apache_mod_perl-101~38::mod_perl-2.0.2.0.2::docs::api::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
# 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.
Note that "Apache2::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 "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 explicitely 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.
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 "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 #---------------package
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 #-----------use
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 #-----------use
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.
perl v5.8.8 apache_mod238::mod_perl-2.0.2::docs::api::Apache2::Reload(3)
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