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Date: Tue, 23 Sep 2008 11:55:44 -0700 (PDT)
From: "Andy Jefferson (JIRA)" <jira@apache.org>
To: jdo-dev@db.apache.org
Subject: [jira] Commented: (JDO-590) Control over transaction isolation
 level
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    [ https://issues.apache.org/jira/browse/JDO-590?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12633845#action_12633845 ] 

Andy Jefferson commented on JDO-590:
------------------------------------

Applied changes for API to api2-legacy. Without this anyone using DataNucleus would be unable to do anything when using JDK1.3/1.4. No plans to apply the TCK changes across to tck2-legacy.

> Control over transaction isolation level
> ----------------------------------------
>
>                 Key: JDO-590
>                 URL: https://issues.apache.org/jira/browse/JDO-590
>             Project: JDO
>          Issue Type: New Feature
>          Components: api2, api2-legacy, specification, tck2, tck2-legacy
>            Reporter: Andy Jefferson
>            Assignee: Craig Russell
>             Fix For: JDO 2 maintenance release 2
>
>         Attachments: jdo-590-pmf_by_api.patch, jdo-590.patch
>
>
> There are 2 sides to this :-
> 1). Standardising a mechanism for specifying the transaction isolation level. 
> This is the primary thing I am referring to, and to do that we need to provide a notional 
> set of isolation levels - not necessarily just the JDBC set, but that was the 
> start point as a basis for comment. As mentioned in other docs (see http://www.cs.umb.edu/~poneil/iso.pdf ) 
> the JDBC set is not complete for our scope, and other totally valid levels should be part of it. In some parts 
> of the JDO interface (e.g value generation) we define some values, and then 
> allow implementations to add on their own additional values if not catered 
> for in the defined list. This is what I would envisage. Suggested levels
> NONE, READ_UNCOMMITTED, READ_COMMITTED, NO_LOST_UPDATES, REPEATABLE_READ, SERIALIZABLE
> 2). Standardising support for these levels in the JDO implementation, so that 
> the user is always guaranteed to be able to use what they specify. I'm not 
> proposing this at all, and see that as unrealistic for an impl to provide 
> anyway. I simply propose that if an underlying datastore doesn't support the 
> level specified then we throw an exception, hence the user always knows if 
> their isolation level is going to be used. This is very much in line with 
> other parts of the JDO spec where the implementation is free to support some 
> or all of the valid values.
> Obviously, where the underlying datastore supports multiple levels then it 
> provides value for the user. Similarly where the underlying datastore 
> supports only a single level then it is something that user would have no 
> need to change.
> jdo-dev mailing list : Christian Romberg wrote
> we have to distinguish optimistic and datastore transactions in this discussion, and also what we want to achieve. Personally I think, we want to provide some behaviour guarantees of the API. Unfortunately, this is not the approach used by SQL for defining isolation levels.
> So for datastore transactions it simply does not work, because one backend might be a versioning database while another is a non-versioning database, and the behaviour will be totally different, although both guarantee the same isolation level.
> On the other hand with JDO optimistic transactions, the behaviour is quite consistent right now (unless flushing is involved), but only a two levels make sense: READ_UNCOMMITTED NO_LOST_UPDATES
> all other levels are either unachievable or implicitly overachieved.
> However, if we want to provide REPEATABLE_READ, then we could do so in that we implicitly include all read (but not modified) objects in the set of objects checked for modifications at commit time.
> Currently a user can do that, by calling "makeTransactional" on read objects.

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