ansaurus

Question

What is "read operations inside transactions can't allow failover" ?

Answer 1

+1 A:

It looks like you're asking for failover reads, using the new support for this, inside a transaction. As the error message explains, this isn't possible, because in order to be transactional, it has to always read the most recent data. You need to disable failover reads for operations inside transactions.

Nick Johnson 2010-06-13 08:32:57

But I didn't intentionally enable such feature as failover reads. I just did a normal transaction as described in the GAE/J doc. So of cause I don't know how to disable it and I can't find any documentation about this either. I use JPA with GAE/J. And for this read which generated the exception message I just did a EntityManager.find(class, primaryKey). Could you specify more about this? Thanks!

Kenyth 2010-06-14 06:27:51

Does "<property name="datanucleus.appengine.datastoreReadConsistency" value="EVENTUAL" />" in the persistence.xml indicate a failover reads in transaction?

Kenyth 2010-06-14 06:31:26

Thanks Nick! With your hints It seems I've solved my problem. I removed that line of configuration above mentioned and deployed to GAE the modified version of my app. At least until now I don't see the exception message.

Kenyth 2010-06-14 07:46:45

See http://stackoverflow.com/questions/3031214/what-is-read-operations-inside-transactions-cant-allow-failover/3035744#3035744 for a clarification of the answer.

Kenyth 2010-06-14 08:02:11

Answer 2

A:

First thanks very much for @Nick's answer. Here I clarify the answer in a more direct way specific to my problem in case anyone else encountering the same problem asks for help here.

It seems with eventual consistency you can't make a query, at least a query by key, within a transaction, otherwise an exception message described in my question will be thrown. In the related official documentation of Google App Engine, it doesn't point out directly, but with some sentences like this:

All update operations (create, update, delete) occur in primary storage. A strongly consistent read of an entity may still be one operation or transaction behind, since a transaction is applied after it is committed.

Eventual consistency may need to read from storage location other than the primary one and may conflict with transactional consistency so it is not allowed an eventual-consistency read within a transaction as I see it.

Strong consistency is used by default without any configuration. For my own problem I just deleted the following line in the persistence.xml (JPA):

<property name="datanucleus.appengine.datastoreReadConsistency" value="EVENTUAL" />

Thanks again, Nick!

Kenyth 2010-06-14 08:01:05

Please feel free to file a bug: the SDK should really force strong consistency inside a transaction.

Nick Johnson 2010-06-14 14:17:29

the issue I filed: http://code.google.com/p/googleappengine/issues/detail?id=3349

Kenyth 2010-06-14 17:41:26

ansaurus

tags:

views:

answers:

What is "read operations inside transactions can't allow failover" ?

related questions