HTML::Mason::Admin(3) User Contributed Perl DocumentationHTML::Mason::Admin(3)NAMEHTML::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;
}
1;
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>
<p>
Looks like our application broke. Whatever you did, don't do it again!
</p>
<p>
If you have further questions, please feel free to contact us at <a
href="mailto:support@example.com">support@example.com</a>.
</p>
<p><a href="/">Click here</a> to continue.</p>
</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@example.com',
To => 'rt@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
SEE ALSO
Mason
AUTHORS
· Jonathan Swartz <swartz@pobox.com>
· Dave Rolsky <autarch@urth.org>
· Ken Williams <ken@mathforum.org>
COPYRIGHT AND LICENSE
This software is copyright (c) 2012 by Jonathan Swartz.
This is free software; you can redistribute it and/or modify it under
the same terms as the Perl 5 programming language system itself.
perl v5.18.2 2014-01-19 HTML::Mason::Admin(3)
