One of the biggest criticisms of Google’s App Engine have been cries of lock-in, that the applications developed for the platform won’t be portable to any other service. This morning, Chris Anderson, the Portland-based cofounder of the Grabb.it MP3 blog service, just released AppDrop — an elegant hack proving that’s not true.

AppDrop is a container for applications developed with the Google App Engine SDK, running entirely on Amazon’s EC2 infrastructure. Just like Google’s Appspot, anyone can use a modified SDK to deploy their App Engine apps directly to Amazon EC2 instead of Google, and they work without modification.

This proof-of-concept was built in only four days and can be deployed in virtually any Linux/Unix hosting environment, showing that moving applications off Google’s servers isn’t as hard as everyone thought.

How does it work? Behind the scenes, AppDrop is simply a remote installation of the App Engine SDK, with the user authentication and identification modified to use a local silo instead of Google Accounts. As a result, any application that works with the App Engine SDK should work flawlessly on AppDrop. For example, here’s Anderson’s Fug This application running on Google App Engine and the identical code running on EC2 at AppDrop.

Of course, this simple portability comes at the cost of scalability. The App Engine SDK doesn’t use BigTable for its datastore, instead relying on a simple flat file on a single server. This means issues with performance and no scalabity to speak of, but for apps with limited resource needs, something as simple as AppDrop would work fine.

I spoke to Chris this morning about his project and where he wants it to go. “AppDrop is open-source just like the Google SDK, so I’m hoping someone will come along and take it to the next level,” he said. “It wouldn’t be hard for a competent hacker to add real database support. It wouldn’t be that hard to write a Python adapter to MySQL that would preserve the BigTable API. And while that wouldn’t be quite as scalable as BigTable, we’ve all seen that MySQL can take you pretty far. On top of that, you could add multiple application machines connecting to the central database, and load-balancing, and all that rigamarole.”

While this is only a hack, it demonstrates that App Engine developers don’t need to live in fear of Google’s reprisal. “The upshot is that if you put a lot of time into an App Engine app, and then run afoul of Google, you have alternatives, even if they are more work.”

Update: Chris announced the project on his own blog, with some design notes.

April 20, 2008: Addressing concerns with data lock-in, Google announced that large-scale data import and export for will be coming soon to App Engine.