Use optimistic concurrency control by default. This setting is available
for Cloud Firestore customers.
OptimisticWithEntityGroups
Use optimistic concurrency control with entity groups by default. This is
the only available setting for Cloud Datastore customers.
This is the default setting for Cloud Datastore.
Pessimistic
Use pessimistic concurrency control by default. This setting is available
for Cloud Firestore customers.
This is the default setting for Cloud Firestore.