I'm trying to port the Sharding Counters example (code.google.com/appengine/articles/sharding_counters.html) to Java. The only problem is that the Java API does not have a call similar to Python's 'get_by_key_name'. This is the basic idea:
Transaction tx = pm.currentTransaction();
Key key = KeyFactory.createKey(CounterShard.class.getSimpleName(), counter + randomIndex);
CounterShard shard = pm.getObjectById(CounterShard.class, key);
if (shard == null) { // API does not work this way...
shard = new CounterShard(key, counter);
}
shard.increment();
pm.makePersistent(shard);
tx.commit();
Unfortunately, this throws a JDOObjectNotFoundException the first time I run it. I could run a query to determine if a counter for a given name exists, but that is not transactional. Another thread could do the same and in the end both would create an object with the same key.
From what I understand, the only operations supported within a transaction (for the Java API) are get and put. So how can I lock an object by key that does not exist yet (i.e. no 'get') and make sure that I am the first and only one creating it?