=head1 NAME

Catalyst::PSGI - How Catalyst and PSGI work together


The L<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.). L<Plack> is an
implementation of the PSGI specification for running Perl applications.

Catalyst used to contain an entire set of C<< 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 L<PSGI> specification, using L<Plack>'s
adaptors to implement that functionality.

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

=head1 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 L<Catalyst::Upgrading> for specifics about your web server

=head1 Writing your own PSGI file.

=head2 What is a .psgi file?

A C<< .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 C<myapp.psgi> file in the root
of your application.

=head2 Why would I want to write my own .psgi file?

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

The simplest C<.psgi> file for an application called C<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 B<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:

=head2 What is in the .psgi file Catalyst generates by default?

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


=item L<Plack::Middleware::LighttpdScriptNameFix>

=item L<Plack::Middleware::IIS6ScriptNameFix>


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

=head1 SEE ALSO

L<Catalyst::Upgrading>, L<Plack>, L<PSGI::FAQ>, L<PSGI>.

=head1 AUTHORS

Catalyst Contributors, see Catalyst.pm


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