I'm going through the Sharded Counters example in Java:
http://code.google.com/appengine/articles/sharding_counters.html
I have a question about the implementation of the increment method. In
python it explicitly wraps the get() and increment in a transaction.
In the Java example it just retrieves it and sets it. I'm not sure I
fully understand the Datastore and transactions but it seems like the
critical update section should be wrapped in a datastore transaction.
Am I missing something?
Original code:
public void increment() {
PersistenceManager pm = PMF.get().getPersistenceManager();
Random generator = new Random();
int shardNum = generator.nextInt(NUM_SHARDS);
try {
Query shardQuery = pm.newQuery(SimpleCounterShard.class);
shardQuery.setFilter("shardNumber == numParam");
shardQuery.declareParameters("int numParam");
List<SimpleCounterShard> shards =
(List<SimpleCounterShard>) shardQuery.execute(shardNum);
SimpleCounterShard shard;
// If the shard with the passed shard number exists, increment
its count
// by 1. Otherwise, create a new shard object, set its count to
1, and
// persist it.
if (
Transactional code:
public void increment() {
PersistenceManager pm = PMF.get().getPersistenceManager();
Random generator = new Random();
int shardNum = generator.nextInt(NUM_SHARDS);
try {
Query shardQuery = pm.newQuery(SimpleCounterShard.class);
shardQuery.setFilter("shardNumber == numParam");
shardQuery.declareParameters("int numParam");
List<SimpleCounterShard> shards =
(List<SimpleCounterShard>) shardQuery.execute(shardNum);
SimpleCounterShard shard;
// If the shard with the passed shard number exists, increment
its count
// by 1. Otherwise, create a new shard object, set its count to
1, and
// persist it.
if (shards != null && !shards.isEmpty()) {
Transaction tx = pm.currentTransaction();
try {
tx.begin();
//I believe in a transaction objects need to be loaded
by ID (can't
use the outside queried entity)
Key shardKey =
KeyFactory.Builder(SimpleCounterShard.class.getSimpleName(),
shards.get(0).getID())
shard = pm.getObjectById(SimpleCounterShard.class,
shardKey);
shard.setCount(shard.getCount() + 1);
tx.commit();
} finally {
if (tx.isActive()) {
tx.rollback();
}
}
} else {
shard = new SimpleCounterShard();
shard.setShardNumber(shardNum);
shard.setCount(1);
}
pm.makePersistent(shard);
} finally {
pm.close();
}
}
}
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