Huh?
As you are only able to do work in a single transactional unit-of-work
on a single entity-group, rollback must be considered "usefull".
In the above mentioned snippet, an Account entity is looked-up,
updated and a child entity TransactionRecord is added, all in a single
entity-group, namely the Account's. So a rollback will undo all
operations.
If you had added TransactionRecord to, lets say a "TransactionRecords"
entity-group, you would have gotten an error if you were operating in
a transactional scope, or no-transaction support at all, hence a
rollback would not have been possible.
Cheers, Lars Borup Jensen
On 1 Jun., 06:21, Didier Durand <durand.did...@gmail.com> wrote:
> Hi,
>
> entity group is not useful at rollback but rather limits you to which
> updates you can do: all the entities you touch in a single transaction
> must be in the same entity group.
>
> So, designing your groups properly is a key design issue in GAE to
> avoid unnecessary complexity when you later need to update at once
> entities that you created before in various groups.
>
> regards
>
> didier
>
> On May 30, 7:57 am, Jacob <jacob.rho...@gmail.com> wrote:
>
>
>
>
>
>
>
> > I am writing some code that needs to do a rollback on a secondary
> > object/table should the transaction fail, I believe this can be done
> > via entity groups, however I am not sure if this is how it would be
> > implemented. I have written some sample code to check if what I would
> > be doing is correct?
>
> > Would the following code ensure the "Account" object is never updated
> > if the insert of the "TransactionRecord" object fails.
>
> > public void addTransaction(String account, Double value, String
> > description, Date date) throws EntityNotFoundException {
> > DatastoreService datastore =
> > DatastoreServiceFactory.getDatastoreService();
>
> > int retries = 3;
> > while (true) {
> > Transaction txn = datastore.beginTransaction();
> > try {
>
> > // Update the bank balance
> > Key key = KeyFactory.createKey("Account",
> > account);
> > Entity e = datastore.get(key);
> > Double balance = (Double)
> > e.getProperty("balance");
> > balance += value;
> > e.setProperty("balance", value);
> > datastore.put(e);
>
> > // Record transaction details
> > Entity d = new Entity("TransactionRecord",
> > key);
> > d.setProperty("account_key", key);
> > d.setProperty("date", date);
> > d.setProperty("value", value);
> > d.setProperty("description", description);
>
> > txn.commit();
> > break;
> > } catch (ConcurrentModificationException e) {
> > if (retries == 0)
> > throw e;
> > --retries;
> > } finally {
> > if (txn.isActive())
> > txn.rollback();
> > }
> > }
>
> > }
>
> > Thanks for any feedback!
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