GAE Problems: Slim3 To The Rescue?

For the past few months we have been working on porting a time-sheet application on Google App Engine. The application we were trying to port had Spring for Dependency Injection, Hibernate for persistence and Wicket as the web framework. Looking at the will it play in app engine Hibernate was an easy call, it is not compatible on Google App Engine, wicket works with some workaround and Spring is fully supported. We decided to go with JPA as it is easier to convert a hibernate application to JPA than to other ORM framework. We decided to continue with Spring and Wicket.

We encountered several problems during porting the application. Most of the effort went in increasing the performance. We used Key-Only-Query, memcache to increase performance. We also had issues with handling consistency of datastore operations across entity groups.

When we select framework for application development, we think in terms of the application performance and the ease of application development. When we are not porting an application but doing a new development we can choose frameworks more freely. For new application development on Google App Engine Slim3 framework seems like a good option.

Google App Engine has a notion of entity groups. It is a very important concept and decides how the Entities will be mapped in JPA for the application.

There is a limitation that an Entity cannot have more than one parent. This can be a major issue while re-defining Entity associations for Google App Engine for the application. The Entity Groups also decide the transaction strategy for an application. Simply put you can only apply transaction on a single Entity Group. Is there a platform which allow us to provide transactions across Entity groups. Slim3 does allow us to work across entity groups.

Slim3 example for transferring amount across two accounts: