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Catalyst::PSGI - How Catalyst and PSGI work together


The PSGI specification defines an interface between web servers and Perl-based web applications and frameworks. It supports the writing of portable applications that can be run using various methods (as a standalone server, or using mod_perl, FastCGI, etc.). Plack is an implementation of the PSGI specification for running Perl applications.

Catalyst used to contain an entire set of Catalyst::Engine::XXXX classes to handle various web servers and environments (e.g. CGI, FastCGI, mod_perl) etc.

This has been changed in Catalyst 5.9 so that all of that work is done by Catalyst implementing the PSGI specification, using Plack's adaptors to implement that functionality.

This means that we can share common code, and share fixes for specific web servers.

I already have an application

If you already have a Catalyst application, then you should be able to upgrade to the latest release with little or no trouble (see the notes in Catalyst::Upgrading for specifics about your web server deployment).

Writing your own PSGI file.

What is a .psgi file?

A .psgi file lets you control how your application code reference is built. Catalyst will automatically handle this for you, but it's possible to do it manually by creating a myapp.psgi file in the root of your application.

Why would I want to write my own .psgi file?

Writing your own .psgi file allows you to use the alternate plackup command to start your application, and allows you to add classes and extensions that implement Plack::Middleware, such as Plack::Middleware::ErrorDocument or Plack::Middleware::AccessLog.

The simplest .psgi file for an application called TestApp would be:

    use strict;
    use warnings;
    use TestApp;

    my $app = TestApp->psgi_app(@_);

Note that Catalyst will apply a number of middleware components for you automatically, and these will not be applied if you manually create a psgi file yourself. Details of these components can be found below.

Additional information about psgi files can be found at:

What is in the .psgi file Catalyst generates by default?

Catalyst generates an application which, if the using_frontend_proxy setting is on, is wrapped in Plack::Middleware::ReverseProxy, and contains some engine-specific fixes for uniform behaviour, as contained in:


If you override the default by providing your own .psgi file, then none of these things will be done automatically for you by the PSGI application returned when you call MyApp->psgi_app. Thus, if you need any of this functionality, you'll need to implement this in your .psgi file yourself.

An apply_default_middlewares method is supplied to wrap your application in the default middlewares if you want this behaviour and you are providing your own .psgi file.

This means that the auto-generated (no .psgi file) code looks something like this:

    use strict;
    use warnings;
    use TestApp;

    my $app = TestApp->apply_default_middlewares(TestApp->psgi_app(@_));


Catalyst::Upgrading, Plack, PSGI::FAQ, PSGI.


Catalyst Contributors, see


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