google app engine - General Concurrency GAE Datastore Transactions -


this might common question, still not clear on solution. using gae (datastore) , interacting objectify. have question concurrency.

let's have object, lets call blarkar. user 'a' starts transaction attempt load blarkar database, modify it, , save modified version database.

i understand in transaction either happens or nothing happens, confused whether blarkar object marked checked out, , whether can or cannot overwritten while transaction, started user 'a', in progress.

for instance, if user, we'll call 'b', tries saved new version of blarkar database, after user 'a' has begun transaction before has ended it, changes overwritten when user 'a's transaction ends?

again, using gae datastore, , interacting with objectify. however, objectify seems implement datastore's transaction api.

many thanks.

read this: https://code.google.com/p/objectify-appengine/wiki/concepts#transactions

you can google optimistic concurrency.


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