Hi Andy, 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. Best regards, Christian --- Versant GmbH European Headquarters Christian Romberg Wiesenkamp 22 b D 22359 Hamburg --- Versant GmbH is incorporated in Germany. Company registration number: HRB 54723, Amtsgericht Hamburg. Registered Office: Wiesenkamp 22b, 22359 Hamburg, Germany. Geschäftsführer: Jochen Witte. From: Andy Jefferson <[EMAIL PROTECTED]> To: [email protected] Cc: JDO Expert Group <[EMAIL PROTECTED]> Date: 17.03.2008 10:21 Subject: Re: JDO 2.2 : transaction isolation Hi Christian, Thx for your input. > these levels are not really appropriate for JDO, which is because of the > nature these are defined in the SQL spec: > I. with a database guarantee of only committed reads and using JDO > optimistic transactions, you effectively have an isolation level stronger > than TRANSACTION_READ_COMMITTED and lower than > TRANSACTION_REPEATABLE_READ because the phenomenon of "unrepeatable reads" > can occur, while the phenomenon of "lost updates" can not occur. > In our (old) implementation we thus introduced an additional isolation > level "NO_LOST_UPDATES". Ok. > II. with a database guarantee of serializibility and using JDO optimistic > transactions, you don't achieve > serializable JDO transactions unless the physical connection is bound to > the PM for the whole JDO transaction duration. Agreed. There are 2 sides to this :- 1). Standardising a mechanism for specifying the transaction isolation level. This is what 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 you say, that 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. Is this realistic? 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. Regards -- Andy (Java Persistent Objects - http://www.jpox.org)
