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NAME

Apache::Reload - Reload Perl Modules when Changed on Disk

Synopsis

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

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

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

Description

Apache::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 Apache::Reload. It can also do the check for modified modules, when a special touch-file has been modified.

Note that Apache::Reload operates on the current context of @INC. Which means, when called as a Perl*Handler it will not see @INC paths added or removed by Apache::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 Apache::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, simply add the following configuration to your httpd.conf:

  PerlModule Apache::Reload
  PerlInitHandler Apache::Reload

Register Modules Implicitly

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

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

Then any modules with the line:

  use Apache::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 Apache::Reload
  PerlInitHandler Apache::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::* Apache::*"

will monitor all modules under the namespaces ModPerl:: and Apache::.

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.

Performance Issues

This modules is perfectly suited for a development environment. Though it's possible that you would like to use it in a production environment, since with Apache::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, Apache::Reload has to stat(2) the touch file on each request, which adds a slight but most likely insignificant overhead to response times. Otherwise Apache::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

Threaded MPM and Multiple Perl Interpreters

If you use Apache::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.x 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 Apache::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 will be removed in 5.10

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.

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.

See Also

Stonehenge::Reload