Apache2::Reload(3)

NAME
Apache2::Reload - Reload Perl Modules when Changed on Disk

Synopsis
  # Monitor and reload all modules in %INC:
  # httpd.conf:
  PerlModule Apache2::Reload
  PerlInitHandler Apache2::Reload

  # when working with protocols and connection filters
  # PerlPreConnectionHandler Apache2::Reload

  # Reload groups of modules:
  # httpd.conf:
  PerlModule Apache2::Reload
  PerlInitHandler Apache2::Reload
  PerlSetVar ReloadAll Off
  PerlSetVar ReloadModules "ModPerl::* Apache2::*"
  #PerlSetVar ReloadDebug On
  #PerlSetVar ReloadByModuleName On

  # Reload a single module from within itself:
  package My::Apache2::Module;
  use Apache2::Reload;
  sub handler { ... }
  1;

Description
"Apache2::Reload" reloads modules that change on the disk.

When Perl pulls a file via "require", it stores the filename in the global hash %INC. The next time Perl tries to "require" the same file, it sees the file in %INC and does not reload from disk. This module's handler can be configured to iterate over the modules in %INC and reload those that have changed on disk or only specific modules that have registered themselves with "Apache2::Reload". It can also do the check for modified modules, when a special touch-file has been modified.

Require-hooks, i.e., entries in %INC which are references, are ignored. The hook should modify %INC itself, adding the path to the module file, for it to be reloaded.

"Apache2::Reload" inspects and reloads the file associated with a given module. Changes to @INC are not recognized, as it is the file which is being re-required, not the module name.

In version 0.10 and earlier the module name, not the file, is re-required. Meaning it operated on the the current context of @INC. If you still want this behavior set this environment variable in httpd.conf:

  PerlSetVar ReloadByModuleName On

This means, when called as a "Perl*Handler", "Apache2::Reload" will not see @INC paths added or removed by "ModPerl::Registry" scripts, as the value of @INC is saved on server startup and restored to that value after each request. In other words, if you want "Apache2::Reload" to work with modules that live in custom @INC paths, you should modify @INC when the server is started. Besides, 'use lib' in the startup script, you can also set the "PERL5LIB" variable in the httpd's environment to include any non-standard 'lib' directories that you choose. For example, to accomplish that you can include a line:

  PERL5LIB=/home/httpd/perl/extra; export PERL5LIB

in the script that starts Apache. Alternatively, you can set this environment variable in httpd.conf:

  PerlSetEnv PERL5LIB /home/httpd/perl/extra

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

Performance Issues
This module is perfectly suited for a development environment. Though it's possible that you would like to use it in a production environment, since with "Apache2::Reload" you don't have to restart the server in order to reload changed modules during software updates. Though this convenience comes at a price:

•   If the "touch" file feature is used, "Apache2::Reload" has to stat(2) the touch file on each request, which adds a slight but most likely insignificant overhead to response times. Otherwise "Apache2::Reload" will stat(2) each registered module or even worse--all modules in %INC, which will significantly slow everything down.

•   Once the child process reloads the modules, the memory used by these modules is not shared with the parent process anymore. Therefore the memory consumption may grow significantly.

Therefore doing a full server stop and restart is probably a better solution.

Debug
If you aren't sure whether the modules that are supposed to be reloaded, are actually getting reloaded, turn the debug mode on:

  PerlSetVar ReloadDebug On

Caveats

  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.

Threaded MPM and Multiple Perl Interpreters
If you use "Apache2::Reload" with a threaded MPM and multiple Perl interpreters, the modules will be reloaded by each interpreter as they are used, not every interpreters at once. Similar to mod_perl 1.0 where each child has its own Perl interpreter, the modules are reloaded as each child is hit with a request.

If a module is loaded at startup, the syntax tree of each subroutine is shared between interpreters (big win), but each subroutine has its own padlist (where lexical my variables are stored). Once "Apache2::Reload" reloads a module, this sharing goes away and each Perl interpreter will have its own copy of the syntax tree for the reloaded subroutines.

Pseudo-hashes
The short summary of this is: Don't use pseudo-hashes. They are deprecated since Perl 5.8 and are removed in 5.9.

Use an array with constant indexes. Its faster in the general case, its more guaranteed, and generally, it works.

The long summary is that some work has been done to get this module working with modules that use pseudo-hashes, but it's still broken in the case of a single module that contains multiple packages that all use pseudo-hashes.

So don't do that.

Copyright
mod_perl 2.0 and its core modules are copyrighted under The Apache Software License, Version 2.0.

Authors
Matt Sergeant, matt@sergeant.org

Stas Bekman (porting to mod_perl 2.0)

A few concepts borrowed from "Stonehenge::Reload" by Randal Schwartz and "Apache::StatINC" (mod_perl 1.x) by Doug MacEachern and Ask Bjoern Hansen.

MAINTAINERS
the mod_perl developers, dev@perl.apache.org