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From "Craig Russell (JIRA)" <j...@apache.org>
Subject [jira] Commented: (JDO-590) Control over transaction isolation level
Date Sun, 14 Sep 2008 18:49:46 GMT

    [ https://issues.apache.org/jira/browse/JDO-590?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12630901#action_12630901
] 

Craig Russell commented on JDO-590:
-----------------------------------

> A slight flaw with my previous suggestion of throwing an exception at PMF creation when
the isolation level is unsupported. When the PMF is created the configuration is not frozen
until the first PM is requested. Consequently the connection to the datastore is not established
until that point, and so it doesn't know what transaction isolation levels are supported until
then.

This is only an issue when configuring the PMF by API. If it's configured by properties, then
the configuration is frozen and the exception can be thrown then.

If configuring the PMF by API, the exception can be thrown upon the first getPM request (when
the configuration is frozen), instead of when the setTransactionIsolationLevel method is called.
Not ideal but maybe better than not allowing configuration of the PMF with a default isolation
level.




> 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.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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