Developers can build their own app and it is not compulsory to use App Engine.

Whispurr is built on Google's App Engine and they will consider inviting developers in June 2010.

The Django web framework and applications running on it can be used on App Engine with modification.

Developers have read-only access to the filesystem on App Engine.

Existing web applications that require a relational database will not run on App Engine without modification.

Developers worry that the applications will not be portable from App Engine and fear being locked into the technology.

Developers will also be able to use App Engine to deploy their own custom instances of the Jaiku server code.

At the opposite end of the spectrum is Google's App Engine.

Conversely, applications on App Engine must be purpose-built for the platform.

Fortunately, it's easy to get started on App Engine if you don't have your own server.