Plack - PSGI reference implementation and utilities


Plack is a set of PSGI reference server implementations and helper utilities for Web application frameworks, exactly like Ruby's Rack.

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



Plack::Server is a namespace for PSGI server implementations. We have Standalone, CGI, FCGI, Apache, AnyEvent, Coro, Danga::Socket and many server environments that you can run PSGI applications on.

See Plack::Server how to write your own server implementation.


Plack::Loader is a loader to load one of Plack::Server backends and run PSGI application code reference with it.


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

.psgi files

PSGI application is a code reference but it's not easy to pass code reference in the command line or configuration files, so Plack uses a convention that you need a file named app.psgi or alike, which would be loaded (via perl's core function do) to return the PSGI application code reference. See eg/dot-psgi directory for the example .psgi files.


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.


PSGI middleware is a PSGI application that wraps existent 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 an 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 email. You could also fork on github ( to make larger fixes.

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.


Tatsuhiko Miyagawa

Yuval Kogman

Tokuhiro Matsuno

Kazuhiro Osawa

Kazuho Oku




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