Create skeleton PSGI application for the SCO project

I know it was short, but at this point I had enough from the testing. I like testing, but having lots of test withouth the real application (even if we are talking about a clone) looks pointless. Besides, I had doubts about the reusability of those tests, so instead of writing more tests, I switched gears, and started to write the web application.

I was wondering a bit which framework should I use for the project. The MetaCPAN Web is written using Catalyst. I know Dancer quite well. I have some experience with Mojolicious, but in the end I thought: Let's not use any of these. Let's start by implementing everything in plain PSGI. It is both a learning experience and at least at this point, the project does not need fancy things the frameworks might provide.

The first thing I wanted to create is a skeleton for the PSGI-based web application. Something simple like when we once started with PSGI, but with more robust layout.

So I created the >app.psgi file in the root of the project with the following content:

#!/usr/bin/perl
use strict;
use warnings;

use lib 'lib';
use MetaCPAN::SCO;

my $app = MetaCPAN::SCO->run;

Instead of implementing the application in the app.psgi file, I am going to implement it in the MetaCPAN::SCO module and call its run method.

The interesting part of the code is in the lib/MetaCPAN/SCO.pm file.

The run method creates, and implicitly returns a reference to a subroutine. (Assigning the anonymous subroution to the $app variable is not required here, but it will be useful later on.)

Inside the anonymous subroutine we create a Plack::Request object. Then using the path_info method we can retreive part of the requested URL that does not contain the name of the machine. Just the path from /. We can use this later to identify the various requests. (These are usually called "routes").

According to the specs of PSGI, this anonymous subroution needs to return an array reference with 3 values. The first one is the HTTP status code. 200 means success, 404 means "Not Found". The 2nd element is the header. In our case we return only the Content-type. When the path_info equals to '/' we return text/plain which means our response will be interpreted as plain text. In all other cases we return 404 - Not Found and the Content-Type is text/html. There is no particular reason for the differences. At this point the content (which is the string in the 3rd element of the array is plain text. So either Content-Type will work.

Later, when both will return HTML, we'll have to make sure that the Content-Type is text/html in both cases.

package MetaCPAN::SCO;
use strict;
use warnings;

use Plack::Request;

our $VERSION = '0.01';

=head1 NAME

SCO - search.cpan.org clone

=cut

sub run {
    my $app = sub {
        my $env = shift;

        my $request = Plack::Request->new($env);
        if ($request->path_info eq '/') {
            return [ '200', [ 'Content-Type' => 'text/plain' ], ['Hello'], ];
        }

        return [ '404', [ 'Content-Type' => 'text/html' ], ['404 Not Found'], ];
    };
}


1;

Once we have this, we can launch the web-application by running plackup in the root directory of the project. It will tell us we can browse to http://localhost:5000/. Try that, and try adding something to the end of the request to see the 404 error message.

To finish this change, we also need to add Plack::Request to the list of prerequisites. This is the change we make in the Makefile.PL:

    PREREQ_PM    => {
       'Plack::Request'   => '0',
    },

$ git add .
$ git commit -m "add app.psgi and some basic code to show a main page and to give 404 otherwise"

commit