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Date: Mon, 4 Aug 2008 06:24:44 -0700 (PDT)
From: "Andy Jefferson (JIRA)" <jira@apache.org>
To: jdo-dev@db.apache.org
Subject: [jira] Commented: (JDO-590) Control over transaction isolation
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    [ https://issues.apache.org/jira/browse/JDO-590?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12619505#action_12619505 ] 

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

My requirements for this item are to provide a *mechanism* for specification of transaction isolation. That is point 1 in the issue above. In the absence of interest in doing the second part I suggest that this is what we do here. Either way, if we do or not the second part we need a way of specifying transaction isolation.

javax.jdo.Transaction
void setIsolationLevel(String level);
String getIsolationLevel();

javax.jdo.PersistenceManagerFactory
void setTransactionIsolation(String);
String getTransactionIsolation();

Add a PMF property
javax.jdo.option.TransactionIsolation that allows definition of the default value to use, again using a String).

So we allow a text string form of isolation level. This then encompasses some standardised values and also allows implementations to define their own particular extensions to cater for the problems Christian Romberg mentioned. We could easily provide the following statics on javax.jdo.Transaction
"TX_NONE", TX_READ_UNCOMMITTED", "TX_READ_COMMITTED", "TX_NO_LOST_UPDATES", "TX_REPEATABLE_READ", "TX_SERIALIZABLE"

Specification of this parameter would mean that the datastore would use that isolation *if it is supported by the datastore*. Some datastores don't support any transaction capabilities. Some could be mimicked by the JDO layer. The only part here that would be desirable to have is an additional method 
String[] getSupportedTransactionIsolations();
to return what the datastore supports. This should go on javax.jdo.Transaction


If there are people who want the second part of this issue then they really ought to put forward their time to flush out what issues they see with it; as I said I have no desire for that part.

> 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
>             Fix For: JDO 2 maintenance release 2
>
>
> 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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