/ 
Plack-1.0016
River stage four • 773 direct dependents • 1922 total dependents
/ Plack::Builder

NAME

Plack::Builder - OO and DSL to enable Plack Middlewares

SYNOPSIS

# in .psgi
use Plack::Builder;

my $app = sub { ... };

builder {
    enable "Deflater";
    enable "Session", store => "File";
    enable "Debug", panels => [ qw(DBITrace Memory Timer) ];
    enable "+My::Plack::Middleware";
    $app;
};

# use URLMap

builder {
    mount "/foo" => builder {
        enable "Foo";
        $app;
    };

    mount "/bar" => $app2;
    mount "http://example.com/" => builder { $app3 };
};

# using OO interface
my $builder = Plack::Builder->new;
$builder->add_middleware('Foo', opt => 1);
$builder->add_middleware('Bar');
$builder->wrap($app);

DESCRIPTION

Plack::Builder gives you a quick domain specific language (DSL) to wrap your application with Plack::Middleware subclasses. The middleware you're trying to use should use Plack::Middleware as a base class to use this DSL, inspired by Rack::Builder.

Whenever you call enable on any middleware, the middleware app is pushed to the stack inside the builder, and then reversed when it actually creates a wrapped application handler. "Plack::Middleware::" is added as a prefix by default. So:

builder {
    enable "Foo";
    enable "Bar", opt => "val";
    $app;
};

is syntactically equal to:

$app = Plack::Middleware::Bar->wrap($app, opt => "val");
$app = Plack::Middleware::Foo->wrap($app);

In other words, you're supposed to enable middleware from outer to inner.

INLINE MIDDLEWARE

Plack::Builder allows you to code middleware inline using a nested code reference.

If the first argument to enable is a code reference, it will be passed an $app and should return another code reference which is a PSGI application that consumes $env at runtime. So:

builder {
    enable sub {
        my $app = shift;
        sub {
            my $env = shift;
            # do preprocessing
            my $res = $app->($env);
            # do postprocessing
            return $res;
        };
    };
    $app;
};

is equal to:

my $mw = sub {
    my $app = shift;
    sub { my $env = shift; $app->($env) };
};

$app = $mw->($app);

URLMap support

Plack::Builder has a native support for Plack::App::URLMap via the mount method.

use Plack::Builder;
my $app = builder {
    mount "/foo" => $app1;
    mount "/bar" => builder {
        enable "Foo";
        $app2;
    };
};

See Plack::App::URLMap's map method to see what they mean. With builder you can't use map as a DSL, for the obvious reason :)

NOTE: Once you use mount in your builder code, you have to use mount for all the paths, including the root path (/). You can't have the default app in the last line of builder like:

my $app = sub {
    my $env = shift;
    ...
};

builder {
    mount "/foo" => sub { ... };
    $app; # THIS DOESN'T WORK
};

You'll get warnings saying that your mount configuration will be ignored. Instead you should use mount "/" => ... in the last line to set the default fallback app.

builder {
    mount "/foo" => sub { ... };
    mount "/" => $app;
}

Note that the builder DSL returns a whole new PSGI application, which means

  • builder { ... } should normally the last statement of a .psgi file, because the return value of builder is the application that is actually executed.

  • You can nest your builder blocks, mixed with mount statements (see "URLMap support" above):

    builder {
    mount "/foo" => builder {
        mount "/bar" => $app;
    }
}

    will locate the $app under /foo/bar, since the inner builder block puts it under /bar and it results in a new PSGI application which is located under /foo because of the outer builder block.

CONDITIONAL MIDDLEWARE SUPPORT

You can use enable_if to conditionally enable middleware based on the runtime environment.

builder {
    enable_if { $_[0]->{REMOTE_ADDR} eq '127.0.0.1' } 'StackTrace', force => 1;
    $app;
};

See Plack::Middleware::Conditional for details.

OBJECT ORIENTED INTERFACE

Object oriented interface supports the same functionality with the DSL version in a clearer interace, probably with more typing required.

# With mount
my $builder = Plack::Builder->new;
$builder->add_middleware('Foo', opt => 1);
$builder->mount('/foo' => $foo_app);
$builder->mount('/' => $root_app);
$builder->to_app;

# Nested builders. Equivalent to:
# builder {
#     mount '/foo' => builder {
#         enable 'Foo';
#         $app;
#     };
#     mount '/' => $app2;
# };
my $builder_out = Plack::Builder->new;
my $builder_in  = Plack::Builder->new;
$builder_in->add_middleware('Foo');
$builder_out->mount("/foo" => $builder_in->wrap($app));
$builder_out->mount("/" => $app2);
$builder_out->to_app;

# conditional. You can also directly use Plack::Middleware::Conditional
my $builder = Plack::Builder->new;
$builder->add_middleware_if(sub { $_[0]->{REMOTE_ADDR} eq '127.0.0.1' }, 'StackTrace');
$builder->wrap($app);

SEE ALSO

Plack::Middleware Plack::App::URLMap Plack::Middleware::Conditional