Hacking Thy Fearful Symmetry

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Going Postal (with Dancer)

September 5, 2011
dancer perl business::canadapost canada post dancer::plugin::cache::chi

The Dancer plugin mechanism primary aims to provide a way to encapsulate pieces of functionality that can be re-used by different applications. But, it's so light-weight and handy, that's it's easy to also use it to encapsulate parts of the application itself. In that way, it's very reminescent of the concept of role.

For example, and that's not the best example as the functionality is probably going to end up on CPAN before long, I'm working on a cart for the Académie des Chasseurs de Primes site. One of the things the cart system has to compute is the postal cost. Since I'm based in Canada, writing that part of the code is easy:

sub postal_fee {
    return "much too much";

... Well, okay. While the return value is correct, it could be a little more precise. Fortunately, Canada Post provides a web service to calculate posting rates. Even better, there is already Business::CanadaPost on CPAN to interface with it. Yay!

Hmm... Wait a second. The module hasn't been updated since 2005, and the web service changed address in the meantime, b0rking it. Booo...

Hold your horses... Ah! The fix is fairly trivial, and once patched, the module seems to work like a beaut. Woohoo!

So, anyway, knowing that I have the tools to get my postage fees, I can abstract the logic in the application itself to something like

sub postal_fee {
    my %args = @_;

    my $key = join '-', @args{qw/
        country city width length height weight

    return cache->compute( $key => sub {
            map { $_ => $args{$_} } qw/ country city /

            map { $_ => $args{$_} } 
                qw/ height width length weight /

        return postage->cheapest_shipping_rate;
    } );

Everything that has to do with the postage is dealt with in the postage object. Because Canada Post's web service is a little slow (and because it doesn't make sense to hit it more than I need too), I'm also caching retrieved postage fees using cache->compute (provided by Dancer::Plugin::Cache::CHI).

Now that the high-level logic has been set in the application, we can implement the details within the plugin. First thing I do is to define the Dancer-specific parts of it:

package Dancer::Plugin::Postage::CanadaPost;

use 5.10.0;

use strict;
use warnings;

use Dancer ':syntax';
use Dancer::Plugin;

use Business::CanadaPost;

use List::Util qw/ min /;

my $singleton;

### Dancer stuff

register postage => sub {
    return $singleton ||= __PACKAGE__->new;

hook 'after' => sub {
    $singleton = undef;


Not a lot there. I define the keyword postage, which will return the singleton object for the class (which is auto-created upon its first call). And, as I don't want previous destinations or items to linger and mess up future calculations, I also add a hook that systematically deletes the singleton after every request.

As a side-note, I could have created more helper keywords instead of using 'postage->destination()', 'postage->add_item' and the like. And I might, in the future, but for now I slightly prefer the *$object*->*$action* type of syntax. I'm OO that way.

After this, what is left to do is to implement the remaining functionality as a good old straight-forward OO wrapper for the underlying Business::CanadaPost work-horse:

### regular object stuff

sub new {
    my $class = shift;
    my $self = {};

    my $conf = plugin_setting();

    $self->{cp} = Business::CanadaPost->new(
        merchantid     => $conf->{merchant_id} || 'CPC_DEMO_XML',
        frompostalcode => $conf->{origin}{postal_code},
        testing        => $conf->{testing} // 1,

    return bless $self, $class;

sub destination {
    my $self = shift;
    my %arg = @_;

    $self->{cp}->setcountry( $arg{country} )         if $arg{country};
    $self->{cp}->settopostalzip( $arg{postal_code} ) if $arg{postal_code};
    $self->{cp}->settocity( $arg{city} )             if $arg{city};

    return $self;

sub add_item {
    my $self = shift;


    return $self;

sub shipping_options {
    my $self = shift;

    my $cp = $self->{cp};

    unless ( $self->{retrieved} ) {
        $cp->getrequest or die $cp->geterror;
        $self->{retrieved} = 1;

    return map { 
            name => $cp->getshipname($_),
            rate => $cp->getshiprate($_),
    } 1..$cp->getoptioncount;

sub cheapest_shipping_rate {
    my $self = shift;

    return min map { $_->{rate} } $self->shipping_options;


And I'm done. My application has its postage object. It's only only created if the request needs it, which is good for performance. Even better, the fees are cached so that the web service has less chances to be a bottle-neck. That's all I need for the time being, and I can move to my next task for that cart: PayPal integration.

... But that, my friend, is the topic of another blog entry.

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About the author

Yanick Champoux
Perl necrohacker , ACP writer, orchid lover. Slightly bonker all around. he/him/his