How to save an entity in GAE datastore only if it does not exist while preventing race conditions

Question

I am trying to save an entity to Google App Engine Datastore only if it does not exist, if it does the thread should not continue.

Main issue is: I want to prevent any r

Answer 1

When calling Datastore from App Engine, the api is different. Use a transaction. First try to get() the entity to see if it's there. If not, put() the entity.

Here's some sample code that demonstrates transactions using the App Engine API: https://github.com/GoogleCloudPlatform/java-docs-samples/blob/master/appengine/datastore/src/test/java/com/example/appengine/TransactionsTest.java

Answer 2

Assuming that the both threads are trying to save an entity with the same key, then this code:

datastore.add(entity);

will do what you want. If an entity already exists in datastore with the same key, then .add() will throw an exception.