mstplbg

In our Hackerspace RaumZeitLabor in Mannheim, Germany, we have a "Network Power Manager 2000" which is a device that has 8 power sockets and is able to turn these sockets on/off when told so via network. We use it to save some power when nobody is at our space.

That sounds quite nice, but we had to reverse-engineer the protocol. Afterwards, we noticed that only one (!) connection to that device is possible. So our first approach of using a little script to communicate with it was not optimal: It had timing problems because the NPM2000 took a little bit longer to close the TCP connection.

Therefore, I decided to write a little webapp using the excellent Dancer framework which talks to the NPM2000 and provides a nice API to the user(s). It also should keep one TCP connection open all the time to save the overhead of creating a new connection and waiting for old connections to close properly on both sides.

For creating and using said TCP connection I decided to use AnyEvent::Socket. The main issue with this approach is that Dancer doesn’t use the AnyEvent mainloop and thus AnyEvent won’t work at all.

The nice people in #dancer suggested taking a look at Twiggy, an AnyEvent HTTP server for PSGI. It can be easily combined with Dancer and uses AnyEvent. So, to run a Dancer application with Twiggy, use:

cpanm Dancer Twiggy
dancer -a npmd
cd npmd
plackup -s Twiggy bin/app.pl

Now that’s great, we have an AnyEvent-based server running our Dancer application. So, let’s use an AnyEvent timer in lib/npmd.pm:

package npmd;
use Dancer ':syntax';
use AnyEvent;

our $VERSION = '0.1';

my $w; 

get '/' => sub {

        $w = AnyEvent->timer(after => 1, interval => 1, cb => sub {

                debug 'timer msg!';

        });

        template 'index';

};

true;

After the first request to the application in your browser you should see the message 'timer msg' on your terminal every second.

Let’s instead create the timer outside a route, so that it runs all the time:

package npmd;
use Dancer ':syntax';
use AnyEvent;

our $VERSION = '0.1';

my $w = AnyEvent->timer(after => 1, interval => 1, cb => sub {

        debug 'timer msg!';

});

get '/' => sub {

    template 'index';

};

true;

Oops. That won’t work. The code is run, but $w falls out of scope immediately because we are not using it in any handler. This causes AnyEvent to delete the timer. There are two ways to fix this: use 'our $w' instead of 'my $w' or use $w in any of your routes.

So, a working version could look like this:

package npmd;
use Dancer ':syntax';
use AnyEvent;

our $VERSION = '0.1';

our $w;

    $w = AnyEvent->timer(after => 1, interval => 1, cb => sub {

        debug 'timer msg';

    });

get '/' => sub {

    template 'index';

};

true;