Last week, I upgraded Google’s App Engine to version 1.5.2.

I was careless and did not perform enough tests, and it turned out there is a bug with App Engine SDK 1.5.2 (I believe same with 1.5.0). There is a memcache error due to missing library.

If you encounter a "javax/cache/CacheException", then you are probably having the same problem. The fix is simply upgrading to the latest version at 1.5.4.

Over that 2 days where the bug infested the SG 4D app, I observed a few interesting behaviors. Here’s what I leant.

Lesson 1: Ads revenue increased 10X

As the app could not display the latest results, users has nothing better to do, so they click the ads.. That resulted in 10X more clicks than usual!

An evil thought: Introduce glitches to drive ads revenue (just kidding!)

Lesson 2: In-app-purchase increased 5X

The in-app-purchase to unlock the premium features increased too! Users thought that the app has “expired”, hence they can’t see the results, hence the purchase.

(this hypothesis is true, and was validated by a couple of emails from users)

Lesson 3: Other 4D apps rise quickly

The effect is immediate. On the first day that my app did not work, users downloaded alternatives. There are 2 other 4D apps, and they were both propelled to top 50!

Surprisingly, my downloads remain the same. That means my downloads are constantly contributed by new users, and is unaffected by the bug.

Lesson 4: Test, test, test!

Remember to test during software upgrade, even when it comes from Google!

This is a tutorial: creating a barebone Play! app and deploying on the free Google App Engine.

Steps to setting up Play! with App Engine

1. Download Play framework (latest is 1.2.1)

2. Unzip and add path

export PATH=$PATH:/YOURPATH/play-1.2.1

3. Install GAE module (latest is 1.4)

play install gae

4. Install siena module (latest is 1.5)

play install siena

5. Edit /YOURAPP/conf/dependencies.yml

require:
    – play
    – play -> gae 1.4
    – play -> siena 1.5

6. Add to conf/application.conf

# —- Google app engine module —-
module.gae=${play.path}/modules/gae-1.4
# —- Siena module —-
module.siena=${play.path}/modules/siena-1.5

7. Restart your app. GAE will start and a /war will be created

8. Edit /war/WEB-INF/appengine-web.xml and enter your appengine appid

9. Deploy

play war MyFirstApp -o MyFirstApp-War
/Applications/eclipse/plugins/appengine-java-sdk-1.5.0/bin/appcfg.sh update MyFirstApp-War/

Go to http://yourapp.appspot.com, and it should run!

Note: This tutorial is only a basic to getting a barebone Play! app to run on Google’s App Engine. Developing a Play! app on App Engine still require your effort. Here is an Easy Bookmark example.

This is a little secret. I have created a mobile website for 4D results at http://sg4d.just2us.com/

There is nothing special about it, other than that it was built with Google’s App Engine. Running on Google’s infrastructure and servers has its bonuses – Google ensures stability, performance and scalability.