Below are 2 ways I like to do tasks on App Engine.
A. Start as a deferred task with out a servlet.
B. Start a task using a servlet.
A. Start a DeferredTask
I like this method the best because it's much simpler to start a task. Although you have to keep in mind that the payload or the class that implements DeferredTask will want to serialize whats included the task. For those who use injection will need to use something like static injection.
1. create a class that implements DeferredTask
2. create a method that wills tart your task.
1. Create a class that implements the interface DeferredTask