Components.pod - Mason Developer's Manual

- NAME
- DESCRIPTION
- WHAT ARE COMPONENTS?
- IN-LINE PERL SECTIONS
  - Examples and Recommended Usage
- CALLING COMPONENTS
  - Components that output HTML
  - Components that compute values
- TOP-LEVEL COMPONENTS
  - dhandlers
  - autohandlers
  - dhandlers vs. autohandlers
- PASSING PARAMETERS
  - In Component Calls
  - In HTTP requests
  - Accessing Parameters
  - Parameter Passing Examples
- OTHER PERL SECTIONS
  - <%init>
  - <%cleanup>
  - <%args>
  - <%once>
  - <%filter>
  - <%doc>
  - <%text>
  - \ at end of line
- DATA CACHING
  - Basic Usage
  - Multiple Keys/Values
  - Expiration
  - Busy Locks
  - Keeping In Memory
  - Caching All Output
  - Guarantees (or lack thereof)
- FILTERING
  - <%filter> section
  - mc_call_self command
  - How filtering works
- ACCESSING SERVER INTERNALS
- SENDING HTTP HEADERS
- USING THE PERL DEBUGGER
  - Using debug files
  - Specifying when to create debug files
  - How debug files work
  - When debug files don't work
- USING THE PERL PROFILER
- THE PREVIEWER
- TIPS AND TRAPS
- AUTHOR
- SEE ALSO

# NAME

Components.pod - Mason Developer's Manual

# DESCRIPTION

This manual is written for content developers who know HTML and at least a little Perl. The goal is to write, run, and debug Mason components.

If you are the webmaster (or otherwise responsible for the Mason installation), you should also read HTML::Mason::Admin. There you will find FAQs about virtual site configuration, performance tuning, component caching, and so on.

I strongly suggest that you have a working Mason to play with as you work through these examples. Other component examples can be found in the `samples/` directory.

# WHAT ARE COMPONENTS?

The component - a mix of Perl and HTML - is Mason's basic building block and computational unit. Under Mason, web pages are formed by combining the output from multiple components. An article page for a news publication, for example, might call separate components for the company masthead, ad banner, left table of contents, and article body. Consider this layout sketch:

        +---------+------------------+
        |Masthead | Banner Ad        |
        +---------+------------------+
        |         |                  |
        |+-------+|Text of Article ..|
        ||       ||                  |
        ||Related||Text of Article ..|
        ||Stories||                  |
        ||       ||Text of Article ..|
        |+-------+|                  |
        |         +------------------+
        |         | Footer           |
        +---------+------------------+

The top level component decides the overall page layout, perhaps with HTML tables. Individual cells are then filled by the output of subordinate components, one for the Masthead, one for the Footer, etc. In practice pages are built up from as few as one, to as many as twenty or more components.

This component approach reaps many benefits in a web environment. The first benefit is consistency: by embedding standard design elements in components, you ensure a consistent look and make it possible to update the entire site with just a few edits. The second benefit is concurrency: in a multi-person environment, one person can edit the masthead while another edits the table of contents. A last benefit is reuseability: a component produced for one site might be useful on another. You can develop a library of generally useful components to employ on your sites and to share with others.

Most components emit chunks of HTML. ``Top level'' components, invoked from a URL, represent an entire web page. Other, subordinate components emit smaller bits of HTML destined for inclusion in top level components.

Components receive form and query data from HTTP requests. When called from another component, they can accept arbitrary parameter lists just like a subroutine, and optionally return values. This enables a type of component that does not print any HTML, but simply serves as a function, computing and returning a result.

Mason actually compiles components down to Perl subroutines, so you can debug and profile component-based web pages with standard Perl tools that understand the subroutine concept, e.g. you can use the Perl debugger to step through components, and Devel::DProf to profile their performance.

# IN-LINE PERL SECTIONS

Here is a simple component example:

        <%perl>
        my $noun = 'World';
        my @time = split /[\s:]/, localtime;
        </%perl>
        Hello <% $noun %>,
        % if ( $time[3] < 12 ) {
        good morning.
        % } else {
        good afternoon.
        % }

After 12 pm, the output of this component is:

        Hello world, good afternoon.

This short example demonstrates the three primary ``in-line'' Perl sections. In-line sections are generally embedded within HTML and execute in the order they appear. Other, specialized Perl sections are tied to component events like initialization and cleanup, argument definition, etc. Those are covered later in Other Perl Sections.

The parsing rules for these Perl sections are as follows:

1. Blocks of the form <% xxx %> are replaced with the result of evaluating xxx as a single Perl expression. These are often used for variable replacement. such as 'Hello, <% `$name` %>!'.

2. Lines beginning with a '%' character are treated as Perl.

3. Multiline blocks of Perl code can be inserted with the <%perl> .. </%perl> tag. The enclosed text is executed as Perl and the return value, if any, is discarded.

   The <%perl> tag is case-insensitive. It may appear anywhere in the text, and may span any number of lines. <%perl> blocks cannot be nested inside one another.

In addition to Perl code, Perl sections may also contain Mason commands. These keywords, identified by their mc_ prefix, collectively provide an interface to Mason services such as data caching, file includes, and so on. HTML::Mason::Commands is the reference for all Mason commands.

## Examples and Recommended Usage

% lines

Most useful for conditional and loop structures - if, while, foreach, , etc. - as well as side-effect commands like assignments. Examples:

o Conditional code

        % my $ua = $r->header_in('User-Agent');
        % if ($ua =~ /msie/i) {
        Welcome, Internet Explorer users
        ...
        % } elsif ($ua =~ /mozilla/i) {
        Welcome, Netscape users
        ...
        % }

o HTML list formed from array

        <ul>
        % foreach $item (@list) {
        <li><% $item %>
        % }
        </ul>

o HTML list formed from hash

        <ul>
        % while (my ($key,$value) = each(%ENV)) {
        <li>
        <b><% $key %></b>: <% $value %>
        % }
        </ul>

o HTML table formed from list of hashes

        <table>
        <tr>
        % foreach my $h (@loh) {
        <td><% $h->{foo} %></td>
        <td bgcolor=#ee0000><% $h->{bar} %></td>
        <td><% $h->{baz} %></td>
        % }
        </tr>
        </table>

For more than three lines of Perl, consider using a <%perl> block.

<% xxx %>

Most useful for printing out variables, as well as more complex expressions. Examples:

      Dear <% $name %>: We will come to your house at <% $address %> in the
      fair city of <% $city %> to deliver your $<% $amount %> dollar prize!

      The answer is <% ($y+8) % 2 %>.

      You are <% $age<18 ? 'not' : '' %> permitted to enter this site.

For side-effect commands like assignments, consider using a % line or <%perl> block instead.

<%perl> xxx <%/perl>

Useful for Perl blocks of more than a few lines. For a very small block, consider using % lines.

# CALLING COMPONENTS

Mason pages often are built not from a single component, but from multiple components that call each other in a hierarchical fashion.

## Components that output HTML

To call one component from another, use the <& &> tag:

        <& compPath, [name=>value, ...], [STORE=>ref] &>

compPath:
  The component path. With a leading '/', the path is relative to the component root (comp_root). Otherwise, it is relative to the location of the calling component.

name=>value pairs:
  Parameters are passed as one or more name=>value pairs, e.g. `player=>'M. Jordan'`.

STORE=>ref:
  The optional STORE parameter takes a scalar reference as an argument, and tells the component to direct its output into the named variable instead of standard output. This is analogous to the difference between sprintf and printf.

Mason uses a bit of magic parsing to eliminate the need for quotes around the component path in common cases. If the first character is one of `[A-Za-z0-9/_.]`, the component path is assumed to be a literal string running up to the first comma or &>. Otherwise, the component path is evaluated as an expression.

Here are some examples:

        # relative component paths
        <& topimage &>
        <& tools/searchbox &>

        # absolute component path
        <& /shared/masthead, color=>'salmon' &>

        # use STORE option to place output in variable
        <& /shared/masthead, color=>'salmon', STORE=>\$mh_text &>

        # this component path MUST have quotes because it contains a comma
        <& "sugar,eggs", mix=>1 &>

        # variable component path
        <& $comp &>

        # variable component and arguments
        <& $comp, %args &>

        # you can use arbitrary expression for component path, but it cannot
        # begin with a letter or number; delimit with () to remedy this
        <& (int(rand(2)) ? 'thiscomp' : 'thatcomp'), id=>123 &>

## Components that compute values

So far you have seen components used solely to output HTML. However, components may also be used to compute a value. For example, you might have a component `isNetscape` that analyzes the user agent to determine whether it is a Netscape browser:

        <%perl>
        mu $ua = $r->header_in('User-Agent');
        return ($ua =~ /Mozilla/i && $ua !~ /MSIE/i) ? 1 : 0;
        </%perl>

Because components are implemented underneath with Perl subroutines, they can return values and even understand scalar/list context.

The <& &> notation only calls a component for its side-effect, and discards its return value, if any. To get at the return value of a component, use the mc_comp command:

        % if (mc_comp('isNetscape')) {
        Welcome, Netscape user!
        % }

Mason adds a `return undef` to the bottom of each component to provide an empty default return value. To return your own value from a component, you must use an explicit `return` statement.

Generally components are divided into two types: those that output HTML, and those that return a value. There is very little reason for a component to do both. For example, it would not be very friendly for `isNetscape` to output ``hi Mom'' while it was computing its value, thereby surprising the `if` statement! Conversely, any value returned by an HTML component would typically be discarded by the <& &> tag that invoked it.

Prior to version 0.6, mc_comp was the only way to call components even for outputting HTML:

        <% mc_comp('/shared/masthead', color=>'salmon') %>

This is still legal, although <& &> is the official syntax. If you have pre-0.6 components that you'd like to convert to use <& &>, check out the utility bin/convert0.6.pl.

# TOP-LEVEL COMPONENTS

The first component invoked for a page (the ``top-level component'') resides within the DocumentRoot and is chosen based on the URL. For example:

        http://www.foo.com/mktg/products?id=372

Apache resolves this URL to a filename, e.g. /usr/local/www/htdocs/mktg/prods.html. Mason loads and executes that file as a component. In effect, Mason calls

        mc_comp('/mktg/products', id=>372)

This component might in turn call other components and execute some Perl code, or it might be nothing more than static HTML.

## dhandlers

What happens when a user requests a component that doesn't exist? In this case Mason scans backward through the URI, checking each directory for a component named dhandler (``default handler''). If found, the dhandler is invoked and is expected to use `$r->path_info` (the virtual location) as the parameter to some access function, perhaps a database lookup or location in another filesystem. In a sense, dhandlers are similar in spirit to Perl's AUTOLOAD feature; they are the ``component of last resort'' when a URL points to a non-existent component.

Consider the following URL, in which `newsfeeds/` exists but not the subdirectory `LocalNews` nor the component `Story1`:

        http://myserver/newsfeeds/LocalNews/Story1

In this case Mason constructs the following search path:

        /newsfeeds/LocalNews/Story1         => no such thing
        /newsfeeds/LocalNews/dhandler       => no such thing
        /newsfeeds/dhandler                 => found! (search ends)
        /dhandler

The found dhandler would read ``/LocalNews/Story1'' from `$r->path_info` and use it as a retrieval key into a database of stories. Optionally, the Mason command mc_dhandler_arg() returns the same path_info stripped of the leading slash (``LocalNews/locStory1.html'').

Here's how a simple /newsfeeds/dhandler might look:

        <& header &>
        <b><% $headline %></b><p>
        <% $body %>
        <& footer &>

        <%init>
        my $arg = mc_dhandler_arg();               # get rest of path
        my ($section,$story) = split("/",$arg);    # split out pieces
        my $sth = $DBH->prepare
            ("SELECT headline,body FROM news WHERE section='$section' AND story='$story'");
        my ($headline,$body) = $sth->fetchrow_array;
        return 404 if !$headline;                  # return "not found" if no such story
        </%init>

## autohandlers

Autohandlers allow you to grab control and perform some action just before Mason calls the top-level component. This might mean adding a standard header and footer, applying an output filter, or setting up global variables.

Autohandlers are directory based. When Mason determines the top-level component, it checks that directory for a component called ``autohandler''; if it exists, it is called instead. After performing its actions, the autohandler typically calls mc_auto_next to transfer control to the original intended component.

mc_auto_next works just like mc_comp except that the component path and arguments are implicit. You can pass additional arguments to mc_auto_next; these are merged with the original arguments, taking precedence in case of conflict. This allows you, for example, to override arguments passed in the URL. The STORE option also works if you want to store and process the component output.

Here is an autohandler that adds a common header and footer to each page in the directory:

        <HTML>
        <HEAD><TITLE>McHuffy Incorporated</TITLE></HEAD>
        <BODY BGCOLOR="salmon">

        <% mc_auto_next %>

        <HR>
        Copyright 1999 McHuffy Inc.
        </BODY>
        </HTML>

Same idea, with separate components for header/footer in the same directory:

        <& header &>
        <% mc_auto_next %>
        <& footer &>

The next autohandler applies a filter to its pages, adding an absolute hostname to relative image URLs:

        <%init>
        my $buf;
        mc_auto_next(STORE=>\$buf);
        $buf =~ s{(<img\s+src=\")/} {$1http://images.mysite.com}ig;
        mc_out($buf);
        </%init>

The same, using a <%filter> tag:

        <% mc_auto_next %>

        <%filter>
        s{(<img\s+src=\")/} {$1http://images.mysite.com}ig;
        </%filter>

Most of the time autohandler can simply call mc_auto_next without needing to know what component was called. However, should you need it, the relative component path is available from mc_auto_comp. For example, here we base the page title partly on the component name:

        <HTML>
        <HEAD><TITLE>McHuffy Incorporated: <% $section %></TITLE></HEAD>
        <% mc_auto_next %>
        </BODY>
        </HTML>

        <%init>
        my $comp = mc_auto_comp();
        my $section = ($comp eq 'mktg.html' ? 'Marketing' : ...);
        </%init>

mc_auto_comp is also useful for calling the component manually, e.g. if you want to suppress one or more original arguments.

By default autohandlers apply only to the current directory, not to its subdirectories. The Mason administrator can turn on the flag allow_recursive_autohandlers that makes autohandlers apply to all subdirectories. Be warned that this mode entails a performance and readability cost. On every request Mason has to scan not only the current directory but all parent directories for a file named ``autohandler''. Likewise, someone trying to determine how a page is rendered has to scan through parent directories.

That said, the applications of recursive autohandlers (e.g. applying a site-wide template) can be compelling enough to overcome these disadvantages.

If you have recursive mode turned on but want a particular autohandler to apply only to the current directory, use a conditional like this:

        <%init>
        return mc_auto_continue if (mc_auto_comp =~ /\//);
        ... rest of autohandler ...
        </%init>

## dhandlers vs. autohandlers

dhandlers and autohandlers both provide a way to exert control over a large set of URLs. However, each specializes in a very different application. The key difference is that dhandlers are invoked only when no appropriate component exists, while autohandlers are invoked only in conjunction with a matching component.

As a rule of thumb: use an autohandler when you have a set of components to handle your pages and you want to augment them with a template/filter. Use a dhandler when you want to create a set of ``virtual URLs'' that don't correspond to any actual components, or to provide default behavior for a directory.

dhandlers and autohandlers can even be used in the same directory. For example, you might have a mix of real URLs and virtual URLs to which you would like to apply a common template/filter.

Prior to version 0.6, dhandlers were the only way to accomplish tasks like applying a standard template across multiple components. However, this application of dhandlers has several disadvantages. First, you must perversely ``hide'' your existing components so that Mason will not find them and instead invoke the dhandler! Second, you must manually check whether the subsequent component exists and handle ``not found'' errors. The upshot is that autohandlers are better suited for many tasks previously solved by dhandlers.

# PASSING PARAMETERS

This section describes Mason's facilities for passing parameters to components (either from HTTP requests or component calls) and for accessing parameter values inside components.

## In Component Calls

Any Perl data type can be passed in a component call:

        <& /sales/header, s=>'dog', l=>[2,3,4], h=>{a=>7,b=>8} &>

This command passes a scalar ($s), a list (@l), and a hash (%h). The list and hash must be passed as references, but they will be automatically dereferenced in the called component.

## In HTTP requests

Consider a CGI-style URL with a query string:

        http://www.foo.com/mktg/prods.html?str=dog&lst=2&lst=3&lst=4

or an HTTP request with some POST content. Mason automatically parses the GET/POST values and makes them available to the component as parameters.

## Accessing Parameters

Component parameters, whether they come from GET/POST or another component, can be accessed in two ways.

1. Declared named arguments: Components can define a `<%args%>` section listing argument names, types, and default values. For example:

        <%args>
        $a
        @b
        %c
        $d=>5
        @e=>('foo','baz')
        %f=>(joe=>1,bob=>2)
        </%args>

Here, $a, @b, and `%c` are required arguments; the component generates an error if the caller leaves them unspecified. $d, @e, and `%f` are optional arguments; they are assigned the specified default values if unspecified. All the arguments are available as lexically scoped (``my'') variables in the rest of the component.

2. `%ARGS` hash: This variable, always available, contains all of the parameters passed to the component. It is especially handy when there are many parameters or when parameter names are determined at run-time. `%ARGS` can be used whether or not you have a <%args> ... </%args> section.

Here's how to pass all of a component's parameters to another component:

        <& template, %ARGS &>

## Parameter Passing Examples

The following examples illustrate the different ways to pass and receive parameters.

1. Passing a scalar id with value 5.

      In a URL: /my/URL?id=5
      In a component call: <& /my/comp, id => 5 &>
      In the called component, if there is a declared argument named...
        $id, then $id will equal 5
        @id, then @id will equal (5)
        %id, then an error occurs
      In addition, $ARGS{id} will equal 5.

2. Passing a list colors with values red, blue, and green.

      In a URL: /my/URL?colors=red&colors=blue&colors=green
      In an component call: <& /my/comp, colors => ['red', 'blue', 'green'] &>
      In the called component, if there is a declared argument named...
        $colors, then $colors will equal ['red', 'blue', 'green']
        @colors, then @colors will equal ('red', 'blue', 'green')
        %colors, then an error occurs
      In addition, $ARGS{colors} will equal ['red', 'blue', 'green'].

3. Passing a hash grades with pairs Alice => 92 and Bob => 87.

      In a URL: /my/URL?grades=Alice&grades=92&grades=Bob&grades=87
      In an component call: <& /my/comp', grades => {Alice => 92, Bob => 87} &>
      In the called component, if there is a declared argument named...
        $grades, then $grades will equal {Alice => 92, Bob => 87}
        @grades, then @grades will equal ('Alice', 92, 'Bob', 87)
        %grades, then %grades will equal (Alice => 92, Bob => 87)
      In addition, $ARGS{grades} will equal {Alice => 92, Bob => 87}.

# OTHER PERL SECTIONS

In this section we describe other specialized sections you can place in your component. Several are tied to phases of the component execution sequence, which goes something like this:

        1. Initialize arguments declared in <%args> section
        2. <%init> section
        3. Output HTTP headers (if not output already)
        4. Primary section (HTML + embedded Perl sections)
        5. <%cleanup> section

Prior to version 0.6, section names were all prefixed with perl_: <%perl_init>, <%perl_cleanup>, etc. For backwards compatibility these names are still recognized, but the short names are preferred. If you have pre-0.6 components that you'd like to convert, a convenient utility exists in bin/convert0.6.pl.

## <%init>

Used for initialization code. For example: connecting to a database and selecting out rows; opening a file and reading its contents into a list.

Technically a <%init> block is equivalent to a <%perl> block at the beginning of the component. However, there is an aesthetic advantage of placing this block at the end of the component rather than the beginning. In the following example, a database query is used to preload the `@persons` list-of-hashes; it lets us hide the technical details at the bottom.

        <H2>Birthdays Next Week</H2>
        <TABLE BORDER=1>
        <TR><TH>Name</TH><TH>Birthday</TH></TR>
        % foreach (@persons) {
            <TR><TD><%$_->{name}%></TD><TD><%$_->{birthday}%></TD></TR>
        % }
        </TABLE>

        <%INIT>
        # Assuming DBI/DBD and Date::Manip are already loaded ...
        # Query MySQL for employees with birthdays next week.
        # Results are stored in the @persons list-of-hashes.

        my (@persons, $name, $birthday);    # local vars

        # Calculate "MM-DD" dates for this and next Sunday
        my $Sun = UnixDate(&ParseDate("Sunday"), "%m-%d");
        my $nextSun = UnixDate(&DateCalc("Sunday", "+7 days"), "%m-%d");

        my $dbh = DBI->connect('DBI:mysql:myDB', 'nobody' );
        my $sth = $dbh->prepare(
           qq{ SELECT name, DATE_FORMAT(birthday, 'm-d')
               FROM emp
               WHERE DATE_FORMAT(birthday,'m-d') BETWEEN '$Sun' AND '$nextSun'
             } );
        $sth->execute;              # other DBDs want this after the bind
        $sth->bind_columns(undef, \($name, $birthday) );

        while ($sth->fetch) {
            push (@persons, {name=>$name, birthday=>$birthday} );
        }
        </%INIT>

Since `<%init>` sections fire before any HTTP headers are sent, they should do their work quickly to prevent dead time on the browser side.

## <%cleanup>

Used for cleanup code. For example: closing a database connection or closing a file handle.

Technically a <%cleanup> block is equivalent to a <%perl> block at the end of the component, but has aesthetic value as marking a cleanup section.

Recall that the end of a component corresponds to the end of a subroutine block. Since Perl is so darned good at cleaning up stuff at the end of blocks, <%cleanup> sections are rarely needed.

## <%args>

xxx contains a list of argument declarations, one per line. Each declaration contains a type character ($, @, or %), a name, and optionally '=>' followed by a default value. The default value must be a valid Perl expression of matching type (scalar, list, hash). See Accessing Parameters above for usage and examples.

## <%once>

This code executes once when the component is loaded. Unlike the other sections, its scope is above the component subroutine instead of inside it. Useful for declaring persistent component-scoped lexical variables (especially objects that are expensive to create), declaring subroutines (both named and anonymous), and initializing state.

You cannot call components or use any mc_ command from this section.

Normally this code will execute individually from every HTTP child that uses the component. However, if the component is preloaded, this code will only execute once in the parent. Unless you have total control over what components will be preloaded, it is safest to avoid initializing variables that can't survive a `fork(),` e.g. DBI handles. For such variables you can use the `$initialized` variable trick:

        <%once>
        my $initialized=0;
        my $dbh;    # declare but don't assign
        ...
        </%once>

        <%init>
        if (!($initialized++)) {
            $dbh = DBI::connect ...
        }
        ...
        </%init>

## <%filter>

This section allows you to filter the output of the component through an arbitrary block of code. Upon entry to this code, `$_` contains the component output, and you are expected to modify it in place. See the FILTERING section for usage and examples.

## <%doc>

Text in this section is treated as a comment and ignored. Most useful for a component's main documentation. One can easily write a program to sift through a set of components and pull out their <%doc> blocks to form a reference page.

Can also be used for in-line comments, though it is an admittedly cumbersome comment marker. Another option is '%#':

        %# this is a comment

These comments differ from HTML comments in that they do not appear in the HTML.

## <%text>

Turns off processing of Mason syntax; text is passed through unmodified. Useful, for example, when documenting Mason itself from a component:

        <%text>
        % This is an example of a Perl line.
        <% This is an example of an expression block. %>
        </%text>

This works for almost everything, but doesn't let you output </%text> itself! When all else fails, use mc_out():

        %mc_out('The tags are <%text> and </%text>.');

## \ at end of line

A \ suppresses the newline before %-lines and section tags. In HTML components, this is mostly useful for fixed width areas like <PRE> tags, since browsers ignore white space for the most part. An example:

        <PRE>
        foo
        %if ($b == 2) {
        bar
        %}
        baz
        </PRE>

outputs

        foo
        bar
        baz

because of the newlines on lines 1 and 3. (Lines 2 and 4 do not generate a newline because the entire line is taken by Perl.) To suppress the newlines:

        <PRE>
        foo\
        %if ($b == 2) {
        bar\
        %}
        baz
        </PRE>

which prints

        foobarbaz

The backslash has no special meaning outside this context. In particular, you cannot use it to escape a newline before a plain text line.

# DATA CACHING

Mason's mc_cache() and mc_cache_self() commands let components save and retrieve the results of computation for improved performance. Anything may be cached, from a block of HTML to a complex data structure.

Each component gets a private data cache. Except under special circumstances, one component does not access another component's cache. Each cached value may be set to expire under certain conditions or at a certain time.

To use data caching, your Mason installation must be configured with a good DBM package like Berkeley DB (DB_File) or GDBM. See HTML::Mason::Admin for more information.

## Basic Usage

Here's the typical usage of mc_cache:

      my $result = mc_cache(action=>'retrieve');
      if (!defined($result)) {
          ... compute $result> ...
          mc_cache(action=>'store', value=>$result);
      }

The first mc_cache call attempts to retrieve this component's cache value. If the value is available it is placed in `$result`. If the value is not available, `$result` is computed and stored in the cache by the second mc_cache call.

The default action for mc_cache is 'retrieve', so the first line can be written as

      my $result = mc_cache();

## Multiple Keys/Values

A cache file can store multiple keys and values. A value can be a scalar, list reference, or hash reference:

      mc_cache(action=>'store',key=>'name',value=>$name);
      mc_cache(action=>'store',key=>'friends',value=>\@lst);
      mc_cache(action=>'store',key=>'map',value=>\%hsh);

The key defaults to 'main' when unspecified, as in the first example above.

Mason uses the MLDBM package to store and retrieve from its cache files, meaning that Mason can cache arbitrarily deep data structures composed of lists, hashes, and simple scalars.

## Expiration

Typical cache items have a useful lifetime after which they must expire. Mason supports three types of expiration:

By Time
  (e.g. the item expires in an hour, or at midnight). To expire an item by time, pass one of these options to the 'store' action.

  expire_at: takes an absolute expiration time, in Perl `time()` format (number of seconds since the epoch)

  expire_in: takes a relative expiration time of the form ``<num><unit>'', where <num> is a positive number and <unit> is one of seconds, minutes, hours, days, or weeks, or any abbreviation thereof. E.g. ``10min'', ``1hour''.

  expire_next: takes a string, either 'hour' or 'day'. It indicates an expiration time at the top of the next hour or day.

  Examples:

          mc_cache(action=>'store', expire_in=>'2 hours');
          mc_cache(action=>'store', expire_next=>'hour');

By Condition
  (e.g. the item expires if a certain file or database table changes). To expire an item based on events rather than current time, pass the 'expire_if' option to the 'retrieve' action.

  expire_if: calls a given anonymous subroutine and expires if the subroutine returns a non-zero value. The subroutine is called with one parameter, the time when the cache value was last written.

  Example:

          # expire the cache if 'myfile' is newer
          mc_cache(action => 'retrieve',
                expire_if => sub { [stat 'myfile']->[9] > $_[0] });

By Explicit Action
  (e.g. a shell command or web interface is responsible for explicitly expiring the item) To expire an item from a Perl script, for any component, use access_data_cache. It takes the same arguments as mc_cache plus one additional argument, cache_file. See the administration manual for details on where cache files are stored and how they are named.

          use HTML::Mason::Utils 'access_data_cache';
          access_data_cache (cache_file=>'/usr/local/mason/cache/foo::bar',
                             action=>'expire' [, key=>'fookey']);

  The 'expire' action can also take multiple keys (as a list reference); this can be used in conjunction with the 'keys' action to expire all keys matching a particular pattern.

          use HTML::Mason::Utils 'access_data_cache';
          my @keys = access_data_cache (cache_file=>'/usr/local/mason/cache/foo::bar',
                                        action=>'keys');
          access_data_cache (cache_file=>'/usr/local/mason/cache/foo::bar',
                             action=>'expire', key=>[grep(/^sales/,@keys)]);

## Busy Locks

The code shown in ``Basic Usage'' above,

      my $result = mc_cache(action=>'retrieve');
      if (!defined($result)) {
          ... compute $result> ...
          mc_cache(action=>'store', value=>$result);
      }

can suffer from a kind of race condition for caches that are accessed frequently and take a long time to recompute.

Suppose that a particular cache value is accessed five times a second and takes three seconds to recompute. When the cache expires, the first process comes in, sees that it is expired, and starts to recompute the value. The second process comes in and does the same thing. This sequence continues until the first process finishes and stores the new value. On average, the value will be recomputed and written to the cache 15 times!

The solution here is to have the first process notify the others that it has started recomputing. This can be accomplished with the busy_lock flag:

            mc_cache(action=>'retrieve',busy_lock=>'10sec',...);

With this flag, the first process sets a lock in the cache that effectively says ``I'm busy recomputing his value, don't bother.'' Subsequent processes see the lock and return the old value. The lock is good for 10 seconds (in this case) and is ignored after that. Thus the time value you pass to busy_lock indicates how long you're willing to allow this component to use an expired cache value.

Would some of your caches would benefit from busy locks? One way to find out is to turn on cache logging in the Mason system logs. If you see large clusters of writes to the same cache in a short time span, then you might want to use busy locks when writing to that cache.

## Keeping In Memory

The keep_in_memory flag indicates that the cache value should be kept in memory after it is stored or retrieved. Since every child process will store its own copy, this flag should be used only for small, frequently retrieved cache values. If used, this flag should be passed to both the store and retrieve commands.

## Caching All Output

Occasionally you will need to cache the complete output of a component. One way to accomplish this is to replace the component with a placeholder that simply calls the component, then caches and prints the result. For example, if the component were named ``foo'', we might rename it to ``foo_main'' and put this component in its place:

        <% $foo_out %>
        <%init>
            my $foo_out;
            if (!defined ($foo_out = mc_cache())) {
                mc_comp('foo_main', STORE=>\$foo_out);
                mc_cache(action=>'store',
                      expire_in=>'3 hours', value=>$foo_out);
            }
        </%init>

This works, but is cumbersome. Mason offers a better shortcut: the mc_cache_self() command that lets a component cache it's own output and eliminates the need for a dummy component. It is typically used right at the top of a `<%init%>` section:

        <%init>
            return if mc_cache_self(expire_in=>'3 hours'[, key=>'fookey']);
            ... <rest of init> ...
        </%init>

mc_cache_self is built on top of mc_cache, so it inherits all the expiration options described earlier.

## Guarantees (or lack thereof)

Mason will make a best effort to cache data until it expires, but will not guarantee it. The data cache is not a permanent reliable store in itself; you should not place in the cache critical data (e.g. user session information) that cannot be regenerated from another source such as a database. You should write your code as if the cache might disappear at any time. In particular,

o If the 'store' action cannot get a write lock on the cache, it simply fails quietly.

o Your Mason administrator will be required to remove cache files periodically when they get too large; this can happen any time.

On the other hand, expiration in its various forms is guaranteed, because Mason does not want you to rely on bad data to generate your content. If you use the 'expire' action and Mason cannot get a write lock, it will repeat the attempt several times and finally die with an error.

# FILTERING

This section describes several ways to apply filtering functions over the results of the current component. By separating out and hiding a filter that, say, changes HTML in a complex way, we allow non-programmers to work in a cleaner HTML environment.

## <%filter> section

The <%filter> section allows you to arbitrarily filter the output of the current component. Upon entry to this code, `$_` contains the component output, and you are expected to modify it in place. The code has access to component arguments and can invoke subroutines, call other components, etc.

This simple filter converts the component output to UPPERCASE:

        <%filter>
        tr/a-z/A-Z/
        </%filter>

The following navigation bar uses a filter to ``unlink'' and highlight the item corresponding to the current page:

        <a href="/">Home</a> | <a href="/products/">Products</a> |
        <a href="/bg.html">Background</a> | <a href="/finance/">Financials</a> |
        <a href="/support/">Tech Support</a> | <a href="/contact.html">Contact Us</a>

        <%filter>
        my $uri = $r->uri;
        s{<a href="$uri/?">(.*?)</a>} {<b>$1</b>}i;
        </%filter>

This allows a designer to code such a navigation bar intuitively without `if` statements surrounding each link! Note that the regular expression need not very robust as long as you have control over what will appear in the body.

## mc_call_self command

This command allows you to filter both the output and the return value of the current component. It is fairly advanced; for most purposes the <%filter> tag above will be sufficient and simpler.

mc_call_self takes two arguments. The first is a scalar reference and will be populated with the component output. The second is either a scalar or list reference and will be populated with the component return value; the type of reference determines whether the component will be called in scalar or list context. Both of these arguments are optional; you may pass undef if you don't care about one of them.

mc_call_self acts like a `fork()` in the sense that it will return twice with different values. When it returns 0, you allow control to pass through to the rest of your component. When it returns 1, that means the component has finished and you can begin filtering the output and/or return value. (Don't worry, it doesn't really do a fork! See next section for explanation.)

The following examples would generally appear at the top of a <%init> section. Here is a no-op mc_call_self that leaves the output and return value untouched:

        if (mc_call_self(my \$output, my \$retval)) {  # assumes Perl 5.005 or greater
            mc_out($output);
            return $retval;
        }

Here is a simple output filter that makes the output all uppercase, just like the <%filter> example above. Note that we ignore both the original and the final return value.

        if (mc_call_self(my \$output, undef)) {
            mc_out(uc($output));
            return;
        }

mc_call_self can even convert output to a return value or vice versa. In the next component we provide a nice friendly format for non-programmers to represent data with, and use a filter to construct and return a corresponding Perl data structure from it:

        # id        lastname        firstname
        59286       Sherman         Karen
        31776       Dawson          Robert
        29482       Lee             Brenda
        ...

        <%perl_init>
        if (mc_call_self(my \$output, undef)) {
            foreach (split("\n",$output)) {
                next if /^#/ || !/\S/;
                my @vals = split(/\s+/);
                push(@people,{id=>$vals[0],last=>$vals[1],first=>$vals[2]});
            }
            return @people;
        }
        </%perl_init>

Now we can get a list of hashes directly from this component.

## How filtering works

mc_call_self (and <%filter>, which is built on it) uses a bit of magic to accomplish everything in one line. If you're curious, here's how it works:

o A component foo calls mc_call_self for the first time.

o mc_call_self sets an internal flag and calls foo again recursively, with a STORE option to capture its content into a buffer.

o foo again calls mc_call_self which, seeing the flag, returns 0 immediately.

o foo goes about its business and generates content into the mc_call_self buffer.

o When control is returned to mc_call_self, it places the content and return value in the references provided, and returns 1.

# ACCESSING SERVER INTERNALS

Mason is built on top of mod_perl, an Apache extension that embeds a persistent Perl interpreter into the web server. Mason makes the powerful `$r` ``request object'' available as a global in all components, granting access to a variety of server internals, HTTP request data, and server API methods.

`$r` is fully described in the Apache documentation -- here is a sampling of methods useful to component developers:

        $r->uri             # the HTTP request URI
        $r->headers_in(..)  # the named HTTP header line
        $r->server->port    # (note two arrows!) port # (usu. 80)
        $r->content_type    # set or retrieve content-type

        $r->content         # don't use this one! (see Tips and Traps)

# SENDING HTTP HEADERS

Mason sends a standard HTTP header with content type text/html when it reaches the primary HTML section of a component (after any `<%init>` section).

That means if you want to send your own HTTP header, you have to do it in the `<%init%>` section. You send headers with Apache commands headers_out and send_http_header.

To prevent Mason from sending out the default header, call mc_suppress_http_header(1). You only need to do this if you are going to call a component before sending your header. Here's an example:

        <%init>
        ...
        mc_suppress_http_header(1);   # necessary because of next line
        my $registered = mc_comp('isUserRegistered');
        if (!$registered) {
             mc_comp('/shared/http/redirect',url=>'/registerScreen');
        }
        ...
        </%init>

The component isUserRegistered returns 0 or 1 indicating whether the user has registered (e.g. by looking for a cookie). If the result is 0, we use an HTTP redirect to go to the registration screen. Mason would normally send the default header upon reaching the primary section of isUserRegistered - that is why we must call mc_suppress_http_header.

To cancel header suppression, call `mc_suppress_http_header(0).`

# USING THE PERL DEBUGGER

The Perl debugger is an indispensable tool for identifying and fixing bugs in Perl programs. Unfortunately, in a mod_perl environment one is normally unable to use the debugger since programs are run from a browser. Mason removes this limitation by optionally creating a debug file for each page request, allowing the request to be replayed from the command line or Perl debugger.

Note: in early 1999 a new module, Apache::DB, was released that makes it substantially easier to use the Perl debugger directly in conjunction with a real Apache server. Since this mechanism is still new, we continue to support Mason debug files, and there may be reasons to prefer Mason's method (e.g. no need to start another Apache server). However we acknowledge that Apache::DB may eventually eliminate the need for debug files. For now we encourage you to try both methods and see which one works best.

## Using debug files

Here is a typical sequence for debugging a Mason page:

1. Find the debug file: When Mason is running in debug mode, requests generate ``debug files'', cycling through filenames ``1'' through ``20''. To find a request's debug file, simply do a ``View Source'' in your browser after the request and look for a comment like this at the very top:

           <!--
           Debug file is '3'.
           Full debug path is '/usr/local/mason/debug/anon/3'.
           -->

2. Run the debug file: Debug files basically contain two things: a copy of the entire HTTP request (serialized with Data::Dumper), and all the plumbing needed to route that request through Mason. In other words, if you simply run the debug file like this:

           perl /usr/local/mason/debug/anon/3

   you should see the HTTP headers and content that the component would normally send to the browser.

3. Debug the debug file: Now you merely add a `-d` option to run the debug file in Perl's debugger -- at which point you have to deal the problem of anonymous subroutines.

   Mason compiles components down to anonymous subroutines which are not easily breakpoint'able (Perl prefers line numbers or named subroutines). Therefore, immediately before each component call, Mason calls a nonce subroutine called `debug_hook` just so you can breakpoint it like this:

           b HTML::Mason::Interp::debug_hook

   Since debug_hook is called with the component name as the second parameter, you can also breakpoint specific components using a conditional on $_[1]:

           b HTML::Mason::Interp::debug_hook $_[1] =~ /component name/

   You can avoid all that typing by adding the following to your ~/.perldb file:

           # Perl debugger aliases for Mason
           $DB::alias{mb} = 's/^mb\b/b HTML::Mason::Interp::debug_hook/';

   which reduces the previous examples to just:

           mb
           mb $_[1] =~ /component name/

The use of debug files opens lots of other debugging options. For instance, you can read a debug file into the Emacs editor, with its nifty interface to Perl's debugger. This allows you to set break points visually or (in trace mode) watch a cursor bounce through your code in single-step or continue mode.

## Specifying when to create debug files

Details about configuring debug mode can be found in HTML::Mason::Admin. In particular, the administrator must decide which of three debugging modes to activate:

never (no debug files)

always (create debug files for each request)

error (only generate a debug file when an error occurs)

## How debug files work

To create a debug file, Mason calls almost every one of the mod_perl API methods (`$r->xxx`), trapping its result in a hash. That hash is then serialized by Data::Dumper and output into a new debug file along with some surrounding code.

When the debug file is executed, a new object is created of the class ``HTML::Mason::FakeApache'', passing the saved hash as initialization. The FakeApache object acts as a fake `$r`, responding to each method by getting or setting data in its hash. For most purposes it is indistinguishable from the original `$r` except that print methods go to standard output. The debug file then executes your `handler()` function with the simulated `$r`.

## When debug files don't work

The vast majority of mod_perl API methods are simple get/set functions (e.g. `$r->uri`, `$r->content_type`) which are easy to simulate. Many pages only make use of these methods and can be successfully simulated in debug mode.

However, a few methods perform tasks requiring the presence of a true Apache server. These cannot be properly simulated. Some, such as `log_error` and `send_cgi_header`, are generally tangential to the debugging effort; for these Mason simply returns without doing anything and hopes for the best. Others, such as `internal_redirect` and `lookup_uri`, perform such integral functions that they cannot be ignored, and for these FakeApache aborts with an error. This category includes any method call expected to return an Apache::Table object.

In addition, FakeApache is playing something of a catch-up game: every time a new mod_perl release comes out with new API methods, those methods will not be recognized by FakeApache until it is updated in the next Mason release.

The combination of these problems and the existence of the new Apache::DB package may eventually lead us to stop further work on FakeApache/debug files. For now, though, we'll continue to support them as best we can.

# USING THE PERL PROFILER

Debug files, mentioned in the previous section, can be used in conjunction with Devel::DProf to profile a web request.

To use profiling, pass the `-p` flag to the debug file:

        % ./3 -p

This executes the debug file under Devel::DProf and, for convenience, runs dprofpp. If you wish you can rerun dprofpp with your choice of options.

Because components are implemented as anonymous subroutines, any time spent in components would normally be reported under an unreadable label like `CODE(0xb6cbc).` To remedy this, the `-p` flag automatically adjusts the tmon.out file so that components are reported by their component paths.

Much of the time spent in a typical debug file is initialization, such as loading Mason and other Perl modules. The effects of initialization can swamp profile results and obscure the time actually spent in components. One remedy is to run multiple iterations of the request inside the debug file, thus reducing the influence of initialization time. Pass the number of desired iterations via the `-r` flag:

        % ./3 -p -r20

Currently there are no special provisions for other profiling modules such as Devel::SmallProf. You can try simply:

        % perl -d:SmallProf ./3 -r20

However, this crashes on our Unix system -- apparently some bad interaction between Mason and SmallProf -- so it is unsupported for now.

# THE PREVIEWER

Mason comes with a web-based debugging utility that lets you test your components by throwing fake requests at them. Adjustable parameters include: UserAgent, Time, HTTP Referer, O/S and so on. For example, imagine a component whose color scheme is supposed to change each morning, noon, and night. Using the Previewer, it would be simple to set the perceived time forward 1,5 or 8 hours to test the component at various times of day.

The Previewer also provides a debug trace of a page, showing all components being called and indicating the portion of HTML each component is responsible for. For pages constructed from more than a few components, these traces are quite useful for finding the component that is outputting a particular piece of HTML.

Your administrator will give you the main Previewer URL, and a set of preview ports that you will use to view your site under various conditions. For the purpose of this discussion we'll assume the Previewer is up and working, that the Previewer URL is http://www.yoursite.com/preview, and the preview ports are 3001 to 3005.

Take a look at the main Previewer page. The top part contains the most frequently used options, such as time and display mode. The middle part contains a table of your saved configurations; if this is your first time using the Previewer, it will be empty. The bottom part contains less frequently used options, such as setting the user agent and referer.

Try clicking ``Save''. This will save the displayed settings under the chosen preview port, say 3001, and redraw the page. Under ``Saved Port Settings'', you should see a single row showing this configuration. Your configurations are saved permanently in a file. If a username/password is required to access the Previewer, then each user has his/her own configuration file.

The ``View'' button should display your site's home page. If not, then the Previewer may not be set up correctly; contact your administrator or see the Administrator's Manual.

Go back to the main Previewer page, change the display mode from ``HTML'' to ``debug'', change the preview port to 3002, and click ``Save'' again. You should now see a second saved configuration.

Click ``View''. This time instead of seeing the home page as HTML, you'll get a debug trace with several sections. The first section shows a numbered hierarchy of components used to generate this page. The second section is the HTML source, with each line annotated on the left with the number of the component that generated it. Try clicking on the numbers in the first section; this brings you to the place in the second section where that component first appears. If there's a particular piece of HTML you want to change on a page, searching in the annotated source will let you quickly determine which component is responsible.

The final section of the debug page shows input and output HTTP headers. Note that some of these are simulated due to your Previewer settings. For example, if you specified a particular user agent in your Previewer configuration, then the User-Agent header is simulated; otherwise it reflects your actual browser.

# TIPS AND TRAPS

Do not call $r->content or ''new CGI``
  Mason calls `$r->content` itself to read request input, emptying the input buffer and leaving a trap for the unwary: subsequent calls to `$r->content` hang the server. This is a mod_perl ``feature'' that may be fixed in an upcoming release.

  For the same reason you should not create a CGI object like

        my $query = new CGI;

  when handling a POST; the CGI module will try to reread request input and hang. Instead, create an empty object:

        my $query = new CGI ("");

  such an object can still be used for all of CGI's useful HTML output functions. Or, if you really want to use CGI's input functions, initialize the object from %ARGS:

        my $query = new CGI (\%ARGS);

Separate Perl from HTML
  In our experience, the most readable components, especially for non-programmer designers and editors, contain full HTML in one continuous block at the top with simple substitutions for dynamic elements (<%$name%>, <%$salary%>) but no distracting blocks of Perl code. At the bottom an <%init> block sets up the substitution variables -- getting `$name` from the database, calculating $salary, etc. This organization allows non-programmers to work with the HTML without getting distracted or discouraged by Perl code.

  This technique does sacrifice some performance for readability.

# AUTHOR

Jonathan Swartz, swartz@transbay.net

# SEE ALSO

HTML::Mason, HTML::Mason::Commands