How to save an entity in GAE datastore only if it does not exist while preventing race conditions

Question

I am trying to save an entity to Google App Engine Datastore only if it does not exist, if it does the thread should not continue.

Main issue is: I want to prevent any r

Answer 1

Assuming that the both threads are trying to save an entity with the same key, then this code:

datastore.add(entity);

will do what you want. If an entity already exists in datastore with the same key, then .add() will throw an exception.