=head1 NAME 

mod_perl - Embed a Perl interpreter in the Apache HTTP server 


=head1 DESCRIPTION

The Apache/Perl integration project brings together the full power of
the Perl programming language and the Apache HTTP server.  This is
achieved by linking the Perl runtime library into the server and
providing an object oriented Perl interface to the server's C language
API. 
These pieces are seamlessly glued together by the `mod_perl' server
plugin, making it is possible to write Apache modules entirely in
Perl.  In addition, the persistent interpreter embedded in the server
avoids the overhead of starting an external interpreter and the
penalty of Perl start-up (compile) time.  

Without question, the most popular Apache/Perl module is
Apache::Registry module.  This module emulates the CGI environment,
allowing programmers to write scripts that run under CGI or
mod_perl without change.  Existing CGI scripts may require some
changes, simply because a CGI script has a very short lifetime of one
HTTP request, allowing you to get away with "quick and dirty"
scripting.  Using mod_perl and Apache::Registry requires you to be
more careful, but it also gives new meaning to the work "quick"!
Apache::Registry maintains a cache of compiled scripts, which happens
the first time a script is accessed by a child server or once again if
the file is updated on disk.

Although it may be all you need, a speedy CGI replacement is only a
small part of this project. Callback hooks are in place for each stage
of a request.  Apache-Perl modules may step in during the handler,
header parser, uri translate, authentication, authorization, access,
type check, fixup and logger stages of a request.     

=head1 FAQ

The mod_perl FAQ is maintained by Frank Cringle
E<lt>fdc@cliwe.ping.deE<gt>: http://perl.apache.org/faq/

=head1 Apache/Perl API

See 'perldoc Apache' for info on how to use the Perl-Apache API.

See the lib/ directory for example modules and L<apache-modlist.html>
for a comprehensive list.

See the eg/ directory for example scripts.

=head1 mod_perl

For using mod_perl as a CGI replacement see the L<cgi_to_mod_perl> document.

You may load modules at server startup via:

    PerlModule Apache::SSI SomeOther::Module

Optionally:

    PerlRequire  perl-scripts/script_to_load_at_startup.pl

A B<PerlRequire> file is commonly used for intialization during server
startup time.  A PerlRequire file name can be absolute or relative to
B<ServerRoot> or a path in C<@INC>.   A B<PerlRequire>'d file must
return a true value, i.e., the end of this file should have a: 

 1; #return true value

See eg/startup.pl for an example to start with.

In an httpd.conf E<lt>Location /fooE<gt> or .htaccess you need:

    PerlHandler sub_routine_name

This is the name of the subroutine to call to handle each request. 
e.g. 
in the PerlModule Apache::Registry this is "Apache::Registry::handler".

If PerlHandler is not a defined subroutine, mod_perl assumes it is a
package name which defines a subroutine named "handler".

    PerlHandler   Apache::Registry

Would load Registry.pm (if it is not already) and call it's subroutine
"handler".

There are several stages of a request where the Apache API allows a
module to step in and do something.  The Apache documentation will
tell you all about those stages and what your modules can do.  
By default, these hooks are disabled at compile time, see the INSTALL
document for information on enabling these hooks.
The following configuration directives take one argument, which is the name
of the subroutine to call.  If the value is not a subroutine name, mod_perl
assumes it is a package name which implements a 'handler' subroutine.

    PerlChildInitHandler          (requires apache_1.3.0 or higher)
    PerlPostReadRequestHandler    (requires apache_1.3.0 or higher)
    PerlInitHandler
    PerlTransHandler    
    PerlHeaderParserHandler       
    PerlAccessHandler
    PerlAuthenHandler
    PerlAuthzHandler
    PerlTypeHandler
    PerlFixupHandler
    PerlHandler
    PerlLogHandler
    PerlCleanupHandler
    PerlChildExitHandler          (requires apache_1.3.0 or higher)

Only ChildInit, ChildExit, PostReadRequest and Trans handlers are not
allowed in .htaccess files.

Modules can check if the code is being run in the parent server during
startup by checking the $Apache::Server::Starting variable.

=head1 RESTARTING

=over 4

=item PerlFreshRestart

By default, if a server is restarted 
(ala kill -USR1 `cat logs/httpd.pid`), Perl scripts and modules are
not reloaded.  To reload B<PerlRequire>'s, B<PerlModule>'s, other
use()'d modules and flush the B<Apache::Registry> cache, enable with 
this command:

 PerlFreshRestart On  

=item PERL_DESTRUCT_LEVEL

With Apache versions 1.3.0 and higher, mod_perl will call the
perl_destruct() Perl API function during the child exit phase.
This will cause proper execution of B<END> blocks found during server
startup along with invoking the B<DESTROY> method on global objects
who are still alive.  It is possible that this operation may take a
long time to finish, causing problems during a restart.  If your code
does not contain and B<END> blocks or B<DESTROY> methods which need to
be run during child server shutdown, this destruction can be avoided by
setting the I<PERL_DESTRUCT_LEVEL> environment variable to C<-1>.

=back

=head1 ENVIRONMENT

Under CGI the Perl hash C<%ENV> is magical in that it inherits
environment variables from the parent process and will set them should
a process spawn a child.  However, with mod_perl we're in the parent
process that would normally setup the common environment variables
before spawning a CGI process.  Therefore, mod_perl must feed these
variables to C<%ENV> directly.  Normally, this does not happen until
the response stage of a request when C<PerlHandler> is called.  If you
wish to set variables that will be available before then, such as for
a C<PerlAuthenHandler>, you may use the C<PerlSetEnv> configuration
directive: 

 PerlSetEnv  SomeKey  SomeValue

You may also use the C<PerlPassEnv> directive to pass an already
existing environment variable to Perl's C<%ENV>:

 PerlPassEnv SomeKey 

=over 4

=item CONFIGURATION

The C<PerlSetVar> and C<PerlAddVar> directives provide a simple
mechanism for passing information from configuration files to Perl
modules or Registry scripts.

The C<PerlSetVar> directive allows you to set a key/value pair.

 PerlSetVar  SomeKey  SomeValue

Perl modules or scripts retrieve configuration values using the
C<$r-E<gt>dir_config> method.

  $SomeValue = $r->dir_config('SomeKey');

The C<PerlAddVar> directive allows you to emulate Perl arrays:

 PerlAddVar  SomeKey  FirstValue
 PerlAddVar  SomeKey  SecondValue
 ...         ...      ...
 PerlAddVar  SomeKey  Nth-Value

In the Perl modules the values are extracted using the
C<$r-E<gt>dir_config-E<gt>get> method.

  @array = $r->dir_config->get('SomeKey');

Alternatively in your code you can extend the setting with:

  $r->dir_config->add(SomeKey => 'Bar');

C<PerlSetVar> and C<PerlAddVar> handle keys case-insensitively.



=item GATEWAY_INTERFACE

The standard CGI environment variable B<GATEWAY_INTERFACE> is set to
C<CGI-Perl/1.1> when running under mod_perl.

=item MOD_PERL

The environment variable `MOD_PERL' is set so scripts can say:

 if(exists $ENV{MOD_PERL}) { 
     #we're running under mod_perl
     ...
 }
 else {
     #we're NOT running under mod_perl
 }

=back

=head1 BEGIN blocks

Perl executes C<BEGIN> blocks during the compile time of code as soon
as possible.  The same is true under mod_perl.  However, since
mod_perl normally only compiles scripts and modules once, in the
parent server or once per-child, C<BEGIN> blocks in that code will
only be run once.  As L<perlmod> explains, once a C<BEGIN> has run, it
is immediately undefined.  In the mod_perl environment, this means
C<BEGIN> blocks will not be run during each incoming request unless
that request happens to be one that is compiling the code.  

Modules and files pulled in via require/use which contain C<BEGIN>
blocks will be executed:
  - only once, if pulled in by the parent process
  - once per-child process if not pulled in by the parent process
  - an additional time, once per-child process if the module is pulled in off of disk again via Apache::StatINC
  - an additional time, in the parent process on each restart if PerlFreshRestart is On
  - unpredictable if you fiddle with C<%INC> yourself

B<Apache::Registry> scripts which contain C<BEGIN> blocks will be
executed: 
  - only once, if pulled in by the parent process via B<Apache::RegistryLoader>
  - once per-child process if not pulled in by the parent process
  - an additional time, once per-child process if the script file has changed on disk
  - an additional time, in the parent process on each restart if pulled in by the
    parent process via B<Apache::RegistryLoader> and PerlFreshRestart is On

=head1 END blocks

As L<perlmod> explains, an C<END> subroutine is executed as late as
possible, that is, when the interpreter is being exited.  In the
mod_perl environment, the interpreter does not exit until the server
is shutdown.  However, mod_perl does make a special case for
B<Apache::Registry> scripts.

Normally, C<END> blocks are executed by Perl during it's C<perl_run()>
function, which is called once each time the Perl program is executed,
e.g. once per (mod_cgi) CGI scripts.  However, mod_perl only calls
C<perl_run()> once, during server startup.  Any C<END> blocks
encountered during main server startup, i.e. those pulled in by the
B<PerlRequire> or by any B<PerlModule> are suspended and run at server
shutdown, aka C<child_exit> (requires apache 1.3.0+).  Any C<END>
blocks that are encountered during compilation of Apache::Registry
scripts are called after the script done is running, including
subsequent invocations when the script is cached in memory.   
All other C<END> blocks encountered during other Perl*Handler callbacks,
e.g. B<PerlChildInitHandler>, will be suspended while the process is
running and called during C<child_exit> when the process is shutting down.   
Module authors may be wish to use C<$r-E<gt>register_cleanup> as an
alternative to C<END> blocks if this behavior is not desirable. 

=head1 MEMORY CONSUMPTION

Don't be alarmed by the size of your httpd after you've linked with
mod_perl.  No matter what, your httpd will be larger than normal to start, 
simply because you've linked with perl's runtime.

Here's I'm just running

 % /usr/bin/perl -e '1 while 1'

   PID USERNAME PRI NICE   SIZE   RES STATE   TIME   WCPU    CPU COMMAND
 10214 dougm     67    0   668K  212K run     0:04 71.55% 21.13% perl

Now with a few random modules:

 % /usr/bin/perl -MDBI -MDBD::mSQL -MLWP::UserAgent -MFileHandle -MIO -MPOSIX -e '1 while 1'

 10545 dougm     49    0  3732K 3340K run     0:05 54.59% 21.48% perl

Here's my httpd linked with libperl.a, not having served a single request:

 10386 dougm      5    0  1032K  324K sleep   0:00  0.12%  0.11% httpd-a

You can reduce this if you configure perl 5.004+ with -Duseshrplib.
Here's my httpd linked with libperl.sl, not having served a single request:

 10393 dougm      5    0   476K  368K sleep   0:00  0.12%  0.10% httpd-s

Now, once the server starts receiving requests, the embedded
interpreter will compile code for each 'require' file it has not seen
yet, each new Apache::Registry subroutine that's compiled, along with
whatever modules it's use'ing or require'ing.  Not to mention
AUTOLOADing.  (Modules that you 'use' will be compiled when the server
starts unless they are inside an eval block.)  httpd will grow just as
big as our /usr/bin/perl would, or a CGI process for that matter, it
all depends on your setup.  The L<mod_perl_tuning> document gives
advice on how to best setup your mod_perl server environment.

The mod_perl INSTALL document explains how to build the Apache::
extensions as shared libraries (with 'perl Makefile.PL DYNAMIC=1').
This may save you some memory, however, it doesn't work on a few
systems such as aix and unixware.

However, on most systems, this strategy will only make the httpd
I<look> smaller.  When in fact, an httpd with Perl linked static with
take up less real memory and preform faster than shared libraries at
the same time.  See the L<mod_perl_tuning> document for details.

=head2 MEMORY TIPS

=over 4

=item Leaks

If you are using a module that leaks or have code of their own that leaks, in
any case using the apache configuration directive 'MaxRequestsPerChild' is 
your best bet to keep the size down.  

=item Perl Options

Newer Perl versions also have other options to reduce runtime memory
consumption.  See Perl's INSTALL file for details on C<-DPACK_MALLOC>
and C<-DTWO_POT_OPTIMIZE>.  With these options, my httpd shrinks down
~150K. 

=item Server Startup

Use the B<PerlRequire> and B<PerlModule> directives to load commonly
used modules such as CGI.pm, DBI, etc., when the server is started.
On most systems, server children will be able to share this space.

=item Importing Functions

When possible, avoid importing of a module functions into your
namespace.  The aliases which are created can take up quite a bit of
space.  Try to use method interfaces and fully qualified
Package::function names instead.
Here's a freshly started httpd who's served one request for a script
using the CGI.pm method interface:

 TTY   PID USERNAME  PRI NI   SIZE   RES  STATE   TIME %WCPU  %CPU COMMAND
   p4  5016 dougm     154 20  3808K  2636K sleep   0:01  9.62  4.07 httpd

Here's a freshly started httpd who's served one request for the same
script using the CGI.pm function interface:

 TTY   PID USERNAME  PRI NI   SIZE   RES  STATE   TIME %WCPU  %CPU COMMAND
   p4  5036 dougm     154 20  3900K  2708K sleep   0:01  3.19  2.18 httpd

Now do the math: take that difference, figure in how many other
scripts import the same functions and how many children you have
running.  It adds up!

=item Global Variables

It's always a good idea to stay away from global variables when
possible.  Some variables must be global so Perl can see them, such as
a module's B<@ISA> or B<$VERSION> variables.  In common practice, a
combination of C<use strict> and C<use vars> keeps modules clean and
reduces a bit of noise.  However, B<use vars> also creates aliases as
the B<Exporter> does, which eat up more space.  When possible, try to
use fully qualified names instead of B<use vars>.  Example:

 package MyPackage;
 use strict;
 @MyPackage::ISA = qw(...);
 $MyPackage::VERSION = "1.00";

vs.

 package MyPackage;
 use strict;
 use vars qw(@ISA $VERSION);
 @ISA = qw(...);
 $VERSION = "1.00";

=item Further Reading

In case I forgot to mention, read Vivek Khera's L<mod_perl_tuning>
document for more tips on improving Apache/mod_perl performance.

=back

=head1 SWITCHES

Normally when you run perl from the command line or have the shell
invoke it with `#!', you may choose to pass perl switch arguments
such as C<-w> or C<-T>.  Since the command line is only parsed once,
when the server starts, these switches are unavailable to mod_perl
scripts.  However, most command line arguments have a perl special
variable equivilant.  For example, the C<$^W> variable coresponds
to the C<-w> switch.  Consult L<perlvar> for more details.  With
mod_perl it is also possible to turn on warnings globaly via the
B<PerlWarn> directive:

 PerlWarn On

The switch which enables taint checks does not have a special
variable, so mod_perl provides the B<PerlTaintCheck> directive to turn
on taint checks.  In httpd.conf, enable with: 

 PerlTaintCheck On

Now, any and all code compiled inside httpd will be checked.

The environment variable B<PERL5OPT> can be used to set additional
perl startup flags such as B<-d> and B<-D>.  See L<perlrun>.

=head1 PERSISTENT DATABASE CONNECTIONS

Another popular use of mod_perl is to take advantage of it's
persistance to maintain open database connections.  The basic idea
goes like so:

 #Apache::Registry script
 use strict;
 use vars qw($dbh);

 $dbh ||= SomeDbPackage->connect(...);

Since C<$dbh> is a global variable, it will not go out of scope,
keeping the connection open for the lifetime of a server process,
establishing it during the script's first request for that process.

It's recommended that you use one of the Apache::* database connection
wrappers.  Currently for DBI users there is C<Apache::DBI> and for
Sybase users C<Apache::Sybase::DBlib>.  These modules hide the
peculiar code example above.  In addition, different scripts may share
a connection, minimizing resource consumption.  Example:

 #httpd.conf has
 # PerlModule Apache::DBI
 #DBI scripts look exactly as they do under CGI
 use strict;
 my $dbh = DBI->connect(...);

Although B<$dbh> shown here will go out of scope when the script ends,
the Apache::DBI module's reference to it does not, keep the connection
open.

B<WARNING:> Do not attempt to open a persistent database connection in
the parent process (via PerlRequire or PerlModule).  If you do,
children will get a copy of this handle, causing clashes when the
handle is used by two processes at the same time.  Each child must
have it's own unique connection handle.

=head1 STACKED HANDLERS

With the mod_perl stacked handlers mechanism, it is possible for more
than one Perl*Handler to be defined and run during each stage of a 
request.  

Perl*Handler directives can define any number of subroutines,
e.g. (in config files)

 PerlTransHandler OneTrans TwoTrans RedTrans BlueTrans

With the method, Apache-E<gt>push_handlers, callbacks can be added to
the stack by scripts at runtime by mod_perl scripts.

Apache-E<gt>push_handlers takes the callback hook name as it's first
argument and a subroutine name or reference as it's second. e.g.:

 Apache->push_handlers("PerlLogHandler", \&first_one);

 $r->push_handlers("PerlLogHandler", sub {
     print STDERR "__ANON__ called\n";
     return 0;
 });

After each request, this stack is cleared out.

All handlers will be called unless a handler returns a status other than
OK or DECLINED, this needs to be considered more.  Post apache-1.2 will
have a DONE return code to signal termiation of a stage, which Rob and
I came up with while back when first discussing the idea of stacked
handlers.  2.0 won't come for quite sometime, so mod_perl will most
likely handle this before then. 

example uses:

CGI.pm maintains a global object for it's plain function interface.
Since the object is global, it does not go out of scope, DESTROY is
never called.  CGI-E<gt>new can call: 

 Apache->push_handlers("PerlCleanupHandler", \&CGI::_reset_globals);

This function will be called during the final stage of a request,
refreshing CGI.pm's globals before the next request comes in.  

Apache::DCELogin establishes a DCE login context which must exist for
the lifetime of a request, so the DCE::Login object is stored in a
global variable.  Without stacked handlers, users must set 

 PerlCleanupHandler Apache::DCELogin::purge

in the configuration files to destroy the context.  This is not
"user-friendly".  Now, Apache::DCELogin::handler can call:

 Apache->push_handlers("PerlCleanupHandler", \&purge);

Persistent database connection modules such as Apache::DBI could push
a PerlCleanupHandler handler that iterates over %Connected, refreshing
connections or just checking that ones have not gone stale.  Remember,
by the time we get to PerlCleanupHandler, the client has what it wants
and has gone away, we can spend as much time as we want here without
slowing down response time to the client.

PerlTransHandlers may decide, based or uri or other condition, whether
or not to handle a request, e.g. Apache::MsqlProxy.  Without stacked
handlers, users must configure:

 PerlTransHandler Apache::MsqlProxy::translate
 PerlHandler      Apache::MsqlProxy

PerlHandler is never actually invoked unless translate() sees the
request is a proxy request ($r-E<gt>proxyreq), if it is a proxy request,
translate() set $r-E<gt>handler("perl-script"), only then will PerlHandler
handle the request.  Now, users do not have to specify 'PerlHandler
Apache::MsqlProxy', the translate() function can set it with
push_handlers().

Includes, footers, headers, etc., piecing together a document,
imagine (no need for SSI parsing!):

 PerlHandler My::Header Some::Body A::Footer

This was my first test:

 #My.pm
 package My;

 sub header {
     my $r = shift;
     $r->content_type("text/plain");
     $r->send_http_header;
     $r->print("header text\n");
 }
 sub body   { shift->print("body text\n")   }
 sub footer { shift->print("footer text\n") }
 1;
 __END__ 
 #in config
 <Location /foo>
 SetHandler "perl-script"
 PerlHandler My::header My::body My::footer
 </Location>

Parsing the output of another PerlHandler?  this is a little more
tricky, but consider:

 <Location /foo>
   SetHandler "perl-script"
   PerlHandler OutputParser SomeApp 
 </Location>
 <Location /bar>
   SetHandler "perl-script"
   PerlHandler OutputParser AnotherApp
 </Location>

Now, OutputParser goes first, but it untie's *STDOUT and re-tie's to
it's own package like so:

 package OutputParser;

 sub handler {
     my $r = shift; 
     untie *STDOUT;	
     tie *STDOUT => 'OutputParser', $r;
 }

 sub TIEHANDLE {
     my($class, $r) = @_;
     bless { r => $r}, $class;
 }

 sub PRINT {
     my $self = shift;
     for (@_) {
         #do whatever you want to $_
	 $self->{r}->print($_ . "[insert stuff]");
     }
 }

 1;
 __END__

To build in this feature, configure with:

 % perl Makefile.PL PERL_STACKED_HANDLERS=1 [PERL_FOO_HOOK=1,etc]

Another method 'Apache-E<gt>can_stack_handlers' will return TRUE if
mod_perl was configured with PERL_STACKED_HANDLERS=1, FALSE
otherwise. 

=head1 PERL METHOD HANDLERS

See L<mod_perl_method_handlers>.

=head1 PERL SECTIONS

With E<lt>PerlE<gt>E<lt>/PerlE<gt> sections, it is possible to
configure your server entirely in Perl.

E<lt>PerlE<gt> sections can contain *any* and as much Perl code as you
wish.  These sections are compiled into a special package who's symbol
table mod_perl can then walk and grind the names and values of Perl
variables/structures through the Apache core config gears.  Most of
the configurations directives can be represented as C<$Scalars> or
C<@Lists>.  A C<@List> inside these sections is simply converted into
a single-space delimited string for you inside.  Here's an example:

 #httpd.conf
 <Perl>
 @PerlModule = qw(Mail::Send Devel::Peek);

 #run the server as whoever starts it
 $User  = getpwuid($>) || $>;
 $Group = getgrgid($)) || $); 

 $ServerAdmin = $User;

 </Perl>

Block sections such as E<lt>LocationE<gt>E<lt>/LocationE<gt> are
represented in a C<%Hash>, e.g.:

 $Location{"/~dougm/"} = {
     AuthUserFile => '/tmp/htpasswd',
     AuthType => 'Basic',
     AuthName => 'test',
     DirectoryIndex => [qw(index.html index.htm)],	
     Limit => {
	 METHODS => 'GET POST',
	 require => 'user dougm',
     },
 };

 #If a Directive can take say, two *or* three arguments
 #you may push strings and the lowest number of arguments
 #will be shifted off the @List
 #or use array reference to handle any number greater than
 #the minimum for that directive

 push @Redirect, "/foo", "http://www.foo.com/";

 push @Redirect, "/imdb", "http://www.imdb.com/";

 push @Redirect, [qw(temp "/here" "http://www.there.com")];

Other section counterparts include C<%VirtualHost>, C<%Directory> and
C<%Files>. 

These are somewhat boring examples, but they should give you the basic
idea.  You can mix in any Perl code your heart desires.
See eg/httpd.conf.pl and eg/perl_sections.txt for some examples.

A tip for syntax checking outside of httpd:

 <Perl>
 #!perl

 #... code here ...

 __END__
 </Perl>

Now you may run C<perl -cx httpd.conf>.

It may be the case that E<lt>PerlE<gt> sections are not completed or
an oversight was made in an certain area.  If they do not behave as
you expect, please send a report to the modperl mailing list.

To configure this feature build with 
 'perl Makefile.PL PERL_SECTIONS=1'

=head1 mod_perl and mod_include integration

As of apache 1.2.0, mod_include can handle Perl callbacks.

A `sub' key value may be anything a Perl*Handler can be:
subroutine name, package name (defaults to package::handler),
Class-E<gt>method call or anonymous sub {}

Example:

 Child <!--#perl sub="sub {print $$}" --> accessed
 <!--#perl sub="sub {print ++$Access::Cnt }" --> times. <br>

 <!--#perl sub="Package::handler" arg="one" arg="two" -->

 #don't forget to escape double quotes!
 Perl is
        <!--#perl sub="sub {for (0..10) {print \"very \"}}"-->
        fun to use!

The B<Apache::Include> module makes it simple to include
B<Apache::Registry> scripts with the mod_include perl directive. 

Example:

 <!--#perl sub="Apache::Include" arg="/perl/ssi.pl" -->

You can also use 'virtual include' to include Apache::Registry scripts
of course.  However, using #perl will save the overhead of making
Apache go through the motions of creating/destroying a subrequest and
making all the necessary access checks to see that the request would
be allowed outside of a 'virtual include' context.

To enable perl in mod_include parsed files, when building apache the
following must be present in the Configuration file:

 EXTRA_CFLAGS=-DUSE_PERL_SSI -I. `perl -MExtUtils::Embed -ccopts`

mod_perl's Makefile.PL script can take care of this for you as well:

 perl Makefile.PL PERL_SSI=1

If you're interested in sprinkling Perl code inside your HTML
documents, you'll also want to look at the Apache::Embperl
(http://perl.apache.org/embperl/), Apache::ePerl and Apache::SSI modules. 

=head1 DEBUGGING

=over 4

=item MOD_PERL_TRACE

To enable mod_perl debug tracing configure mod_perl with the
PERL_TRACE option:

 perl Makefile.PL PERL_TRACE=1

The trace levels can then be enabled via the B<MOD_PERL_TRACE>
environment variable which can contain any combination of:

 d - Trace directive handling during configuration read
 s - Trace processing of perl sections
 h - Trace Perl*Handler callbacks
 g - Trace global variable handling, intepreter construction, END blocks, etc.
 all - all of the above

=item spinning httpds

To see where an httpd is "spinning", try adding this to your script or a
startup file:

  use Carp ();
  $SIG{'USR1'} = sub { 
     Carp::confess("caught SIGUSR1!");
  };

Then issue the command line:

  kill -USR1 <spinning_httpd_pid>

=back

=head1 PROFILING

It is possible to profile code run under mod_perl with the
B<Devel::DProf> module available on CPAN.  However, you must have
apache version 1.3.0 or higher and the C<PerlChildExitHandler>
enabled.  When the server is started, B<Devel::DProf> installs an
C<END> block to write the I<tmon.out> file, which will be run when the
server is shutdown.  Here's how to start and stop a server with the
profiler enabled:

 % setenv PERL5OPT -d:DProf
 % httpd -X -d `pwd` &
 ... make some requests to the server here ...
 % kill `cat logs/httpd.pid`
 % unsetenv PERL5OPT
 % dprofpp

See also: B<Apache::DProf>

=head1 BENCHMARKING

How much faster is mod_perl that CGI?  There are many ways to
benchmark the two, see the C<benchmark/> directory for some examples.

See also: B<Apache::Timeit>

=head1 WARNINGS

See L<mod_perl_traps>.

=head1 SUPPORT

See the L<SUPPORT> file.

=head1 Win32

See L<INSTALL.win32> for building from sources.

Info about win32 binary distributions of mod_perl are available
from: 

 http://perl.apache.org/distributions/

=head1 REVISION

$Id: mod_perl.pod,v 1.22 2002/01/02 09:41:23 stas Exp $

=head1 AUTHOR

Doug MacEachern 



