[hibernate-dev] Lambda usage to run a code block in an isolated transaction

[Steve Ebersole]

The problem with "execute in isolation" here is that the "isolation" aspect
refers to being isolated from any current transaction.  It says nothing
about whether that stuff-to-execute should itself be transacted.  This is
why, for example, you see IsolationDelegate accept a `transacted` boolean
argument.

How would you propose we pass such a flag in this case?  Or are you
proposing that this always start a (new) transaction?

On Wed, Sep 14, 2016 at 1:39 PM Sanne Grinovero <sanne at hibernate.org> wrote:

> Today porting some benchmark code to Hibernate ORM 5.2 I had several
> difficulties around the fact that the code now needs to be different
> depending on transactions being container managed or not.
>
> My goal was to have a single benchmark test which I could compile once
> and run in either JavaSE or CMT; with some help from Steve I figured
> the necessary incantations out but ... it looks very unpractical.
>
> One way is to use an isolation delegate, which looks like this:
>
> final SessionImplementor session = (SessionImplementor) s;
> session.getTransactionCoordinator().createIsolationDelegate().delegateWork(
>    new WorkExecutorVisitable() {
>       @ Override
>       public Object accept(WorkExecutor executor, Connection
> connection) throws SQLException {
>          /// Some work with PreparedStatement on Connection..
>       }
> }, true );
>
> This worked fine for some raw SQL used for the benchmark
> initialization, but in another case I'd prefer to use the Session API
> rather than dealing with PreparedStatements and native connections;
> it looks like we don't have an equivalent "run code in isolation" for
> the Session ?
>
> It would be great if I could just pass a lambda to a Session and have
> this executed on a "child Session" in the scope of a "child
> Transaction", or just start and commit a transaction if there isn't
> one.
>
> s.executeInIsolation( session -> session.save(...) );
>
> So I'd expect that details like how to begin the transaction, how it
> should be committed (or rolled back in case of exceptions), how to
> lookup a TransactionManager, and especially how to not leak resources
> should be handled for the user.
>
> Obviously the inner Session instance is a different one than the
> outer, so any data returned by this block should be considered
> detached; maybe this limitation would be clearer if the method was
> hosted on SessionFactory or StatelessSession instead?
> Although it wouldn't necessarily have the limitations of a
> StalessSession, and it would be nice to have the inner transaction
> behave as a nested one when there's already one in the host Session.
>
> Looking forward for comments and improvement ideas :)
>
> Thanks,
> Sanne
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