Question

I have a similar code below that I’m trying to figure out transaction locking:

DAOT.repeatInTransaction(new Transactable() {
        @Override
        public void run(DAOT daot)
        {
                Points points = daot.ofy().find(Points.class, POINTS_ID);
                // do something with points
                takes_a_very_long_time_delay(); // perhaps 10 secs
                daot.ofy().put(points);
        }
});

The code above is executed from within a Java servlet. The operation is expected to work for 10 seconds for example. In between that time, I have a test that will invoke another servlet that will delete a Points entity, I was expecting that the delete operation would fail or at least delete the entity after the transaction above has finished.

However the entity was deleted during the period that the above code is executing. In my real application, I added exception handling to throw exception when trying to access or edit a entity that does not exist.

From there, the application is throwing “Entity not found" exception just after I executed the servlet that will delete the Entity in the code above.

Although I am using GAE Transactions already, however I think I am still missing something that’s why my test fails.

Code for the delete Transaction from withing the Delete servlet:

DAOT.repeatInTransaction(new Transactable() {
        @Override
        public void run(DAOT daot)
        {
                Points points = daot.ofy().find(Points.class, POINTS_ID);
                daot.ofy().delete(points);
        }
});

How can I ensure that a new operation like a delete for a entity will wait until the current operation is happening on a entity during a transaction?

Answer 1

App Engine uses optimistic concurrency, not locking. That is, a transaction on a group of entities will not prevent other processes from modifying those entities while the transaction runs. Instead, when the transaction attempts to commit, it will check if any modifications were made while the transaction was executing, and if it has, discard any changes and run your function again from the beginning.