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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.


Config.pm contains global configuration options for Mason. Makefile.PL will initially create the file based on your environment, placing it in the lib/HTML/Mason subdirectory of the distribution. After that, you can edit it by hand, following the comments inside.

"make install" copies Config.pm to your Perl library directory (e.g. /usr/lib/perl5/site_perl/HTML/Mason) along with the other module files. This allows Mason internally to grab the configuration data with "use HTML::Mason::Config".

Currently this file controls:

o Whether or not certain optional modules, such as Time::HiRes, should be loaded for enhanced features.

o The type of DBM and the serialization method used for Mason's data caching. If you plan to use data caching, make sure that the DBM package is a good-quality one (DB_File or GDBM_File).

httpd.conf, srm.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
    DefaultType text/html
    <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. The next section discusses in detail how to configure this file.


Handler.pl is the most important file in your Mason configuration. It is responsible for creating the three Mason objects and supplying the many parameters that control how your components are parsed and executed. It also provides the opportunity to execute arbitrary code at three important junctures: the server initialization, the beginning of a request, and the end of a request. A wide set of behaviors can be implemented with a mere few lines of well-placed Perl in your handler.pl. In this section we present the basics of setting up handler.pl as well as some ideas for more advanced applications.

Creating the Mason objects

handler.pl creates three Mason objects: the Parser, Interpreter, and ApacheHandler. 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 possible parameters. Only two of them are required, comp_root and data_dir; these are discussed in the next two subsections. The various parameters are documented in the individual reference manuals for each object: HTML::Mason::Parser, HTML::Mason::Interp, and HTML::Mason::ApacheHandler.

The advantage of embedding these parameters in objects is that advanced configurations can create more than one set of objects, choosing which set to use at request time. For example, suppose you have a staging site and a production site running on the same web server, distinguishing between them with a configuration variable called version:

    # Create Mason objects for staging site
    my $parser1 = new HTML::Mason::Parser;
    my $interp1 = new HTML::Mason::Interp (parser=>$parser1, ...);
    my $ah1 = new HTML::Mason::ApacheHandler (interp=>$interp1);

    # Create Mason objects for production site
    my $parser2 = new HTML::Mason::Parser;
    my $interp2 = new HTML::Mason::Interp (parser=>$parser2, ...);
    my $ah2 = new HTML::Mason::ApacheHandler (interp=>$interp2);

    sub handler {

        # Choose the right ApacheHandler
        if ($r->dir_config('version') eq ' staging') {
        } else {

Component root

Given 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.

When Mason handles a request, the request filename ($r->filename) must be underneath your component root -- that way Mason has a legitimate component to start with. If the filename is not under the component root, Mason will place a warning in the error logs and return a 404. Unfortunately if your component root or document root goes through a soft link, Mason will have trouble comparing the paths and will return 404. To fix this, set your document root to the true path.

Data directory

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
 preview:  preview settings files

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

External modules

Components will often need access to external Perl modules. Any such modules that export symbols should be 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 (perl.apache.org/tuning) for details.

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 );

Persistent user sessions

With just a few lines in handler.pl you can make a global hash (e.g. %session) available to all components containing persistent user session data. If you set a value in the hash, you will see the change in future visits by the same user. The Apache::Session module, available from CPAN, provides convenient facilities for this. The following lines, commented out in the default handler.pl, implement a cookie-based session with user data stored on the filesystem:

    my %session;
    my $cookie = $r->header_in('Cookie');
    $cookie =~ s/SESSION_ID=(\w*)/$1/;
    tie %session, 'Apache::Session::File', $cookie, {'Directory' => '/tmp/session'};
    $r->header_out("Set-Cookie" => "SESSION_ID=$session{_session_id};") if ( !$cookie );
    local *HTML::Mason::Commands::session = \%session;

This code is customizable; you can change the user ID location (e.g. URL instead of cookie), the user data storage mechanism (e.g. DBI database), and the name of the global hash.

Using global variables

Global variables generally make programs harder to read, maintain, and debug, and this is no less true for Mason. Due to the persistent mod_perl environment, globals require extra initialization and cleanup care. And the highly modular nature of Mason pages does not mix well with globals: it is no fun trying to track down which of twenty components is stepping on your variable. With the ability to pass parameters and declare lexical (my) variables in components, there is very little need for globals at all.

That said, there are times when it is very useful to make a value available to all Mason components. One example is the Apache request object; this is automatically made available as the global $r. Other examples are a DBI database handler or a hash of user session information. Usually you initialize the global in your handler.pl, either outside the handler() subroutine (if you only need to set it once) or inside (if you need to set it every request).

Mason by default parses components in strict mode, so you can't simply start referring to a new global or you'll get a fatal warning. The solution is to invoke use vars inside the package that components execute in, by default HTML::Mason::Commands:

    { package HTML::Mason::Commands;
      use vars qw($dbh %session);

Alternatively you can use the Parser/allow_globals parameter or method:

    my $parser = new HTML::Mason::Parser (..., allow_globals => [qw($dbh %session)]);
    $parser->allow_globals(qw($foo @bar))

The only advantage to allow_globals is that it will do the right thing if you've chosen a different package for components to run in (via the Parser/in_package Parser parameter.)

Similarly, to initialize the variable in handler.pl, you need to set it in the component package:

   $HTML::Mason::Commands::dbh = DBI->connect(...);

Alternatively you can use the Interp/set_global interpreter method:

    $interp->set_global(dbh => DBI->connect(...));

Again, set_global will do the right thing if you've chosen a different package for components.

Now when referring to these globals inside components, you can use the plain variable name:



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) and one of several serialization mechanisms (Data::Dumper, FreezeThaw or Storable). Mason's Config.pm controls which packages are used.

The most important task is selecting a good DBM package. Most standard DBM packages (SDBM, ODBM, NDBM) are unsuitable for data caching due to significant limitations on the size of keys and values. Perl only comes with SDBM, so you'll need to obtain a good-quality package if you haven't already. At this time the best options are Berkeley DB (DB_File) version 2.x, available at www.sleepycat.com, and GNU's gdbm (GDBM), available at GNU mirror sites everywhere. Stay away from Berkeley DB version 1.x on Linux which has a serious memory leak (and is unfortunately pre-installed on many distributions).

As far as the serialization methods, all of them should work fine. Data::Dumper is probably simplest: it comes with the latest versions of Perl, is required by Mason anyway, and produces readable output (possibly useful for debugging cache files). On the other hand Storable is significantly faster than the other options according to the MLDBM documentation.


Once set up, 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.

If some reason you want to disable data caching, specify use_data_cache=>0 to the Interp object. This will cause all mc_cache calls to return undef without doing anything.


A debug file is a Perl script that creates a fake Apache request object ($r) and calls the same PerlHandler that Apache called. Debug files are created under data_dir/debug/<username> for authenticated users, otherwise they are placed in data_dir/debug/anon. 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).


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,

When replaying a request through a debug file, the global variable $HTML::Mason::IN_DEBUG_FILE will be set to 1. This is useful if you want to omit certain flags (like preloading) in handler.pl when running under debug. For example:

    my %extra_flags = ($HTML::Mason::IN_DEBUG_FILE) ? () : (preloads=>[...]);
    my $interp = new HTML::Mason::Interp (..., %extra_flags);


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. In your handler.pl, add the line

        use HTML::Mason::Preview;

    somewhere underneath "use HTML::Mason". Then add code to your handler routine 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.

System logs

Mason will log various events to a system log file if you so desire. This can be useful for performance monitoring and debugging.

The format of the system log was designed to be easy to parse by programs, although it is not unduly hard to read for humans. Every event is logged on one line. Each line consists of multiple fields delimited by a common separator, by default ctrl-A. The first three fields are always the same: time, the name of the event, and the current pid ($$). These are followed by one or more fields specific to the event.

The events are:


 REQ_START      start of HTTP request           request number, URL + query string
 REQ_END        end of HTTP request             request number, error flag (1 if
                                                error occurred, 0 otherwise)
 CACHE_READ     attempt to read from            component path, cache key, success
                data cache (mc_cache)           flag (1 if item found, 0 otherwise)
 CACHE_STORE    store to data cache             component path, cache key
 COMP_LOAD      component loaded into memory    component path
                for first time  

The request number is an incremental value that uniquely identifies each request for a given child process. Use it to match up REQ_START/REQ_END pairs.

To turn on logging, specify a string value to system_log_events containing one or more event names separated by '|'. In additional to individual event names, the following names can be used to specify multiple events:

 ALL = All events

For example, to log REQ_START, REQ_END, and COMP_LOAD events, you could use system_log_events => "REQUEST|COMP_LOAD" Note that this is a string, not a set of constants or'd together.

Configuration Options

By default, the system log will be placed in data_dir/etc/system.log. You can change this with system_log_file.

The default line separator is ctrl-A. The advantage of this separator is that it is very unlikely to appear in any of the fields, making it easy to split() the line. The disadvantage is that it will not always display, e.g. from a Unix shell, making the log harder to read casually. You can change the separator to any sequence of characters with system_log_separator.

The time on each log line will be of the form "seconds.microseconds" if you are using Time::HiRes, and simply "seconds" otherwise. See Config.pm section.

Sample Log Parser

Here is a code skeleton for parsing the various events in a log. You can also find this in eg/parselog.pl in the Mason distribution.

   while (<LOG>) {
       my (@fields) = split("\cA");
       my ($time,$event,$pid) = splice(@fields,0,3);
       if ($event eq 'REQ_START') {
           my ($reqnum,$url) = @fields;
       } elsif ($event eq 'REQ_END') {
           my ($reqnum,$errflag) = @fields;
       } elsif ($event eq 'CACHE_READ') {  
           my ($comp,$key,$hitflag) = @fields;
       } elsif ($event eq 'CACHE_STORE') { 
           my ($comp,$key) = @fields;
       } elsif ($event eq 'COMP_LOAD') {
           my ($comp) = @fields;
       } else {
           warn "unrecognized event type: $event\n";

Suggested Uses

Performance: REQUEST events are useful for analyzing the performance of all Mason requests occurring on your site, and identifying the slowest requests. (You cannot measure this with standard Apache logs since they only record the end time of the request.) eg/perflog.pl in the Mason distribution is a log parser that outputs the average compute time of each unique URL, in order from slowest to quickest.

Server activity: REQUEST events are useful for determining what your web server children are working on, especially when you have a runaway. For a given process, simply tail the log and find the last REQ_START event with that process id. (You can also use the Apache status page for this, of course.)

Cache efficiency: CACHE events are useful for monitoring cache "hit rates" (number of successful reads over total number of reads) over all components that use a data cache. Because stores to a cache are more expensive than reads, a high hit rate is essential for the cache to have a beneficial effect. If a particular cache hit rate is too low, you may want to consider changing how frequently it is expired or whether to use it at all.

Load frequency: COMP_LOAD events are useful for determining which components are loaded most often and therefore good candidates for preloading.


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. If for some reason you don't want to cache every component (e.g. to save memory) you can modify this behavior via the code_cache_mode parameter.

  • 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. Besides improving performance, object files are essential for debugging and interpretation of compilation errors. However, if you don't want Mason to create object files (e.g. if disk space is scarce), you can turn them off by passing use_object_files=>0 to the Interp object.

Both entities are recomputed if the component source file changes.

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";
    print("Hello ");
    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:

    <& top &>
    ... 20K of HTML ...
    <& center &>
    ... 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 "Parser/source_refer_predicate" 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

Normally, every time Mason executes a component, it checks the last modified time of its source file to see if it needs to be reloaded. These file checks are convenient for development, but for a production site they degrade performance unnecessarily.

To remedy this, Mason has an accelerated mode that changes its behavior in two ways:

1. Does not check component source files at all, relying solely on object files. This means the developer or an automated system is responsible for recompiling any components that change and recreating object files, using the Parser Parser/parse method.

2. Rather than continuously checking whether object files have changed, Mason monitors a "reload file" containing an ever-growing list of components that have changed. Whenever a component changes, the developer or an automated system is responsible for appending the component path to the reload file. The reload file is kept in data_dir/etc/reload.lst.

You can activate this mode with the Interp/use_reload_file Interp method.

The advantage of using this mode is that Mason stats one file per request instead of ten or twenty. The disadvantage is a increase in maintenance costs as the object and reload files have to be kept up-to-date. Automated editorial tools, and cron jobs that periodically scan the component hierarchy for changes, are two possible solutions. The Mason content management system automatically handles this task.


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 message 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 Mason/single site configuration 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