Day 19: Cascade multiple applications

Conditional middleware and URLMap app have something in common: they're PSGI applications but both takes PSGI application or middleware and dispatch them. This is the beauty of PSGI application and middleware architecture and today's application is another example of this.

Cascading multiple applications

Cascading can be useful if you have a couple of applications and runs in order, then try until it returns a successful response. This is sometimes called Chain of responsibility design pattern and often used in web applications such as mod_perl handlers.

Cascade Application

Plack::App::Cascade allows you to composite multiple applications in order and runs until it returns non-404 responses.

use Plack::App::Cascade;
use Plack::App::File;
use Plack::App::URLMap;

my @paths = qw(
    /home/www/static
    /virtualhost/example.com/htdocs/static
    /users/miyagawa/public_html/images
);

my $app = Plack::App::Cascade->new;
for my $path (@paths) {
    my $file = Plack::App::File->new(root => $path);
    $app->add($file);
}

my $map = Plack::App::URLMap->new;
$map->mount("/static" => $app);
$map->to_app;

This application is mapped to /static using URLMap, and all requests will try the three directories specified in @paths using App::File application and returns the first found files. It might be useful if you want to serve static files but want to cascade from multiple directories like this.

Cascade different apps

use CatalystApp;
CatalystApp->setup_engine('PSGI');
my $app1 = sub { CatalystApp->run(@_) };

use CGI::Application::PSGI;
use CGIApp;
my $app2 = sub {
    my $app = CGIApp->new({
        QUERY => CGI::PSGI->new($_[0]),
    });
    CGI::Application::PSGI->run($app);
};

use Plack::App::Cascade;
Plack::App::Cascade->new(apps => [ $app1, $app2 ])->to_app;

This will create two applications, one with Catalyst and the other with CGI::Application and runs two applications in order. Suppose you have an overlapping URL structure and /what/ever.cat served with the Catalyst application and /what/ever.cgiapp served with the CGI::Application app.

Well that might sound crazy and i guess it's better to use URLMap to map two applications in different paths, but if you really want to cascade them, this is the way to go :)

Comments

[Jakub Narebski]

There is no DSL for cascading, like there is for stacking middleware?

[miyagawa]

Jakub: Good question: i've been thinking about it for a while but couldn't find a nice DSL to implement that yet. But yes that's the plan.

[Bernhard Schmalhofer]

In the first example, shouldn't it say:

$app->add($file);

?

[miyagawa]

Bernhard: Fixed. Thanks!