appengine transaction lock

Question

I want make sure that one thing about the transaction lock of GAE. In the documentation it said:

All datastore operations in a transaction must operate on entities in the same entity group if the transaction is a single group transaction, ....

So while a txn is running, it will lock only every entries of that single entity group, won't lock every entry of models which are used in the transaction. Is it right?

Answer 1

GAE uses optimistic locking - that means that nothing is locked in transaction, instead it check last update time, and if it's after time you have started your transactions - it will throw exceptions (and rollback your data). It's described at 'What Can Be Done In a Transaction'