Monitor All Modules in %INC

To monitor and reload all modules in %INC at the beginning of request's processing, simply add the following configuration to your httpd.conf:

  PerlModule Apache2::Reload
  PerlInitHandler Apache2::Reload

When working with connection filters and protocol modules "Apache2::Reload" should be invoked in the pre_connection stage:

  PerlPreConnectionHandler Apache2::Reload

See also the discussion on "PerlPreConnectionHandler".

Register Modules Implicitly

To only reload modules that have registered with "Apache2::Reload", add the following to the httpd.conf:

  PerlModule Apache2::Reload
  PerlInitHandler Apache2::Reload
  PerlSetVar ReloadAll Off
  # ReloadAll defaults to On

Then any modules with the line:

  use Apache2::Reload;

Will be reloaded when they change.

Register Modules Explicitly

You can also register modules explicitly in your httpd.conf file that you want to be reloaded on change:

  PerlModule Apache2::Reload
  PerlInitHandler Apache2::Reload
  PerlSetVar ReloadAll Off
  PerlSetVar ReloadModules "My::Foo My::Bar Foo::Bar::Test"

Note that these are split on whitespace, but the module list must be in quotes, otherwise Apache tries to parse the parameter list.

The "*" wild character can be used to register groups of files under the same namespace. For example the setting:

  PerlSetVar ReloadModules "ModPerl::* Apache2::*"

will monitor all modules under the namespaces "ModPerl::" and "Apache2::".

Monitor Only Certain Sub Directories

To reload modules only in certain directories (and their subdirectories) add the following to the httpd.conf:

  PerlModule Apache2::Reload
  PerlInitHandler Apache2::Reload
  PerlSetVar ReloadDirectories "/tmp/project1 /tmp/project2"

You can further narrow the list of modules to be reloaded from the chosen directories with "ReloadModules" as in:

  PerlModule Apache2::Reload
  PerlInitHandler Apache2::Reload
  PerlSetVar ReloadDirectories "/tmp/project1 /tmp/project2"
  PerlSetVar ReloadAll Off
  PerlSetVar ReloadModules "MyApache2::*"

In this configuration example only modules from the namespace "MyApache2::" found in the directories /tmp/project1/ and /tmp/project2/ (and their subdirectories) will be reloaded.

Special "Touch" File

You can also declare a file, which when gets touch(1)ed, causes the reloads to be performed. For example if you set:

  PerlSetVar ReloadTouchFile /tmp/reload_modules

and don't touch(1) the file /tmp/reload_modules, the reloads won't happen until you go to the command line and type:

  % touch /tmp/reload_modules

When you do that, the modules that have been changed, will be magically reloaded on the next request. This option works with any mode described before.

Unregistering a module

In some cases, it might be necessary to explicitly stop reloading a module.

  Apache2::Reload->unregister_module('Some::Module');

But be carefull, since unregistering a module in this way will only do so for the current interpreter. This feature should be used with care.

Problems With Reloading Modules Which Do Not Declare Their Package Name

If you modify modules, which don't declare their "package", and rely on "Apache2::Reload" to reload them, you may encounter problems: i.e., it'll appear as if the module wasn't reloaded when in fact it was. This happens because when "Apache2::Reload" "require()"s such a module all the global symbols end up in the "Apache2::Reload" namespace! So the module does get reloaded and you see the compile time errors if there are any, but the symbols don't get imported to the right namespace. Therefore the old version of the code is running.

Failing to Find a File to Reload

"Apache2::Reload" uses %INC to find the files on the filesystem. If an entry for a certain filepath in %INC is relative, "Apache2::Reload" will use @INC to try to resolve that relative path. Now remember that mod_perl freezes the value of @INC at the server startup, and you can modify it only for the duration of one request when you need to load some module which is not in on of the @INC directories. So a module gets loaded, and registered in %INC with a relative path. Now when "Apache2::Reload" tries to find that module to check whether it has been modified, it can't find since its directory is not in @INC. So "Apache2::Reload" will silently skip that module.

You can enable the "Debug|/Debug" mode to see what "Apache2::Reload" does behind the scenes.

Problems with Scripts Running with Registry Handlers that Cache the Code

The following problem is relevant only to registry handlers that cache the compiled script. For example it concerns "ModPerl::Registry" but not "ModPerl::PerlRun".

The Problem

Let's say that there is a module "My::Utils":

  #file:My/Utils.pm
  #----------------
  package My::Utils;
  BEGIN { warn __PACKAGE__ , " was reloaded\n" }
  use base qw(Exporter);
  @EXPORT = qw(colour);
  sub colour { "white" }
  1;

And a registry script test.pl:

  #file:test.pl
  #------------
  use My::Utils;
  print "Content-type: text/plain\n\n";
  print "the color is " . colour();

Assuming that the server is running in a single mode, we request the script for the first time and we get the response:

  the color is white

Now we change My/Utils.pm:

  -  sub colour { "white" }
  +  sub colour { "red" }

And issue the request again. "Apache2::Reload" does its job and we can see that "My::Utils" was reloaded (look in the error_log file). However the script still returns:

  the color is white

The Explanation

Even though My/Utils.pm was reloaded, "ModPerl::Registry"'s cached code won't run '"use My::Utils;"' again (since it happens only once, i.e. during the compile time). Therefore the script doesn't know that the subroutine reference has been changed.

This is easy to verify. Let's change the script to be:

  #file:test.pl
  #------------
  use My::Utils;
  print "Content-type: text/plain\n\n";
  my $sub_int = \&colour;
  my $sub_ext = \&My::Utils::colour;
  print "int $sub_int\n";
  print "ext $sub_ext\n";

Issue a request, you will see something similar to:

  int CODE(0x8510af8)
  ext CODE(0x8510af8)

As you can see both point to the same CODE reference (meaning that it's the same symbol). After modifying My/Utils.pm again:

  -  sub colour { "red" }
  +  sub colour { "blue" }

and calling the script on the secondnd time, we get:

  int CODE(0x8510af8)
  ext CODE(0x851112c)

You can see that the internal CODE reference is not the same as the external one.

The Solution

There are two solutions to this problem:

Solution 1: replace "use()" with an explicit "require()" + "import()".

 - use My::Utils;
 + require My::Utils; My::Utils->import();

now the changed functions will be reimported on every request.

Solution 2: remember to touch the script itself every time you change the module that it requires.