Plack Handbook - Convert cgi apps to psgi

Day 6: Convert CGI apps to PSGI

The most popular web server environments to run web applications for Perl have been CGI, FastCGI, and mod_perl. CGI.pm is one of the Perl core modules that happens to run fine in any of these environments (with some tweaks). This means many web applications and frameworks use CGI.pm to deal with environment differences because it's the easiest.

CGI::PSGI is a CGI module subclass that makes it easy to migrate existing CGI.pm based applications to PSGI. Imagine you have the following CGI application:

 use CGI;

 my $q = CGI->new;
 print $q->header('text/plain'),
     "Hello ", $q->param('name');

This is a very simple CGI application and converting this to PSGI is easy using the CGI::PSGI module:

 use CGI::PSGI;

 my $app = sub {
     my $env = shift;
     my $q = CGI::PSGI->new($env);
     return [
         $q->psgi_header('text/plain'),
         [ "Hello ", $q->param('name') ],
     ];
 };

CGI::PSGI->new($env) takes the PSGI environment hash and creates an instance of CGI::PSGI, which is a subclass of CGI.pm. All methods including param(), query_string, etc. do the right thing to get the values from the PSGI environment rather than CGI's ENV values.

psgi_header is a utility method that works just like CGI's header method and returns the status code and an array reference containing the list of HTTP headers.

Tomorrow I'll talk about how to convert existing web frameworks that use CGI.pm to PSGI using CGI::PSGI.