=head1 NAME

Mod_perl_faq - frequently asked questions about mod_perl ($Date: 2000/03/05 11:57:55 $)

=head1 DESCRIPTION

Mod_perl allows an Apache Web Server to directly execute perl code.  This
document is designed to answer questions that arise when designing new
applications, and converting existing applications, to run in the mod_perl
environment.

=head1 QUESTIONS & ANSWERS

=head2 What is mod_perl?

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.

Mod_perl is a bundle of software.  One part of the bundle is designed to
be compiled and linked together with Apache and Perl.  The remainder is
perl code that provides the object-oriented interface to the "perl-enabled"
web server.

The primary advantages of mod_perl are power and speed.  You have full
access to the inner-workings of the web server and can intervene at any
stage of request-processing.  This allows for customized processing of (to
name just a few of the phases) URI->filename translation, authentication,
response generation and logging.  There is very little run-time overhead.
In particular, it is not necessary to start a separate process, as is
often done with web-server extensions.  The most wide-spread such
extension mechanism, the Common Gateway Interface (CGI), can be replaced
entirely with perl-code that handles the response generation phase of
request processing.  Mod_perl includes a general purpose module for this
purpose (Apache::Registry) that can transparently run existing perl CGI
scripts.

=head2 Where can I get mod_perl?

Mod_perl can be found at
http://www.perl.com/CPAN/modules/by-module/Apache/

=head2 What else do I need?

=over 4

=item Perl

http://www.perl.com/CPAN/src/latest.tar.gz

Win32 users note: at the time of writing, ActiveState's Perl cannot be
used with mod_perl, because it is based on an old version of perl
(perl-5.003_07, build 316).  There have been no reports of success
with mod_perl and newer versions of ActiveState (April 1999).

=item Apache

http://www.apache.org/dist/

=back

=head2 How do I install it?

Configuring and installing apache with mod_perl is a complex process,
so it is really not a good idea to attempt to do it manually.  If you
are used to configuring third-party modules into Apache using the
apache configuration process, please realize that running mod_perl's
Makefile.PL with the right parameters does this for you.

Read the INSTALL* files in the top-level mod_perl distribution
directory and then choose one of the INSTALL.simple* recipes that is
close to your requirements, as a starting point.  When you succeed in
compiling and linking an httpd, a quick way to check that everything
is configured according to plan is to run it with the C<-l> (list
compiled-in modules) and C<-V> (show paths) flags.

=head2 What documentation should I read?

The mod_perl documentation in mod_perl.pod.  After you have installed
mod_perl you can read it with the command: C<perldoc mod_perl>.

The complete list of available documentation can be found at the end
of mod_perl's README file.

If you are using mod_perl to extend the server functionality, you will
need to read C<perldoc Apache> and the Apache API notes, which can be
found in apache_x.x.x/htdocs/manual/misc/API.html.

Existing (perl-) CGI scripts should run as-is under mod_perl.  There are a
number of reasons why they may need to be adjusted, and these are
discussed later in this FAQ.  If you are developing a new CGI script it is
probably best to use CGI.pm.  It is part of the standard perl distribution
and its documentation can be read with the command: C<perldoc CGI>.

=head2 How do I run CGI scripts under mod_perl?

Refer to L<mod_perl_cgi> for tips on writing and converting CGI
scripts for mod_perl.

=head2 How do I access the Apache API from mod_perl?

Interfacing with Apache is discussed in L<mod_perl_api>.

=head2 How secure are mod_perl scripts?

Because mod_perl runs within an httpd child process, it runs with the
user-id and group-id specified in the httpd.conf file.  This user/group
should have the lowest possible privileges.  It should only have access
to world readable files.  Even so, careless scripts can give away
information.  You would not want your /etc/passwd file to be readable over
the net, for instance.

Different mod_perl scripts run successively using the same Perl
interpreter instance. So, in addition to classical CGI mischiefs, a
malicious mod_perl script can redefine any Perl object and change the
behavior of other mod_perl scripts.

If you turn on tainting checks, perl can help you to avoid the pitfalls of
using data received from the net.  Setting the C<-T> switch on the first line
of the script is not sufficient to enable tainting checks under mod_perl.
You have to include the directive C<PerlTaintCheck On> in the httpd.conf
file.

=head2 What if my script needs higher privileges?

You will have to start a new process that runs under a suitable user-id
(or group-id).  If all requests handled by the script will need the higher
privileges, you might as well write it as a suid CGI script.  Read the
documentation about suEXEC in the Apache documentation.

Alternatively, pre-process the request with mod_perl and fork a suid
helper process to handle the privileged part of the task.

=head2 Why is httpd using so much memory?

Read the section on "Memory Consumption" in the mod_perl.pod.

Make sure that your scripts are not leaking memory.  Global variables
stay around indefinitely, lexical variables (declared with my()) are
destroyed when they go out of scope, provided there are no references
to them from outside of that scope.  The Apache::Leak module can warn
about some types of memory leak.

To get information about the modules that have been loaded and their
symbol-tables, use the Apache::Status module.  It is enabled by adding
these lines to the httpd configuration file.

  <Location /perl-status>
  SetHandler  perl-script
  PerlHandler Apache::Status
  </Location>

Then look at the URL http://www.your.host/perl-status

Joel Wagner reports that calling an undefined subroutine in a module
can cause a tight loop that consumes all memory.  Here is a way to
catch such errors.  Define an autoload subroutine

  sub UNIVERSAL::AUTOLOAD {
  	  my $class = shift;
  	  warn "$class can't `$UNIVERSAL::AUTOLOAD'!\n";
  }

It will produce a nice error in error_log, giving the line number of
the call and the name of the undefined subroutine.

=head2 Do I have to restart the server when I change my Perl code?

Apache::Registry checks the timestamp of scripts that it has loaded
and reloads them if they change.  Other handlers and library modules
are not automatically reloaded by mod_perl, but you can use the
Apache::StatINC module to do this for you.

=head2 So how do I use mod_perl in conjunction with ErrorDocument?

Andreas Koenig writes:

=over 4

=item *

Set up your testing engine:

LWP comes with debugging capabilities that are sometimes better than
your browser, sometimes your browser is the better testing
device. Make sure you can call lwp-request from the command line and
have your browser ready before you start. I find the C<-x> switch
(extended debugging) and the C<-d> switch (do not display content) most
useful.

=item *

Test your server with

    lwp-request -xd http://your.server/test/file.not_there

Carefully examine if the status is 404 and if the headers look good.

If you try 'lwp-request -es', the HTML output will not be the one you
are sending, instead lwp-request will send its own cooked HTML text
(as of version libwww-perl-5.09). Check the real text either with the
C<-x> switch or with telnet or your browser.

=item *

Set up your Errordocument configuration in the testing area. I have
this in my .htaccess file:

    ErrorDocument 404 /perl/errors/err404-01

The /perl/ directory is configured to

    <Location /perl>
    SetHandler perl-script
    PerlHandler Apache::Registry::handler
    Options ExecCGI
    </Location>

I have no PerlSendHeader and no PerlNewSendHeader directive in any
configuration file.

=item *

Repeat step 2 (Test your server)

=item *

Write your error handler in mod_perl. You have to be prepared that you
have to tell both apache *and* the browser the right thing. Basically
you have to tell the browser what the error is, but you have to
pretend to apache that everything was OK. If you tell apache the error
condition, it will handle the situation on its own and add some
unwanted stuff to the output that goes to the browser.

The following works fine for me:

    my $r = Apache->request;
    $r->content_type('text/html; charset=ISO-8859-1');
    $r->send_http_header;
    $r->status(200);
    ...send other HTML stuff...

At the time of the send_http_header we have an error condition of type
404--this is what gets sent to the browser. After that I set status to
200 to silence the apache engine.

I was not successful in trying to do the same with CGI.pm, but I
didn't try very hard.

=item *

Repeat step 2 (Test your server)

=item *

The above is tested with mod_perl/0.98 and 0.99

=item *

Open questions I could not find documentation for (except RTFS): what
exactly is PerlSendHeaders and PerlNewSendHeaders. What is the default
setting for those? How do these cooperate with CGI.pm, Apache.pm,
Apache::Registry?

=back

=head2 How can I reference private library modules?

If you put your modules into one of the directories on perl's search
path (the @INC array), they will be found automatically.
Traditionally, site-specific modules go in /usr/lib/perl5/site_perl/.
Newer versions of mod_perl add the directory $ServerRoot/lib/perl to
@INC on startup so that is a good place for modules that are only used 
by mod_perl scripts.

If you need to load files from other non-standard locations, you can
add directories to the @INC array with a 'use lib' statement in a
startup script.  See L<mod_perl_tuning> for an example.

=head2 How can I pass arguments to a SSI script?

Following the documentation, I have put the following in the html
file:

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

I want to send an argument to the ssi.pl script.  How?

It won't work with Apache::Include.  Instead of a script, define a
subroutine that's pulled in with PerlRequire or PerlModule, like so:

  sub My::ssi {
     my($r, $one, $two, $three) = @_;
     ...
  }

In the html file:

  <!--#perl sub="My::ssi" arg="one" arg="two" arg="three" -->

=head2 Why is image-file loading so slow when testing with httpd C<-X> ?

If you use Netscape while your server is running in single-process
mode, the "KeepAlive" feature gets in the way.  Netscape tries to open
multiple connections and keep them open.  Because there is only one
server process listening, each connection has to time-out before the
next succeeds.  Turn off KeepAlive in httpd.conf to avoid this effect.

=head2 What can cause a subroutine or variable to be sporadically undefined?

If you sometimes see error messages like this:

  [Thu Sep 11 11:03:06 1997] Undefined subroutine
  &Apache::ROOT::perl::script1::sub_foo called at
  /some/path/perl/script2 line 42.

despite the fact that script2 normally works just fine, it looks like
you have a namespace problem in a library file.  If sub_foo is located
in a file that is pulled in by 'require' and both script1 and script2
require it, you need to be sure that the file containing sub_foo sets
a package name.  Otherwise, sub_foo gets defined in the namespace that
is active the first time it is required, and the next require is a
no-op because that file is already in %INC.  The same problem can
happen with global variables.

The solution is simple, set up your require'd file something along
these lines:

  package SomeName;

  sub sub_foo {...}

Now, have scripts call SomeName::sub_foo() instead of sub_foo().

=head2 Is there a bug that causes httpd processes to crash?

You may see httpd child processes crashing with segmentation fault
when you restart the server with a HUP or USR1 signal.  This is not a
bug in mod_perl.  If you have 'PerlFreshRestart On' in the
configuration, the main httpd daemon reloads all the perl modules that
it has preloaded when it gets a HUP or USR1 signal.  Unfortunately,
not all perl modules are robust enough to survive this, for them,
unusual situation.

=head2 What could be causing sporadic errors "in cleanup"?

Some people have seen error messages such as this:

   [Fri Sep 26 10:50:08 1997]      (in cleanup) no dbproc key in hash
   at /usr/lib/perl5/site_perl/Apache/Registry.pm line 119.

Doug writes:

"I have yet to figure out why, but there have been a few arbitrary
cases where Perl (in mod_perl) _insists_ on finding and/or calling a
DESTROY method for an object.  Defining an empty sub DESTROY has been
the bandaid for these few cases."

If the specific error message gives you a hint about which object is
causing difficulty, put the C<sub DESTROY { }> in the module that
defines that object class.

=head2 How can I test that my script is running under mod_perl?

There are 2 environment variables you can test.

  exists $ENV{"MOD_PERL"}   # if running under mod_perl

  $ENV{"GATEWAY_INTERFACE"} eq "CGI-Perl/1.1"

The MOD_PERL variable gets set immediately when the perl interpreter
starts up, whereas GATEWAY_INTERFACE may not be set yet when BEGIN
blocks are being processed.

=head2 Why don't "format" and "write" work under mod_perl?

The Perl tie'd filehandle interface is not complete, format/write is
one of the missing pieces.  If you configure Perl with sfio, write()
should work just fine.

=head2 Where can I get help that I did not find in here?

There is a mailing-list dedicated to mod_perl.  It is archived at
http://outside.organic.com/mail-archives/modperl/ and at
http://forum.swarthmore.edu/epigone/modperl (which has a search
engine) and also at
http://www.progressive-comp.com/Lists/?l=apache-modperl&r=1#apache-modperl
(threaded and indexed).

You can subscribe to the list by sending a mail to
C<modperl-subscribe@apache.org> and responding to the confirmation
message that you will receive.  To unsubscribe, send mail to
C<modperl-unsubscribe@apache.org> B<from the address you are
subscribed at> and reply to the confirmation message.  Look at the
full headers of mails that you receive from the list to see the
address that they were sent to.  The address is embedded in the
C<Return-Path> header (you will probably have to activate a "show full 
headers" function in your mail reader to see it).  To find the
address, delete C<modperl-return-nnnn-> from the front of the return path 
and C<@apache.org> from the back, then replace the C<=> with C<@>.

Remember: the mailing list is for questions about and discussion of
mod_perl.  Quetions about perl programming in general should be asked
in the newsgroup comp.lang.perl.misc, after consulting the fine perl
faqs.  There is a whole set of newsgroups dedicated to web authoring,
web servers etc.: comp.infosystems.www.*

The mod_perl homepage http://perl.apache.org/ has links to other
mod_perl resources.

The pod source of this FAQ is available at
http://www.ping.de/~fdc/mod_perl/mod_perl_faq.tar.gz

=head2 Where do I send suggestions and corrections concerning this FAQ?

mailto:fdc@cliwe.ping.de
