With app-engine-patch a major part of Django works on App Engine without any modifications. The most important change is that you have to use Google's Model class because the development model is too different from Django (at least with Django's current API).
Well, it turns out there's a very powerful feature baked into Django which can handle the task for us.
Django includes an internal "dispatcher" which allows objects to listen for signals from other objects.
In our case, our tumblelog app is going to "listen" to our Entry and Link models. Every time a new Entry or Link is saved, those models will send out a signal. When the tumblelog app gets the signal it will automatically update itself.
Mamona is a Django application for handling online payments. It can work with any existing setup without a need of changing other applications' models.