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Plack - Perl Superglue for Web frameworks and Web Servers (PSGI toolkit)


Plack is a set of tools for using the PSGI stack. It contains middleware components, a reference server and utilities for Web application frameworks. Plack is like Ruby's Rack or Python's Paste for WSGI.

See PSGI for the PSGI specification and PSGI::FAQ to know what PSGI and Plack are and why we need them.



Plack::Handler and its subclasses contains adapters for web servers. We have adapters for the built-in standalone web server HTTP::Server::PSGI, CGI, FCGI, Apache1, Apache2 and HTTP::Server::Simple included in the core Plack distribution.

There are also many HTTP server implementations on CPAN that have Plack handlers.

See Plack::Handler when writing your own adapters.


Plack::Loader is a loader to load one Plack::Handler adapter and run a PSGI application code reference with it.


Plack::Util contains a lot of utility functions for server implementors as well as middleware authors.

.psgi files

A PSGI application is a code reference but it's not easy to pass code reference via the command line or configuration files, so Plack uses a convention that you need a file named app.psgi or similar, which would be loaded (via perl's core function do) to return the PSGI application code reference.

  # Hello.psgi
  my $app = sub {
      my $env = shift;
      # ...
      return [ $status, $headers, $body ];

If you use a web framework, chances are that they provide a helper utility to automatically generate these .psgi files for you, such as:

  # MyApp.psgi
  use MyApp;
  my $app = sub { MyApp->run_psgi(@_) };

It's important that the return value of .psgi file is the code reference. See eg/dot-psgi directory for more examples of .psgi files.

plackup, Plack::Runner

plackup is a command line launcher to run PSGI applications from command line using Plack::Loader to load PSGI backends. It can be used to run standalone servers and FastCGI daemon processes. Other server backends like Apache2 needs a separate configuration but .psgi application file can still be the same.

If you want to write your own frontend that replaces, or adds functionalities to plackup, take a look at the Plack::Runner module.


PSGI middleware is a PSGI application that wraps an existing PSGI application and plays both side of application and servers. From the servers the wrapped code reference still looks like and behaves exactly the same as PSGI applications.

Plack::Middleware gives you an easy way to wrap PSGI applications with a clean API, and compatibility with Plack::Builder DSL.


Plack::Builder gives you a DSL that you can enable Middleware in .psgi files to wrap existent PSGI applications.

Plack::Request, Plack::Response

Plack::Request gives you a nice wrapper API around PSGI $env hash to get headers, cookies and query parameters much like Apache::Request in mod_perl.

Plack::Response does the same to construct the response array reference.


Plack::Test is a unified interface to test your PSGI application using standard HTTP::Request and HTTP::Response pair with simple callbacks.


Plack::Test::Suite is a test suite to test a new PSGI server backend.


Patches and Bug Fixes

Small patches and bug fixes can be either submitted via nopaste on IRC irc:// or the github issue tracker. Forking on github is another good way if you intend to make larger fixes.

See also when you think this document is terribly outdated.

Module Namespaces

Modules added to the Plack:: sub-namespaces should be reasonably generic components which are useful as building blocks and not just simply using Plack.

Middleware authors are free to use the Plack::Middleware:: namespace for their middleware components. Middleware must be written in the pipeline style such that they can chained together with other middleware components. The Plack::Middleware:: modules in the core distribution are good examples of such modules. It is recommended that you inherit from Plack::Middleware for these types of modules.

Not all middleware components are wrappers, but instead are more like endpoints in a middleware chain. These types of components should use the Plack::App:: namespace. Again, look in the core modules to see excellent examples of these (Plack::App::File, Plack::App::Directory, etc.). It is recommended that you inherit from Plack::Component for these types of modules.

DO NOT USE Plack:: namespace to build a new web application or a framework. It's like naming your application under CGI:: namespace if it's supposed to run on CGI and that is a really bad choice and would confuse people badly.


Tatsuhiko Miyagawa


The following copyright notice applies to all the files provided in this distribution, including binary files, unless explicitly noted otherwise.

Copyright 2009-2013 Tatsuhiko Miyagawa


Tatsuhiko Miyagawa (miyagawa)

Tokuhiro Matsuno (tokuhirom)

Jesse Luehrs (doy)

Tomas Doran (bobtfish)

Graham Knop (haarg)


Yuval Kogman (nothingmuch)

Kazuhiro Osawa (Yappo)

Kazuho Oku

Florian Ragwitz (rafl)

Chia-liang Kao (clkao)

Masahiro Honma (hiratara)

Daisuke Murase (typester)

John Beppu

Matt S Trout (mst)

Shawn M Moore (Sartak)

Stevan Little

Hans Dieter Pearcey (confound)


Mark Stosberg

Aaron Trevena


The PSGI specification upon which Plack is based.

The Plack wiki:

The Plack FAQ:


This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.