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Jonathan Swartz


HTML::Mason::Admin - Mason Administrator's Guide


This guide is intended for the sys admin/web master in charge of installing, configuring, or tuning a Mason system.


This section discusses the various files and directories that play a part in Mason's configuration.


After installing Mason, edit this file to configure global options, following the comments inside. Currently the only options to configure are the type of DBM and the serialization method to use for Mason's caching mechanisms.

httpd.conf (srm.conf, access.conf)

Directives must be added to Apache's configuration files to specify which requests should be handled through Mason, and the handler used for those requests. As described in HTML::Mason, a simple configuration looks like:

    DocumentRoot /usr/local/www/htdocs
    PerlRequire /usr/local/mason/handler.pl
    <Location />
        SetHandler perl-script
        PerlHandler HTML::Mason

This file contains startup code that initializes the parent Apache process. It also defines the handler used by each child process to field Mason requests. See the synopsis in HTML::Mason for a simple example.

handler.pl creates three Mason objects: the Parser, Interpreter, and Apache handler. The Parser compiles components into Perl subroutines; the Interpreter executes those compiled components; and the Apache handler routes mod_perl requests to Mason. These objects are created once in the parent httpd and then copied to each child process.

These objects have a fair number of initial parameters, only two of which are required: comp_root and data_dir. The various parameters are documented in the individual reference manuals for each object: HTML::Mason::Parser, HTML::Mason::Interp, and HTML::Mason::ApacheHandler.

Components will often need access to external Perl modules. Any such modules that export symbols should by listed in handler.pl, rather than the standard practice of using a PerlModule configuration directive. This is because components are executed inside the HTML::Mason::Commands package, and can only access symbols exported to that package. Here's sample module list:

    { package HTML::Mason::Commands;
      use CGI ':standard';
      use LWP::UserAgent;
      ... }

In any case, for optimal memory utilization, make sure all Perl modules are used in the parent process, and not in components. Otherwise, each child allocates its own copy and you lose the benefit of shared memory between parent processes and their children. See Vivek Khera's mod_perl tuning FAQ for details.

Another parent/child consideration is file ownership. Web servers that run on privileged ports like 80 start with a root parent process, then spawn children running as the 'User' and 'Group' specified in httpd.conf. This difference leads to permission errors when child processes try to write files or directories created by the parent process.

To work around this conflict, Mason remembers all directories and files created at startup, returning them in response to $interp->files_written. This list can be fed to a chown() at the end of the startup code in handler.pl:

    chown ( [getpwnam('nobody')]->[2], [getgrnam('nobody')]->[2],
            $interp->files_written );
Component space (comp_root)

The component space is a tree of component source files. The top of the tree is called the component root and is set via the comp_root parameter. In simple Mason configurations the component root is the same as the server's DocumentRoot. More complex configurations may specify several different document roots under a single component root.

Data directory (data_dir)

The data directory is where Mason keeps various files to help implement caching, debugging, etc. You specify a single data directory via the data_dir parameter and Mason creates subdirectories underneath it as needed:

 cache:    data cache files
 debug:    debug files
 etc:      miscellaneous files
 obj:      compiled components

These directories will be discussed in appropriate sections throughout this manual.


This section explains how standard Mason features work and how to administer them.

Data caching


Cache files are implemented using MLDBM, an interface for storing persistent multi-level data structures. MLDBM, in turn, uses one of several DBM packages (DB_File, GDBM, etc.) and one of several serialization mechanisms (Data::Dumper, FreezeThaw or Storable). Mason's Config.pm contains stubs for several combinations; you will at least want to replace the default NDBM with a faster, less limited package.


Data caching requires little administration. When a component calls mc_cache or mc_cache_self for the first time, Mason automatically creates a new cache file under data_dir/cache, replacing slashes in the component path with "::". For example, the cache file for component /foo/bar is data_dir/cache/foo::bar.

Currently Mason never deletes cache files, not even when the associated component file is modified. (This may change in the near future.) Thus cache files hang around and grow indefinitely. You may want to use a cron job or similar mechanism to delete cache files that get too large or too old. For example:

    # Shoot cache files more than 30 days old
    foreach (<data_dir/cache>) {    # path to cache directory
        unlink $_ if (-M >= 30);

In general you can feel free to delete cache files periodically and without warning, because the data cache mechanism is explicitly not guaranteed -- developers are warned that cached data may disappear anytime and components must still function.


A debug file is a Perl script that creates a fake Apache request object ($r) and calls the same PerlHandler that Apache called. Several ApacheHandler parameters are required to activate and configure debug files:


The debug_mode parameter indicates which requests should produce a debug file: "all", "none", or "error" (only if a error occurs). Debug files are created under data_dir/debug/<username> for authenticated users, otherwise they are placed in data_dir/debug/anon.


The full path to your Perl binary -- e.g. /usr/bin/perl. This is used in the Unix "shebang" line at the top of each debug file.


The full path to your handler.pl script. Debug files invoke handler.pl just as Apache does as startup, to load needed modules and create Mason objects.


The name of the request handler defined in handler.pl. This routine is called with the saved Apache request object.

Here's a sample ApacheHandler constructor with all debug options:

    my $ah = new HTML::Mason::ApacheHandler (interp=>$interp,


The previewer is a web based utility that allows site developers to:

  1. View a site under a variety of simulated client conditions: browser, operating system, date, time of day, referer, etc.

  2. View a debug trace of a page, showing the component call tree and indicating which parts of the page are generated by which components.

The web-based previewer interface (a single component, actually) allows the developer to select a variety of options such as time, browser, and display mode. The set of these options together is called a previewer configuration. Configurations can be saved under one of several preview ports. For more information on how the previewer is used, see HTML::Mason::Components.

Follow these steps to activate the Previewer:

  1. Choose a set of preview ports, for example, 3001 to 3005.

  2. In httpd.conf, put a Listen in for each port. E.g.

      Listen your.site.ip.address:3001
      Listen your.site.ip.address:3005

    You'll also probably want to restrict access to these ports in your access.conf. If you have multiple site developers, it is helpful to use username/password access control, since the previewer will use the username to keep configurations separate.

  3. Add code to your handler routine (in handler.pl) to intercept Previewer requests on the ports defined above. Your handler should end up looking like this:

        sub handler {
            my ($r) = @_;
            # Compute port number from Host header
            my $host = $r->header_in('Host');
            my ($port) = ($host =~ /:([0-9]+)$/);
            $port = 80 if (!defined($port));
            # Handle previewer request on special ports
            if ($port >= 3001 && $port <= 3005) {
                my $parser = new HTML::Mason::Parser(...);
                my $interp = new HTML::Mason::Interp(...);
                my $ah = new HTML::Mason::ApacheHandler (...);
                return HTML::Mason::Preview::handle_preview_request($r,$ah);
            } else {
                $ah->handle_request($r);    # else, normal request handler

    The three "new" lines inside the if block should look exactly the same as the lines at the top of handler.pl. Note that these separate Mason objects are created for a single request and discarded. The reason is that the previewer may alter the objects' settings, so it is safer to create new ones every time.

  4. Copy the Previewer component ("samples/preview") to your component root (you may want to place it at the top level so that http://www.yoursite.com/preview calls up the previewer interface). Edit the "CONFIGURATION" block at the top to conform to your own Mason setup.

To test whether the previewer is working: restart your server, go to the previewer interface, and click "View". You should see your site's home page.


This section explains Mason's various performance enhancements and how to administer them.

Code Caching/Object Files

When Mason encounters a component for the first time, it compiles the component into a Perl subroutine. To preserve the fruits of its labor, Mason will:

  • store a reference to the subroutine in an in-memory hash table. The current server process can use this for future references to the same component.

  • store the subroutine body in an object file under data_dir/obj/component-path. Future server processes can eval the object file and save time on parsing. Both entities are recomputed if the component source file changes.

Besides improving performance, object files are essential for debugging and interpretation of compilation errors.

Source References

Mason's parser translates plain HTML in components to simple print statements. For example, the following component:

    %my $name = "Jon";
    Hello <% $name %>, how are you?

translates to something like:

    my $name = "Jon";
    $r->print("Hello ");
    $r->print(", how are you?");

The amount of memory taken up by a compiled component is therefore at least as large as the combined size of its HTML blocks. If a component has 50K of HTML, that means 50K of storage for each child process that loads the component. Multiply that by ten processes and twenty such components and you've got some noticeable memory overhead.

To reduce this overhead Mason generates, in certain cases, code that reads from the source file at runtime. For example, the following component:

    <%mc_comp(' top')%>
    ... 20K of HTML ...
    ... 30K of HTML ...

translates to something like:

    my $_srctext = mc_file('/usr/local/www/htdocs/foo/bar');

The resulting code is a bit slower but more memory efficient. Mason decides whether to use these "source references" by first measuring both the total size and the amount of HTML in a component. Those values are then examined by a customizable ""source_refer_predicate" in Parser" which makes a determination based on local policy, say "more than 50% HTML", or "more than 20K of HTML".

Pure text components

A component with no Perl and no Mason constructs -- all text and HTML -- is known as a pure text component. Mason optimizes this special case by creating a zero size object file. The dummy object file signifies that the results should simply be obtained by reading the component's source file.

This feature requires no administration; I mention it simply so that you are not surprised to see zero size object files.


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


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


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.)

Reload file

Even if a component has been preloaded or cached in memory, Mason still checks the last modified time of its source file every time it runs to see if it needs to be reloaded. If the average page consists of twenty components, that means twenty file stats per page, a potential performance concern.

To prevent these constant file checks, Mason can monitor a single "reload file" of modified components. When a component changes, you append its component path to the reload file, one path per line. At the beginning of each request Mason checks to see if the reload file has changed; if so, it reads the new paths and invalidates their cache entries, which in turn forces a recompile the next time those components are requested.

The reload file is kept in data_dir/etc/reload.lst. You can activate reload file monitoring with interp->use_reload_file.

The advantage of using a reload file is that Mason stats one file per request instead of ten or twenty. The disadvantage is a major increase in maintenance costs as the reload file has to be kept up-to-date. If developers on your site use editorial tools to access and trigger components, you can update the reload file as part of these tools. Or you might run a cron job or similar timed task that periodically scans the component hierarchy, updating the reload file if anything has changed.


Site builders often maintain two versions of their sites: the production (published) version visible to the world, and the development (staging) version visible internally. Developers try out changes on the staging site and push the pages to production once they are satisfied.

The priorities for the staging site are rapid development and easy debugging, while the main priority for the production site is performance. This section describes various ways to adapt Mason for each case.

Output mode

Mason can spew data in two modes. "Batch" mode means that Mason computes the entire page in memory and then transmits it all at once. "Stream" mode means that Mason outputs data as soon as it is computed. (This is only Mason's point of view; it does not take buffering done by Perl or the O/S into account.)

Which is better, batch or stream? It depends on the context.

For production web servers, stream mode is better because it gets data to the browser more quickly. A browser can only process and display data at a certain rate--streaming the data allows the browser to start working in parallel with the server, while waiting to the end serializes the task (first the server does all its work, then the browser does all its work). From a user perspective the initial bytes are especially important: until the browser receives some data, it simply displays a "waiting" message. Serving a computationally intense page in batch mode makes the server look unresponsive and tempts users to hit Stop, whereas in stream mode the browser at least acknowledges an answer and draws a background.

For development or staging web servers, batch mode has the advantage of better error handling. Suppose an error occurs in the middle of a page. In stream mode, the error message interrupts existing output, often appearing in an awkward HTML context such as the middle of a table which never gets closed. The user may see a partial page and have to "View source" to see the error message. In batch mode, the error message is output neatly and alone.

You control output mode by setting ah->output_mode to "batch" or "stream".

Error mode

When an error occurs, Mason can respond by:

  • showing a detailed error message in the browser

  • die'ing, which sends a 501 to the browser and lets the error go to the error logs. The first option is ideal for development, where you want immediate feedback on the error. The second option is usually desired for production so that users are not exposed to messy error messages. You control this option by setting ah->error_mode to "html" or "fatal" respectively.

Debug mode

As discussed in the debugging section, you can control when Mason creates a debug file. While creating a debug file is not incredibly expensive, it does involves a bit of work and the creation of a new file, so you probably want to avoid doing it on every request to a frequently visited site. I recommend setting debug_mode to 'all' in development, and 'error' or 'none' in production.

Reload files

Consider reload files only for frequently visited production sites.


The example below extends the "single site configuration" in Mason example in HTML::Mason.

When configuring Mason to serve multiple virtual hosts, Mason's comp_root must be separated from the DocumentRoot (since DocumentRoot changes per virtual server). In this case you'll want to collect all of your DocumentRoots inside a single component space:

    # httpd.conf
    PerlRequire /usr/local/mason/handler.pl

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

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

In contrast to these big changes to httpd.conf, the Mason bootstrap in handler.pl stays the same:

    my $interp = new HTML::Mason::Interp (parser=>$parser,

The <Location> directives in this example now route all requests through Mason--every page is dynamic. The directory structure for this scenario might looks like this:

    /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, then, is a private directory for use by components, inaccessible from the Web.


Jonathan Swartz, swartz@transbay.net


HTML::Mason, HTML::Mason::Parser, HTML::Mason::Interp, HTML::Mason::ApacheHandler