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From "Andy Jefferson (JIRA)" <j...@apache.org>
Subject [jira] Created: (JDO-590) Control over transaction isolation level
Date Fri, 11 Apr 2008 13:42:06 GMT
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
            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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