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Are there any ways (better than using UrlFetch) to connect to any external database from AppEngine?
I already know about hosted SQL databases in App Engine for Business, but they are not free and will be avialable in Q2 2011, so it's not an option right now.
I like datastore very much, but customers don't like vendor lock or have other reasons.
The only way to connect external databases to AppEngine is by exposing them via a web API.
Basically you need to create a webservices layer upon your Database, allowing your GAE Application to interact with the remote db through UrlFetch Http calls.