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From "Craig Russell (JIRA)" <j...@apache.org>
Subject [jira] Commented: (JDO-590) Control over transaction isolation level
Date Wed, 06 Aug 2008 17:12:44 GMT

    [ https://issues.apache.org/jira/browse/JDO-590?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12620322#action_12620322

Craig Russell commented on JDO-590:

The structure of this proposal makes sense: add get/set TransactionIsolation to Transaction
and PersistenceManagerFactory; and add a property (and xml attribute to jdoconfig).

The values for the Strings might be context sensitive, so need a bit of thought. In an xml
file, "read-committed" is probably a good value to use, and TX_READ_COMMITTED is a good name
for the public final String variable (in javax.jdo.Constants).

I've read (again) this excellent and uncontroverted paper on isolation levels, and I'd recommend
it for others: http://www.cs.umb.edu/~poneil/iso.pdf

I'm not convinced of the need for TX_NONE, as I'm not sure what it means.

I'm not sure that we should mix isolation level names with phenomena, so TX_NO_LOST_UPDATES
might not be needed. 

It might be useful to introduce TX_SNAPSHOT and TX_CURSOR_STABILITY into the mix to accommodate
non-ANSI-SQL datastores. These would also allow users to request specific handling of some
long-running cursored queries and the use of mvcc if available on the datastore.


1. We do need to be clear what the application guarantees are if they choose to use specific
transaction isolation settings. We could do what the ANSI and JDBC standards require: that
if the user asks for an isolation that the underlying datastore doesn't support, the request
is "upgraded" to a setting that is supported. This would accommodate users asking for SNAPSHOT
on datastores that don't support it by mapping it to SERIALIZABLE.

2. We need to think about how to use the isolation setting with Optimistic transactions. Does
it even make sense? The idea of optimistic is that a non-transactional connection is used
for all operations before flush. So with optimistic transactions, does the isolation level
apply only to the final (transactional) connection?

> 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
> 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
> 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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