Google Datastore Transactions Optimistic Concurrency Control or not?

Question

The questions is fairly simple.

Does Google Datastore Transactions Optimistic Concurrency Control or not?

One part of the documentations says that it does:

When a transaction starts, App Engine uses optimistic concurrency control by checking the last update time for the entity groups used in the transaction. Upon commiting a transaction for the entity groups, App Engine again checks the last update time for the entity groups used in the transaction. If it has changed since our initial check, an exception is thrown. Source

Another part of the documentation indicates that it doesn't:

When a transaction is started, the datastore rejects any other attempts to write to that entity group before the transaction is complete. To illustrate this, say you have an entity group consisting of two entities, one at the root of the hierarchy and the other directly below it. If these entities belonged to separate entity groups, they could be updated in parallel. But because they are part of the same entity group, any request attempting to update one of the entities will necessarily prevent a simultaneous request from updating any other entity in the same group until the original request is finished. Source

As I understand it, the first quote tells me that it is fine to start a transaction, read an entity and ignore closing the transaction, if I saw no reason for updating the entity.

The second quote tells me that, if I start a transaction and read an entity, then I should always remember to close it again, otherwise I cannot start a new on the same entity.

Which part of the documentation is correct?

BTW. In case the correct quote is the second one, I am using Objectify to handle all my transactions. Will this remember to close all started transactions, even though no changes was made?

Answer 1

The commenter (Greg) is correct. Whether or not you explicitly close a transaction, all transactions are closed by the container at the end of a request. You can't "leak" transactions (although you could screw up transactions within a single request).

Furthermore, with Objectify's transaction API, transactions are automatically opened and closed for you when you execute a unit of Work. You don't manage transactions yourself.

To answer your root question: Yes, all transactions in the GAE datastore are optimistic. There is no pessimistic locking in the datastore; you can start as many transactions as you want on a single entity group but only the first commit will succeed. All subsequent attempts to commit will rollback with ConcurrentModificationException.