In Google App Engine the transaction isolation is said to be SNAPSHOT isolation, where you do not see previous deletes or puts within the transaction itself, but only the state of the datastore when the transaction began (https://developers.google.com/appengine/docs/python/datastore/transactions). In an older article it says that the transaction level is actually SERIALIZABLE (https://developers.google.com/appengine/articles/transaction_isolation).
The Google Test Compatibility Kit (TCK) shows that it is indeed SNAPSHOT isolation, but in the aforementioned article it says "Inside transactions, on the other hand, the isolation level is SNAPSHOT by default, with the option of changing to SERIALIZABLE".
My question is, how do I enable the isolation level to become SERIALIZABLE?