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HTML::Mason::Admin(3pm) User Contributed Perl Documentation HTML::Mason::Admin(3pm)

NAME
HTML::Mason::Admin - Mason Administrator's Manual

VERSION
version 1.54

DESCRIPTION
This manual is written for the sysadmin/webmaster in charge of installing, configuring, or
tuning a Mason system. The bulk of the documentation assumes that you are using mod_perl.
See RUNNING OUTSIDE OF MOD_PERL for more details. For more details on mod_perl, visit the
mod_perl website at http://perl.apache.org/.

SITE CONFIGURATION METHODS
Mason includes a module specifically designed to integrate Mason and mod_perl (1 and 2),
"HTML::Mason::ApacheHandler". By telling mod_perl to hand content requests to this
module, you can use Mason to generate web pages. There are two ways to configure Mason
under mod_perl.

· Basic

Mason provides reasonable default behavior under mod_perl, so using Mason can be as
simple as adding two directives to your Apache configuration file. Throughout this
document, we will assume that your Apache configuration file is called httpd.conf. By
adding more configuration parameters to this file you can implement more complex
behaviors.

· Advanced

If the basic method does not provide enough flexibility for you, you can wrap Mason in
a custom mod_perl handler. The wrapper code you write can create its own Mason
objects, or it can take advantage of httpd.conf configuration parameters and let Mason
create the objects it needs by itself.

We recommend that you start with the basic method and work your way forward as the need
for flexibility arises.

Mason is very flexible, and you can replace parts of it by creating your own classes.
This documentation assumes that you are simply using the classes provided in the Mason
distribution. Subclassing is covered in the Subclassing document. The two topics are
orthogonal, as you can mix the configuration techniques discussed here with your own
custom subclasses.

BASIC CONFIGURATION VIA httpd.conf DIRECTIVES
The absolutely most minimal configuration looks like this:

PerlModule HTML::Mason::ApacheHandler

<Location />
SetHandler perl-script
PerlHandler HTML::Mason::ApacheHandler
</Location>

This configuration tells Apache to serve all URLs through Mason (see the next section for
a more realistic strategy). We use the PerlModule line to tell mod_perl to load Mason
once at startup time, saving time and memory. This example does not set any Mason
configuration parameters, so Mason uses its default values.

If this is your first time installing and using Mason, we recommend that you use the above
configuration in a test webserver to start with. This will let you play with Mason under
mod_perl with a minimum of fuss. Once you've gotten this working, then come back and read
the rest of the document for further possibilities.

Controlling Access via Filename Extension
As it turns out, serving every URL through Mason is a bad idea for two reasons:

1. Mason should be prevented from handling images, tarballs, and other binary files. Not
only will performance suffer, but binary files may inadvertently contain a Mason
character sequence such as "<%". These files should be instead served by Apache's
default content handler.

2. Mason should be prevented from serving private (non-top-level) Mason components to
users. For example, if you used a utility component for performing arbitrary sql
queries, you wouldn't want external users to be able to access it via a URL. Requests
for private components should simply result in a 404 NOT_FOUND.

The easiest way to distinguish between different types of files is with filename
extensions. While many naming schemes are possible, we suggest using "normal" extensions
for top-level components and adding an "m" prefix for private components. For example,

Top-level Private

Component outputs HTML .html .mhtml
Component outputs text .txt .mtxt
Component executes Perl .pl .mpl

This scheme minimizes the chance of confusing browsers about content type, scales well for
new classes of content (e.g. .js/.mjs for javascript), and makes transparent the fact that
you are using Mason versus some other package.

Here is a configuration that enforces this naming scheme:

PerlModule HTML::Mason::ApacheHandler

<LocationMatch "(\.html|\.txt|\.pl)$">
SetHandler perl-script
PerlHandler HTML::Mason::ApacheHandler
</LocationMatch>

<LocationMatch "(\.m(html|txt|pl)|dhandler|autohandler)$">
SetHandler perl-script
PerlInitHandler Apache::Constants::NOT_FOUND
</LocationMatch>

The first block causes URLs ending in .html, .txt, or .pl to be served through Mason. The
second block causes requests to private components to return 404 NOT_FOUND, preventing
unscrupulous users from even knowing which private components exist. Any other file
extensions (e.g. .gif, .tgz) will be served by Apache's default content handler.

You might prefer "FilesMatch" to "LocationMatch". However, be aware that "LocationMatch"
will work best in conjunction with Mason's dhandlers.

Configuration Parameters
Mason allows you to flexibly configure its behavior via httpd.conf configuration
parameters.

These configuration parameters are set via mod_perl's "PerlSetVar" and "PerlAddVar"
directives. Though these parameters are all strings in your httpd.conf file, Mason treats
different directives as containing different types of values:

· string

The variable's value is simply taken literally and used. The string should be
surrounded by quotes if the it contains whitespace. The quotes will be automatically
removed by Apache before Mason sees the variable.

· boolean

The variable's value is used as a boolean, and is subject to Perl's rules on
truth/falseness. It is recommended that you use 0 (false) or 1 (true) for these
arguments.

· code

The string is treated as a piece of code and "eval"'ed. This is used for parameters
that expect subroutine references. For example, an anonymous subroutine might look
like:

PerlSetVar MasonOutMode "sub { ... }"

A named subroutine reference would look like this:

PerlSetVar MasonOutMode "\&Some::Module::handle_output"

· list

To set a list parameter, use "PerlAddVar" for the values, like this:

PerlAddVar MasonPreloads /foo/bar/baz.comp
PerlAddVar MasonPreloads /foo/bar/quux.comp

· hash_list

Just like a list parameter, use "PerlAddVar" for the values. However, in the case of
a hash_list, each element should be a key/value pair separated by "=>":

PerlAddVar MasonDataCacheDefaults "cache_class => MemoryCache"
PerlAddVar MasonDataCacheDefaults "namespace => foo"

Take note that the right hand side of the each pair should not be quoted.

See HTML::Mason::Params for a full list of parameters, and their associated types.

GENERAL SERVER CONFIGURATION
Component Root
The component root (comp_root) marks the top of your component hierarchy. When running
Mason with the ApacheHandler or CGIHandler modules, this defaults to your document root.

The component root defines how component paths are translated into real file paths. If
your component root is /usr/local/httpd/docs, a component path of /products/index.html
translates to the file /usr/local/httpd/docs/products/index.html.

One cannot call a component outside the component root. If Apache passes a file through
Mason that is outside the component root (say, as the result of an Alias) you will get a
404 and a warning in the logs.

You may also specify multiple component roots in the spirit of Perl's @INC. Each root is
assigned a key that identifies the root mnemonically. For example, in httpd.conf:

PerlAddVar MasonCompRoot "private => /usr/home/joe/comps"
PerlAddVar MasonCompRoot "main => /usr/local/www/htdocs"

This specifies two component roots, a main component tree and a private tree which
overrides certain components. The order is respected ala @INC, so private is searched
first and main second.

The component root keys must be unique in a case-insensitive comparison. The keys are used
in several ways. They help to distinguish component caches and object files between
different component roots, and they appear in the "title()" of a component.

Data Directory
The data directory (data_dir) is a writable directory that Mason uses for various features
and optimizations. By default, it is a directory called "mason" under your Apache server
root. Because Mason will not use a default data directory under a top-level directory,
you will need to change this on certain systems that assign a high-level server root such
as /usr or /etc.

Mason will create the directory on startup, if necessary, and set its permissions
according to the web server User/Group.

External Modules
Components will often need access to external Perl modules. There are several ways to load
them.

· The httpd PerlModule directive:

PerlModule CGI
PerlModule LWP

· In the "<%once>" section of the component(s) that use the module.

<%once>
use CGI ':standard';
use LWP;
</%once>

Each method has its own trade-offs:

The first method ensures that the module will be loaded by the Apache parent process at
startup time, saving time and memory. The second method, in contrast, will cause the
modules to be loaded by each server child. On the other hand this could save memory if the
component and module are rarely used. See the mod_perl guide's tuning section and Vivek
Khera's mod_perl tuning guide for more details on this issue.

The second method uses the modules from inside the package used by components
("HTML::Mason::Commands"), meaning that exported method names and other symbols will be
usable from components. The first method, in contrast, will import symbols into the
"main" package. The significance of this depends on whether the modules export symbols and
whether you want to use them from components.

If you want to preload the modules in your httpd.conf file, and still have them export
symbols into the "HTML::Mason::Commands" namespace, you can do this:

<Perl>
{ package HTML::Mason::Commands;
use CGI;
use LWP;
}
</Perl>

A Perl section will also work for including local library paths:

<Perl>
use lib '/path/to/local/lib';
</Perl>

Allowing Directory Requests
By default Mason will decline requests for directories, leaving Apache to serve up a
directory index or a FORBIDDEN as appropriate. Unfortunately this rule applies even if
there is a dhandler in the directory: /foo/bar/dhandler does not get a chance to handle a
request for /foo/bar/.

If you would like Mason to handle directory requests, set decline_dirs to 0. The dhandler
that catches a directory request is responsible for setting a reasonable content type via
"$r->content_type()".

Configuring Virtual Sites
These examples extend the single site configurations given so far.

Multiple sites, one component root

If you want to share some components between your sites, arrange your httpd.conf so that
all DocumentRoots live under a single component space:

# Web site #1
<VirtualHost www.site1.com>
DocumentRoot /usr/local/www/htdocs/site1
<LocationMatch ...>
SetHandler perl-script
PerlHandler HTML::Mason::ApacheHandler
</LocationMatch>
</VirtualHost>

# Web site #2
<VirtualHost www.site2.com>
DocumentRoot /usr/local/www/htdocs/site2
<LocationMatch ...>
SetHandler perl-script
PerlHandler HTML::Mason::ApacheHandler
</LocationMatch>
</VirtualHost>

# Mason configuration
PerlSetVar MasonCompRoot /usr/local/www/htdocs
PerlSetVar MasonDataDir /usr/local/mason
PerlModule HTML::Mason::ApacheHandler

The directory structure for this scenario might look like:

/usr/local/www/htdocs/ # component root
+- shared/ # shared components
+- site1/ # DocumentRoot for first site
+- site2/ # DocumentRoot for second site

Incoming URLs for each site can only request components in their respective DocumentRoots,
while components internally can call other components anywhere in the component space. The
shared/ directory is a private directory for use by components, inaccessible from the Web.

Multiple sites, multiple component roots

If your sites need to have completely distinct component hierarchies, e.g. if you are
providing Mason ISP services for multiple users, then the component root must change
depending on the site requested.

<VirtualHost www.site1.com>
DocumentRoot /usr/local/www/htdocs/site1

# Mason configuration
PerlSetVar MasonCompRoot /usr/local/www/htdocs/site1
PerlSetVar MasonDataDir /usr/local/mason/site1

<LocationMatch ...>
SetHandler perl-script
PerlHandler HTML::Mason::ApacheHandler
</LocationMatch>
</VirtualHost>

# Web site #2
<VirtualHost www.site2.com>
DocumentRoot /usr/local/www/htdocs/site2

# Mason configuration
PerlSetVar MasonCompRoot /usr/local/www/htdocs/site2
PerlSetVar MasonDataDir /usr/local/mason/site2

<LocationMatch ...>
SetHandler perl-script
PerlHandler HTML::Mason::ApacheHandler
</LocationMatch>
</VirtualHost>

ADVANCED CONFIGURATION
As mentioned previously, it is possible to write a custom mod_perl content handler that
wraps around Mason and provides basically unlimited flexibility when handling requests.
In this section, we show some basic wrappers and re-implement some of the functionality
previously discussed, such as declining image requests and protecting private components.

In addition, we discuss some of the possibilities that become available when you create a
custom wrapper around Mason's request handling mechanism. This wrapper generally consists
of two parts. The initialization portion, run at server startup, will load any needed
modules and create objects. The other portion is the "handler()" subroutine, which
handles web page requests.

Writing a Wrapper
To create a wrapper, you simply need to define a "handler()" subroutine in the package of
your choice, and tell mod_perl to use it as a content handler. The file that defines the
"handler()" subroutine can be a module, or you can simply load a simple file that contains
this subroutine definition. The latter solution was, for a long time, the only way to
configure Mason, and the file used was traditionally called handler.pl.

Nowadays, we recommend that you create a custom module in the appropriate namespace and
define your "handler()" subroutine there. The advantage to this approach is that it uses
well-known techniques for creating and installing modules, but it does require a bit more
work than simply dropping a script file into the Apache configuration directory. But
because the process is better defined, it may "feel" more solid to some folks than the
script approach.

The eg/ directory of the Mason distribution contains a couple sample modules that define
"handler()" subroutines. Let's assume that your module, like the example, defines a
"handler()" in the package "MyApp::Mason". In this case, your Apache configuration would
look like this:

PerlModule MyApp::Mason

<LocationMatch ...>
SetHandler perl-script
PerlHandler MyApp::Mason
</LocationMatch>

You may still see references to a handler.pl file in the Mason users list archives, as
well as the FAQ. These references will generally be applicable to any custom code
wrapping Mason.

Wrappers and PerlSetVar-style configuration

Sometimes people attempt to write a wrapper and configure Mason with "PerlSetVar"
directives in their Apache configuration file. This does not work. When you give
mod_perl this configuration:

PerlHandler HTML::Mason::ApacheHandler

it will dispatch directly to the "HTML::Mason::ApacheHandler->handler()" method, without
ever executing your wrapper code. However, you can mix the two methods. See Mixing
httpd.conf Configuration with a Wrapper

Wrapping with a <Perl> block
You can also put your wrapper code in a "<Perl>" block as part of your httpd.conf file.
The result is no different than loading a file via the "PerlRequire" directive.

The Wrapper Code
Regardless of how you load your wrapper code, it will always work the same way. The
"handler()" subroutine should expect to receive the Apache request object representing the
current request. This request object is used by the ApacheHandler module to determine
what component is being called.

Let's look at the guts of some wrapper code. Here's a first version:

package MyApp::Mason;

use strict;
use HTML::Mason::ApacheHandler;

my $ah =
HTML::Mason::ApacheHandler->new
( comp_root => '/path/to/comp/root',
data_dir => '/path/to/data/dir' );

sub handler {
my ($r) = @_;

return $ah->handle_request($r);
}

This wrapper is fully functional, but it doesn't actually do anything you couldn't do more
easily by configuring Mason via the httpd.conf file. However, it does serve as a good
skeleton to which additional functionality can easily be added.

External Modules Revisited
Since you are loading an arbitrary piece of code to define your wrapper, you can easily
load other modules needed for your application at the same time. For example, you might
simple add these lines to the wrapper code above:

{
package HTML::Mason::Commands;

use MIME::Base64;
}

Explicitly setting the package to "HTML::Mason::Commands" makes sure that any symbols that
the loaded modules export (constants, subroutines, etc.) get exported into the namespace
under which components run. Of course, if you've changed the component namespace, make
sure to change the package name here as well.

Alternatively, you might consider creating a separate piece of code to load the modules
you need. For example, you might create a module called "MyApp::MasonInit":

{
package HTML::Mason::Commands;

use Apache::Constants qw(:common);
use Apache::URI;
use File::Temp;
}

This can be loaded via a "PerlModule" directive in the httpd.conf file, or in the wrapper
code itself via "use".

Example: Controlling access with component attributes

An example of something you can only do with wrapper code is deciding at run-time whether
a component can be accessed at the top-level based on a complex property of the component.
For example, here's a piece of code that uses the current user and a component's
"access_level" attribute to control access:

sub handler {
my ($r) = @_;

my $req = $ah->prepare_request($r);

my $comp = $req->request_comp;

# this is done via magic hand-waving ...
my $user = get_user_from_cookie();

# remember, attributes are inherited so this could come from a
# component higher up the inheritance chain
my $required_access = $comp->attr('access_level');

return NOT_FOUND
if $user->access_level < $required_access;

return $req->exec;
}

Wrappers with Virtual Hosts
If you had several virtual hosts, each of which had a separate component root, you'd need
to create a separate ApacheHandler object for each host, one for each host. Here's some
sample code for that:

my %ah;
foreach my $site ( qw( site1 site2 site3 ) ) {
$ah{$site} =
HTML::Mason::ApacheHandler->new
( comp_root => "/usr/local/www/$site",
data_dir => "/usr/local/mason/$site" );
}

sub handler {
my ($r) = @_;

my $site = $r->dir_config('SiteName');

return DECLINED unless exists $ah{$site};

return $ah{$site}->handle_request($r);
}

This code assumes that you set the "SiteName" variable via a "PerlSetVar" directive in
each "VirtualHost" block, like this:

<VirtualHost site1.example.com>
PerlSetVar SiteName site1

<LocationMatch ...>
SetHandler perl-script
PerlHandler MyApp::Mason
</LocationMatch>
</VirtualHost>

Creating apachehandler objects on the fly

You might also consider creating ApacheHandler objects on the fly, like this:

my %ah;
sub handler {
my ($r) = @_;
my $site = $r->dir_config('SiteName');

return DECLINED unless $site;

unless exists($ah{$site}) {
$ah{$site} = HTML::Mason::ApacheHandler->new( ... );
}

$ah{$site}->handle_request($r);
}

This is more flexible but you lose the memory savings of creating all your objects during
server startup.

Other uses for a wrapper

If you have some code which must always run after a request, then the only way to
guarantee that this happens is to wrap the "$ah->handle_request()" call in an "eval {}"
block, and then run the needed code after the request returns. You can then handle errors
however you like.

Mixing httpd.conf Configuration with a Wrapper
You can take advantage of Mason's httpd.conf configuration system while at the same time
providing your own wrapper code. The key to doing this is not creating your own
ApacheHandler object. Instead, you call the "HTML::Mason::ApacheHandler->handler()" class
method from your "handler()" subroutine. Here's a complete wrapper that does this:

package MyApp::Mason;

use strict;
use HTML::Mason::ApacheHandler;

sub handler {
my ($r) = @_;

return HTML::Mason::ApacheHandler->handler($r);
}

The "HTML::Mason::ApacheHandler->handler" method will create an ApacheHandler object based
on the configuration directives it finds in your httpd.conf file. Obviously, this wrapper
is again a skeleton, but you could mix and match this wrapper code with any of the code
shown above.

Alternately you could subclass the "HTML::Mason::ApacheHandler" class, and override the
"handler()" method it provides. See the Subclassing documentation for more details. Of
course, you could even create a subclass and write a wrapper that called it.

DEVELOPMENT
This section describes how to set up common developer features.

Global Variables
Global variables can make programs harder to read, maintain, and debug, and this is no
less true for Mason components. Due to the persistent mod_perl environment, globals
require extra initialization and cleanup care.

That said, there are times when it is very useful to make a value available to all Mason
components: a DBI database handle, a hash of user session information, the server root for
forming absolute URLs.

Because Mason by default parses components in "strict" mode, you'll need to declare a
global if you don't want to access it with an explicit package name. The easiest way to
declare a global is with the allow_globals parameter.

Since all components run in the same package, you'll be able to set the global in one
component and access it in all the others.

Autohandlers are common places to assign values to globals. Use the "<%once>" section if
the global only needs to be initialized at load time, or the "<%init>" section if it needs
to be initialized every request.

Sessions
Mason does not have a built-in session mechanism, but you can use the
"MasonX::Request::WithApacheSession" module, available from CPAN, to add a session to
every request. It can also automatically set and read cookies containing the session id.

Data Caching
Data caching is implemented with DeWitt Clinton's "Cache::Cache" module. For full
understanding of this section you should read the documentation for "Cache::Cache" as well
as for relevant subclasses (e.g. "Cache::FileCache").

Cache files
By default, "Cache::FileCache" is the subclass used for data caching, although this
may be overridden by the developer. "Cache::FileCache" creates a separate subdirectory
for every component that uses caching, and one file some number of levels underneath
that subdirectory for each cached item. The root of the cache tree is
data_dir/"cache". The name of the cache subdirectory for a component is determined by
the function "HTML::Mason::Utils::data_cache_namespace".

Default constructor options
Ordinarily, when "$m->cache" is called, Mason passes to the cache constructor the
"namespace", and "cache_root" options, along with any other options given in the
"$m->cache" method.

You may specify other default constructor options with the data_cache_defaults
parameter. For example,

PerlSetVar MasonDataCacheDefaults "cache_class => SizeAwareFileCache"
PerlAddVar MasonDataCacheDefaults "cache_depth => 2"
PerlAddVar MasonDataCacheDefaults "default_expires_in => 1 hour"

Any options passed to individual "$m->cache" calls override these defaults.

Disabling data caching
If for some reason you want to disable data caching entirely, set the default
"cache_class" to "NullCache". This subclass faithfully implements the cache API but
never stores data.

PERFORMANCE
This section explains Mason's various performance enhancements and how to administer them.
One of the best ways to maximize performance on your production server is run in
static_source mode; see the third subsection below.

Code Cache
When Mason loads a component, it places it in a memory cache. By default, the cache has no
limit, but you can specify a maximum number of components to cache with the
code_cache_max_size parameter. In this case, Mason will free up space as needed by
discarding components. The discard algorithm is least frequently used (LFU), with a
periodic decay to gradually eliminate old frequency information. In a nutshell, the
components called most often in recent history should remain in the cache.

Previous versions of Mason attempted to estimate the size of each component, but this
proved so inaccurate as to be virtually useless for cache policy. The max size is now
specified purely in number of components.

Mason can use certain optimizations with an unlimited cache, especially in conjunction
with static_source, so don't limit the cache unless experience shows that your servers are
growing too large. Many dynamic sites can be served comfortably with all components in
memory.

You can prepopulate the cache with components that you know will be accessed often; see
Preloading Components. Note that preloaded components possess no special status in the
cache and can be discarded like any others.

Naturally, a cache entry is invalidated if the corresponding component source file
changes.

To turn off code caching completely, set code_cache_max_size to 0.

Object Files
The in-memory code cache is only useful on a per-process basis. Each process must build
and maintain its own cache. Shared memory caches are conceivable in the future, but even
those will not survive between web server restarts.

As a secondary, longer-term cache mechanism, Mason stores a compiled form of each
component in an object file under data_dir/obj. Any server process can eval the object
file and save time on parsing the component source file. The object file is recreated
whenever the source file changes.

The object file pathname is formed from three parts:

· the compiler "object_id" - this prevents different versions of Mason or compilers from
using the same object file, such as after an upgrade

· the component path

· object_file_extension, by default ".obj"

Besides improving performance, object files can be useful for debugging. If you feel the
need to see what your source has been translated into, you can peek inside an object file
to see exactly how Mason converted a given component to a Perl object. This was crucial
for pre-1.10 Mason, in which error line numbers were based on the object file rather than
the source file.

If for some reason you don't want Mason to create object files, set use_object_files to 0.

Static Source Mode
In static_source mode, Mason assumes that the component hierarchy is unchanging and thus
does not check source timestamps when using an in-memory cached component or object file.
This significantly reduces filesystem stats and other overhead. We've seen speedups by a
factor of two or three as a result of this mode, though of course YMMV.

When in static_source mode, you must remove object files and call
$interp->flush_code_cache in order for the server to recognize component changes. The
easiest way to arrange this is to point static_source_touch_file to a file that can be
touched whenever components change.

We highly recommend running in this mode in production if you can manage it. Many of
Mason's future optimizations will be designed for this mode. On development servers, of
course, it makes sense to keep this off so that components are reloaded automatically.

Disabling Autoflush
To support the dynamic autoflush feature, Mason has to check for autoflush mode after
printing every piece of text. If you can commit to not using autoflush, setting
enable_autoflush to 0 will allow Mason to compile components more efficiently. Consider
whether a few well-placed "$m->flush_buffer" calls would be just as good as autoflush.

Write a handler subroutine
Writing your own "handler()" subroutine which uses an ApacheHandler object (or objects)
created during server startup is slightly faster (around 5% or so) than configuring mason
via your httpd.conf file and letting Mason create its own ApacheHandler objects
internally.

Preloading Components
You can tell Mason to preload a set of components in the parent process, rather than
loading them on demand, using the preloads parameter. Each child server will start with
those components loaded in the memory cache. The trade-offs are:

time
a small one-time startup cost, but children save time by not having to load the
components

memory
a fatter initial server, but the memory for preloaded components are shared by all
children. This is similar to the advantage of using modules only in the parent
process.

Try to preload components that are used frequently and do not change often. (If a
preloaded component changes, all the children will have to reload it from scratch.)

Preallocating the Output Buffer
You can set buffer_preallocate_size to set the size of the preallocated output buffer for
each request. This can reduce the number of reallocations Perl performs as components
output text.

ERROR REPORTING AND EXCEPTIONS
When an error occurs, Mason can respond by:

· showing a detailed error message in the browser in HTML.

· die'ing, which sends a 500 status to the browser and lets the error message go to the
error logs.

The first behavior is ideal for development, where you want immediate feedback on the
error. The second behavior is usually desired for production so that users are not
exposed to messy error messages. You choose the behavior by setting error_mode to
"output" or "fatal" respectively.

Error formatting is controlled by the error_format parameter. When showing errors in the
browser, Mason defaults to the "html" format. When the error_mode is set to "fatal", the
default format is "line", which puts the entire error message on one line in a format
suitable for web server error logs. Mason also offers other formats, which are covered in
the Request class documentation.

Finally, you can use Apache's "ErrorDocument" directive to specify a custom error handler
for 500 errors. In this case, you'd set the error_mode to "fatal". The URL specified by
the "ErrorDocument" directive could point to a Mason component.

Exceptions Under the Hood
The way that Mason really reports errors is through the use of exception objects, which
are implemented with the "Exception::Class" module from CPAN, and some custom code in the
HTML::Mason::Exceptions module.

If, during the execution of a component, execution stops because some code calls "die()",
then Mason will catch this exception. If the exception being thrown is just a string,
then it will be converted to an "HTML::Mason::Exception" object. If the exception being
thrown is an object with a "rethrow()" method, then this method will be called.
Otherwise, Mason simply leaves the exception untouched and calls "die()" again.

Calling a Component to Handle Errors

Returning to the topic of wrapper code that we covered earlier, what if you wanted to
handle all request errors by calling an error handling component? There is no way to do
this without wrapper code. Here's an example "handler()" subroutine that does this:

sub handler {
my ($r) = @_;

my $return = eval { $ah->handle_request($r) };

if ( my $err = $@ )
{
$r->pnotes( error => $err );
$r->filename( $r->document_root . '/error/500.html' );

return $ah->handle_request($r);
}

return $return;
}

First, we wrap our call to "$ah->handle_request()" in an "eval{}" block. If an error
occurs, we store it in the request object using the "$r->pnotes()" method. Then we change
the filename property of the Apache request object to point to our error-handling
component and call the "$ah->handle_request()" method again, passing it the altered
request object. We could have put the exception in "$r->args", but we want to leave this
untouched so that the error-handling component can see the original arguments.

Here's what that component error-handling component might look like:

<html>
<head>
<title>Error</title>
</head>

<body>

Looks like our application broke. Whatever you did, don't do it again!

If you have further questions, please feel free to contact us at <a
href="mailto:support AT example.com">support AT example.com</a>.

<a href="/">Click here</a> to continue.

</body>
</html>

<%init>
my $error = $r->pnotes('error');

my $error_text = "Page is " . $r->parsed_uri->unparse . "\n\n";

$error_text .= UNIVERSAL::can( $error, 'as_text' ) ? $error->as_text : $error;

$r->log_error($error_text);

my $mail =
MIME::Lite->new
( From => 'error-handler AT example.com',
To => 'rt AT example.com',
Subject => 'Application error',
Data => $error_text,
);

$r->register_cleanup( sub { $mail->send } );
</%init>

<%flags>
inherit => undef
</%flags>

This component does several things. First of all, it logs the complete error to the
Apache error logs, along with the complete URL, including query string, that was
requested. The "$r->parsed_uri()" method that we use above is only available if the
"Apache::URI" module has been loaded.

The component also sends an email containing the error, in this case to an RT
installation, so that the error is logged in a bug tracking system. Finally, it displays
a less technical error message to the user.

For this to work properly, you must set error_mode to "fatal", so that Mason doesn't just
display its own HTML error page.

RUNNING OUTSIDE OF MOD_PERL
Although Mason is most commonly used in conjunction with mod_perl, the APIs are flexible
enough to use in any environment. Below we describe the two most common alternative
environments, CGI and standalone scripts.

Using Mason from a CGI Script
The easiest way to use Mason via a CGI script is with the CGIHandler module module.

Here is a skeleton CGI script that calls a component and sends the output to the browser.

#!/usr/bin/perl
use HTML::Mason::CGIHandler;

my $h = HTML::Mason::CGIHandler->new
(
data_dir => '/home/jethro/code/mason_data',
);

$h->handle_request;

The relevant portions of the httpd.conf file look like:

DocumentRoot /path/to/comp/root
ScriptAlias /cgi-bin/ /path/to/cgi-bin/

<LocationMatch "\.html$">
Action html-mason /cgi-bin/mason_handler.cgi
AddHandler html-mason .html
</LocationMatch>
<LocationMatch "^/cgi-bin/">
RemoveHandler .html
</LocationMatch>
<FilesMatch "(autohandler|dhandler)$">
Order allow,deny
Deny from all
</FilesMatch>

This simply causes Apache to call the mason_handler.cgi script every time a URL ending in
".html" under the component root is requested.

To exclude certian directories from being under Mason control, you can use something like
the following:

<LocationMatch "^/(dir1|dir2|dir3)/">
RemoveHandler .html
</LocationMatch>

This script uses the CGIHandler class to do most of the heavy lifting. See that class's
documentation for more details.

Using Mason from a Standalone Script
Mason can be used as a pure text templating solution -- like Text::Template and its
brethren, but with more power (and of course more complexity).

Here is a bare-bones script that calls a component file and sends the result to standard
output:

#!/usr/bin/perl
use HTML::Mason;
use strict;

my $interp = HTML::Mason::Interp->new ();
$interp->exec(<relative path to file>, <args>...);

Because no component root was specified, the root is set to your current working
directory. If you have a well defined and contained component tree, you'll probably want
to specify a component root.

Because no data directory was specified, object files will not be created and data caching
will not work in the default manner. If performance is an issue, you will want to specify
a data directory.

Here's a slightly fuller script that specifies a component root and data directory, and
captures the result in a variable rather than sending to standard output:

#!/usr/bin/perl
use HTML::Mason;
use strict;

my $outbuf;
my $interp = HTML::Mason::Interp->new
(comp_root => '/path/to/comp_root',
data_dir => '/path/to/data_dir',
out_method => \$outbuf
);
$interp->exec(<component-path>, <args>...);

# Do something with $outbuf