would.</p></li>

<li><p><tt class="literal">__END__</tt>
<a name="INDEX-3313" /> and <tt class="literal">__DATA__</tt> cannot be used within CGI scripts with
Apache::Registry. They will cause your scripts to
fail.</p></li>

<li><p>Typically, your scripts should not call exit in
<em class="emphasis">mod_perl</em>, or it will cause Apache to exit
instead (remember, the Perl interpreter is embedded within the web
server). However, <em class="emphasis">Apache::Registry</em> overrides the
standard <tt class="function">exit</tt>
<a name="INDEX-3314" /> command so it is safe for
these scripts.</p>
</li>
</ul>

<p>If it's too much of a hassle to convert your application to run
effectively under Apache::Registry, then you should investigate the
<a name="INDEX-3315" />
<a name="INDEX-3316" />Apache::PerlRun
module. This module uses the Perl interpreter embedded within Apache,
but doesn't
<a name="INDEX-3317" />cache compiled versions of your code. As a
result, it can run sloppy CGI scripts, but without the full
performance improvement of Apache::Registry. It will, nonetheless, be
faster than a typical CGI application.</p>


<p>Increasing the speed of CGI scripts is only part of what
<em class="emphasis">mod_perl</em> can do. It also allows you do write
code in Perl that interacts with the Apache response cycle, so you
can do things like handle authentication and authorization yourself.
A full discussion of <em class="emphasis">mod_perl</em> is certainly
beyond the scope of this book. If you want to learn more about
<em class="emphasis">mod_perl</em>, then you should definitely start with
Stas Bekman's
<em class="emphasis">mod_perl</em>
<a name="INDEX-3318" /> guide, available at
<a href="http://perl.apache.org/guide/">http://perl.apache.org/guide/</a>.
Then look at <em class="citetitle">Writing Apache Modules with Perl and
C</em>, which provides <a name="INDEX-3319" /> <a name="INDEX-3320" />a very thorough, <a name="INDEX-3321" /> <a name="INDEX-3322" /> <a name="INDEX-3323" />although technical,
<a name="INDEX-3324" />
<a name="INDEX-3325" />
<a name="INDEX-3326" />overview
<a name="INDEX-3327" />
<a name="INDEX-3328" />
<a name="INDEX-3329" />of
<em class="emphasis">mod_perl</em>.</p>
</div>


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