CPAN Testers is a pretty big project with a long, storied history. At its heart is a data warehouse holding all the test reports made by people installing CPAN modules. Around that exists an ecosystem of tools and visualizations that use this data to provide useful insight into the status of CPAN distributions.

For the CPAN Testers webapp project, I needed a way to show off some pre-release tools with some context about what they are and how they might be made ready for release. I needed a "beta" website with a front page that introduced the beta projects. But, I also needed the same Mojolicious application to serve (in the future) as a production website. The front page of the production website would be completely different from the front page of the beta testing website.

To achieve this, I used Mojolicious's template variants feature.

First, I created a variant of my index.html template for my beta site and called it index.html+beta.ep.

<h1>CPAN Testers Beta</h1>
<p>This site shows off some new features currently being tested.</p>
<h2><a href="/chart.html">Release Dashboard</a></h2>

Next, I told Mojolicious to use the "beta" variant when in "beta" mode by passing $app->mode to the variant stash variable.

# myapp.pl
use Mojolicious::Lite;
get '/*path', { path => 'index' }, sub {
    my ( $c ) = @_;
    return $c->render(
        template => $c->stash( 'path' ),
        variant => $c->app->mode,
    );
};
app->start;

The mode is set by passing the -m beta option to Mojolicious's daemon or prefork command.

$ perl myapp.pl daemon -m beta

This gives me the new landing page for beta.cpantesters.org.

$ perl myapp.pl get / -m beta
<h1>CPAN Testers Beta</h1>
<p>This site shows off some new features currently being tested.</p>
<h2><a href="/chart.html">Release Dashboard</a></h2>

But now I also need to replace the original landing page (index.html.ep) so it can still be seen on the beta website. I do this with a simple trick: I created a new template called web.html+beta.ep that imports the original template and unsets the variant stash variable. Now I can see the main index page on the beta site at http://beta.cpantesters.org/web.

%= include 'index', variant => undef

$ perl myapp.pl get /web -m beta
<h1>CPAN Testers</h1>
<p>This is the main CPAN Testers application.</p>

Template variants are a useful feature in some edge cases, and this isn't the first time I've found a good use for them. I've also used them to provide a different layout template in "development" mode to display a banner saying "You're on the development site". Useful for folks who are undergoing user acceptance testing. The best part is that if the desired variant for that specific template is not found, Mojolicious falls back to the main template. I built a mock JSON API application which made extensive use of this fallback feature, but that's another blog post for another time.