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Prevent your Heroku Play! apps from idling

Disclaimer: we understand that the following may be a teeny weeny bit disrespectful to the free service Heroku provides, but asking $35/month is a bit steep for a website with less than 24 requests per day. If it gets popular some day, we will ofcourse reconsider our strategy.

While looking for a cloudservice to host our libwat.ch Play! (Java) app, we stumbled upon Heroku.com. It’s a really nice service which supports all kinds of (free and paid) configurations.

Cheap Dutch basterds as we are, we wanted the free service -so that means 1 webworker which falls asleep after an hour of inactivity. The app isn’t that popular yet (c’mon, sign up!), so it dozes off very frequently.

This means the request (person) that awakes the app must wait some 20 seconds while the app steps out of bed, brushes its teeth and combs its hair… not good!

Fortunately, only the webworker is put to sleep, so any background jobs are never idled, which leaves us with a nice opportunity to insomnatise the webworker 🙂

Just add this job to your Play app* :

public class KeepSiteAwakeJob extends Job {

  public void doJob() throws IOException {
    new URL("http://libwat.ch").getContent();


*) Note that if you’re not using Play!, you could still use one of the free scheduler plugins Heroku offers and have that invoke the website every sub-60 minutes.

Keeping your PhoneGap / Cordova app in sync with your RESTful backend

These days I’m working on a PhoneGap Apache Callback Apache Cordova app. I’m having tons of fun, and thought I’d share an issue we encountered and the lightweight but effective solution we came up with.

Cordova: speeding up your app

Cordova apps are distributed via the Application store of your mobile device. This means the static content (HTML, JS, CSS) is stored on the mobile device. This means a huge performance gain over typical desktop-targeted websites which render all HTML serverside. Even if you don’t need the awesome Javascript-to-hardware-feature bridge Cordova provides you with, you can still use it to distribute your frontend to your clients. By the way: if you don’t need the hardware features (camera, contacts, gps, etc) then don’t include cordova.js, for it saves your clientbrowsers loading a 125KB library.

You need dynamic content, right?

What’s a (web)app with only static screens? Likely not a very popular one. So we need to get (preferably JSON formatted) data from some backend. Your Javascript controller interprets this modal and updates the view accordingly.

The client doesn’t bother which backend architecture it connects to, but just to let you know, the project I mentioned uses an implementation of a JAX-WS REST API which indeed returns nicely JSON formatted strings to the client.

Problem: keeping your PhoneGap / Cordova app in sync with your RESTful backend

Now consider the following scenario: you have 10.000 users with version 1.0 to 1.3 of your awesome application. You completely rewrite the client and want to force all users to update to version 2.0. They should not use 1.x anymore, because you had to change most of your backend as well.

How would we do that, as we need the user to update the app via the AppStore/Google Play/etc.

The solution: clientside version.txt, serverside check ‘minimal required version’

What we did (and it’s no rocket science and most certainly not something lots of others have not already done in some similar way) is bundling a version.txt file with the app which contains a single line, reading ‘1.0’ or ‘4’. The version.txt can be assigned to a JS variable with a simple synchronous $.ajax call.

On the serverside we have a REST service ‘http://<server>/services/versioncheck’ which we pass the client’s version.txt when the app is started. The service returns false in case the clientversion is lower than the ‘minimal required version’ we’ve defined in the backend.

So, now that we’ve updated our app to version 2.0, the service returns false for any not yet upgraded clients and the Javascript callback will urge the user to update the app.

Asynchronous jobs with Play Framework

Ever needed to run some standalone asynchronous job from Java and found it to be a bit tedious? Read on..

Declaring a Job in Play that runs every 3 seconds  and increments a property is as simple as this:

public class PublishJob extends Job {

  private int i;

  public void doJob() throws Exception {

Q: W00t! It can’t be that simple!
A: Sorry, it really is.