Building Blocks

I just added a Cron job to “ping” my application every 1 minute. After a short discussion with Paul Cusch, whom commented on one of my previous posts regarding the load request issues Grails on AppEngine is seeing. I decided to give it a try. This solution is entirely based on Paul’s idea I’m just writing it up.

Grails AppEngine plugin is missing a feature to use cron out of the box. I hope to submit a patch for it. This is how it works. You need to have a cron.xml file next to your appengine-web.xml file. In the cron file you describe which url and how often a GET operation should be done on the specified url.

Details can be found here.

I simply added a request to a specified url, which I added to my URL mapping:

"/today"(controller: "user", action = "ping")

Then added a cron file to ping it every minute:

<?xml version="1.0" encoding="UTF-8"?>
<cronentries>
    <cron>
        <url>/today</url>
        <description>Keep the app alive by ping every 1 minutes</description>
        <schedule>every 1 minutes</schedule>
    </cron>
</cronentries>

The cron.xml file I keep in my grails-app/conf directory, following the conventions used by the AppEngine plugin, for similar files.

The user controller unfortunately got the honor for hosting the ping request.

def ping = {
  render "ping ${new Date().toString()}"
}

And there it is. Next time that application is uploaded to the AppEngine, the ping method on the user controller will be executed every 1 minute. Helping to keep the application loaded.

This method comes with a few drawbacks however. The application will still hit loading requests every now and then, and those requests will take from your quota. This setup can easily eat up all your free quota.

Also, if everyone sets up this cron job. Then a lot of quota is wasted in general, which might affect the service for other applications. This workaround tries to fix a problem that should be fixed in the inner workings of AppEngine it self (not excluding changes to Grails initialization).

There is however a query that is ongoing. Google is considering the possibility to allow people to pay for keeping their application in memory. If you would consider such an option then star the this issue. I myself starred it, I think it’s quite acceptable.

I turned the cron loading task off in my application, instead seeing it as an experiment with Cron on Google AppEngine. The payoff for this solution wasn’t that great and for me it comes with the expense of some of my code quality.