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Plack::Component - Base class for PSGI endpoints


  package Plack::App::Foo;
  use parent qw( Plack::Component );

  sub call {
      my($self, $env) = @_;
      # Do something with $env

      my $res = ...; # create a response ...

      # return the response
      return $res;


Plack::Component is the base class shared between Plack::Middleware and Plack::App::* modules. If you are writing middleware, you should inherit from Plack::Middleware, but if you are writing a Plack::App::* you should inherit from this directly.


call ($env)

You are expected to implement a call method in your component. This is where all the work gets done. It receives the PSGI $env hash-ref as an argument and is expected to return a proper PSGI response value.


new (%opts | \%opts)

The constructor accepts either a hash or a hashref and uses that to create the instance. It will call no other methods and simply return the instance that is created.


This method is called by to_app and is meant as a hook to be used to prepare your component before it is packaged as a PSGI $app.


This is the method used in several parts of the Plack infrastructure to convert your component into a PSGI $app. You should not ever need to override this method; it is recommended to use prepare_app and call instead.


This is a wrapper for response_cb in Plack::Util. See "RESPONSE CALLBACK" in Plack::Middleware for details.


Objects for the derived classes (Plack::App::* or Plack::Middleware::*) are created at the PSGI application compile phase using new, prepare_app and to_app, and the created object persists during the web server lifecycle, unless it is running on the non-persistent environment like CGI. call is invoked against the same object whenever a new request comes in.

You can check if it is running in a persistent environment by checking psgi.run_once key in the $env being true (non-persistent) or false (persistent), but it is best for you to write your middleware safely for a persistent environment. To accomplish that, you should avoid saving per-request data like $env in your object.


The Plack::Middleware module used to inherit from Class::Accessor::Fast, which has been removed in favor of the Plack::Util::Accessor module. When developing new components it is recommended to use Plack::Util::Accessor like so:

  use Plack::Util::Accessor qw( foo bar baz );

However, in order to keep backwards compatibility this module provides a mk_accessors method similar to Class::Accessor::Fast. New code should not use this and use Plack::Util::Accessor instead.


Plack Plack::Builder Plack::Middleware