Dancer As A Persistent Interpreter Node

Dancer is first and foremost a web framework. But lightness and flexibility allows it to be much, much more than that. In a way, Dancer is a lot like rum during the Holidays: it can be savored for its own goodness, or it can be used to spike the eggnog, or saturate the fruitcake with yumminess, or as a tool to get the children to sleep, or to bribe the Child Support Representant who comes to inquire about those unethical sleep-inducing methods.

For example, let's say that we have scripts that are heavy on the compile-time department. To make them more palatable, we would like to have a daemon that would stay alive between script runs so that the compile time penalty is only hit once. Well, a Dancer app is persistent, and it has this nifty HTTP protocol to talk with clients. So let's use it in a little daemon called santa.pl:

package Santa;

use Path::Class;

use Dancer 2 ':syntax';

post '/**' => sub {
    my $path = dir('/')->subdir( map @$_ => splat )->stringify;

    return send_error( "path '$path' is not an executable file" )
        unless -x $path;

    local *STDOUT;
    open STDOUT, '>', \my $output;

    eval { do $path; 1 }
        or return send_error( $@, 500 );

    return $output;
};

dance;

To interact with that daemon, let's build ourselves an elf.pl:

#!/usr/bin/env perl 

use 5.10.0;

use Path::Class;

my( $script, @args ) = @ARGV;

my $santa_url = $ENV{NORTH_POLE} or run_local(@ARGV);

require LWP::UserAgent;
my $agent = LWP::UserAgent->new;

my $response = $agent->post($santa_url . file($script)->absolute, { @args });

unless ( $response->is_success ) {
    warn "Santa failed us, we are on our own\n";
    run_local(@ARGV);
}

say $response->content;

sub run_local {
    warn "running locally\n";
    exec @_;
}

And we are done. If we fire up santa.pl on our machine, we can now harness the power of Christmas to run our scripts:

$ time ./elf.pl /home/yanick/work/perl-modules/Galuga/harvest_entries.pl
running locally
<html><body>well, hello there</body></html>
real    0m1.383s
user    0m1.312s
sys     0m0.072s

$ time NORTH_POLE=http://localhost:3000 ./elf.pl /home/yanick/work/perl-modules/Galuga/harvest_entries.pl
<html><body>well, hello there</body></html>

real    0m1.299s
user    0m0.108s
sys     0m0.016s

$ time NORTH_POLE=http://localhost:3000 ./elf.pl /home/yanick/work/perl-modules/Galuga/harvest_entries.pl
<html><body>well, hello there</body></html>

real    0m0.159s
user    0m0.112s
sys     0m0.012s

This extremely scientific sample of three script runs shows that the team of the elves and Santa are doing exactly what we want. Without a server available, the elf runs the script directly (which in this example takes 1.3s). If the server is there, the first run has a comparable time (as the code is interpreted for the first time), but the subsequent runs see their running time dropping from 1.3 seconds to 0.159 seconds. Not too shabby an improvement, for a full server/client system taking less than 40 lines of code...

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This article has been written by Yanick Champoux for the Perl Dancer Advent Calendar 2012.

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Dancer As A Persistent Interpreter Node

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