Man, i've been there. Check out sharding - you can only update a
single entity a couple of times a second or else you'll get a
ConcurrentModificationException - you have to shard your entities.
Ben
nimbits.com
On Jan 3, 11:20 pm, Harshad <harshad...@gmail.com> wrote:
> Hi,
>
> I am reading the official GAE documentation on transactions and I
> can't understand when a ConcurrentModificationException is thrown.
>
> Look at one of the examples which I am copy-pasting here:
>
> int retries = 3;
> while (true) {
> Transaction txn = datastore.beginTransaction();
> try {
> Key boardKey = KeyFactory.createKey("MessageBoard",
> boardName);
> Entity messageBoard = datastore.get(boardKey);
>
> long count = (Long) messageBoard.getProperty("count");
> ++count;
> messageBoard.setProperty("count", count);
> datastore.put(messageBoard);
>
> txn.commit();
> break;
> } catch (ConcurrentModificationException e) {
> if (retries == 0) {
> throw e;
> }
> // Allow retry to occur
> --retries;
> } finally {
> if (txn.isActive()) {
> txn.rollback();
> }
> }
>
> }
>
> Now, all the writes to the datastore (in this example) are wrapped
> under a transaction. So why would a ConcurrentModificationException be
> thrown?
>
> Does it happen when some other code which is not wrapped in a
> transaction updates the same entity that is being modified by the
> above code? If I ensure that all code that updates an Entity is always
> wrapped in a transaction, is it guaranteed that I won't get a
> ConcurrentModificationException?
>
> thanks,
> Harshad
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