Dynamic AJAX charting with Dancer and Highcharts

Let's build a real simple AJAX-based application using Dancer to feed data to Highcharts to display a real-time graph of the load average of the webserver the application is running on.

We'll serve up a page with the appropriate Javascript to utilise Highcharts, polling via AJAX requests to fetch fresh data from our Dancer app regularly.

First, we'll need to write a simple Dancer app to serve up the HTML, along with handling AJAX requests for the current load average.

I scaffolded the app with dancer -a ajaxchartingdemo, then edited lib/ajaxchartingdemo.pm to contain the following simple code:

package ajaxchartingdemo;
use Dancer ':syntax';
use Dancer::Plugin::Ajax;

use Unix::Uptime;

get '/' => sub {
    template 'index';
};

ajax '/getloadavg' => sub {
    {
        timestamp => time,
        loadavg => ( Unix::Uptime->load )[0]
    };
};

true;

I want that hashref the ajax handler is returning to be serialised to JSON for me, so I edited the config.yml file and added:

serializer: 'JSON'

With that done, I then had to download HighCharts into public/javascripts/highcharts, and edit views/index.tt to contain the appropriate HTML and mostly Javascript to display the chart.

I'm not going to repeat the whole of that here, as that part isn't Dancer-related, and you can see it on GitHub anyway, but it was simple stuff; the part which fetched the loadavg using an AJAX request (courtesy of jQuery) was as simple as:

setInterval(function() {
    $.getJSON('/getloadavg', function(response) {
        var point = [ 
            response.timestamp * 1000,
            response.loadavg - 0
        ];;
        series.addPoint(point, true, shiftalong);
    })
}, 2000);

With all this done, we're ready to see what we get!

During the advent calendar period, you can try it out live at:

http://advent.perldancer.org:4600/

(This will likely be discontinued in the near future, but the demo app will still be available on GitHub for you to try yourself.)

You should be presented with a pretty graph, which updates automatically:

Getting the example app code

The example application is available on GitHub:

https://github.com/bigpresh/dancer-ajaxloadavgchart-example

AUTHOR

David Precious, with thanks to ua on IRC for the idea and odyniec for help whilst I was hurriedly writing this post.

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