We create the following script that we call echo_routing.psgi and we run it using plackup echo_routing.psgi. If we browse to http://localhost:5000/ we see an input box and a button. We type in "hello" and click on the button. The URL changes to http://localhost:5000/echo?field=hello and we get back: You said: hello.

If we click on the "Back" button of the browser, clear the input box and click on the button again the URL changes to http://localhost:5000/echo?field= and the browser displays: You did not say anything..

If we now change the URL to be http://localhost:5000/abc, the browser will display 404 Not Found.

Details of the usage

Please, note after clicking on the button the URL changes. Both the actual path that now includes the word "echo", and it also has some parameters after it. The parameters are just as they were in the earlier echo example. The new thing is the change in the path. This happens because in the form in the HTML below (in the get_html function) we added an action="/echo" attribute. By default a form will submit to the same URL where it came from, but the action attribute can control the URL.

The code

For each route we create a subroutine. server_root will serve the request to / and serve_echo will serve the request to /echo. If you look at the code, it is quite straight forward returning the 3-element array ref as required by the PSGI specification. It is explained in the getting started with PSGI article.

The interesting part is that we have created a hash called %ROUTING which is a dispatch-table. The keys are the possible routes, the values are references to functions handling the give route.

The main part of the code is the anonymous subroutine assigned to $app. We use the Plack::Request module to fetch the path_info, which is essentially the route. We check if it is an existing key in the %ROUTING hash and if it has a value: my $route = $ROUTING{$request->path_info};. The variable $route will either be undef if the route the user accessed was not defined (as is in the case of /abc), or it will be a reference to a function. If it is a reference to a function we de-reference the variable and call the function. Using the $route->($env) syntax we also pass the $env variable as the first parameter.

If the path_info did not match any of the keys in the routing table, we return the HTTP Error code 404 Not Found.

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

use Plack::Request;

my %ROUTING = (
    '/'      => \&serve_root,
    '/echo'  => \&serve_echo,
);


my $app = sub {
    my $env = shift;
 
    my $request = Plack::Request->new($env);
    my $route = $ROUTING{$request->path_info};
    if ($route) {
        return $route->($env);
    }
    return [
        '404',
        [ 'Content-Type' => 'text/html' ],
        [ '404 Not Found' ],
    ];
};

sub serve_root {
    my $html = get_html();
    return [
        '200',
        [ 'Content-Type' => 'text/html' ],
        [ $html ],
    ];
} 

sub serve_echo {
    my $env = shift;
 
    my $request = Plack::Request->new($env);
    my $html;
    if ($request->param('field')) {
        $html = 'You said: ' . $request->param('field');
    } else {
        $html = 'You did not say anything.';
    }
    return [
        '200',
        [ 'Content-Type' => 'text/html' ],
        [ $html ],
    ];
}

sub get_html {
    return q{
      <form action="/echo">

      <input name="field">
      <input type="submit" value="Echo">
      </form>
      <hr>
    }
}

Comments

Would be nice to list (and compare?) here also url dispatching modules from CPAN like Path::Router, Path::Dispatcher, HTTP::Router, Router::Simple, Router::Boom, Router::R3, ....