Plack::Builder(3) 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 interface, 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);

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