[jboss-as7-dev] Switch to use TransactionSynchronizationRegistry.registerInterposedSynchronization?

[Jonathan Halliday]

On 06/02/2011 01:41 PM, Scott Marlow wrote:

> My understanding of "Too late in the game", would be attempting to
> register the sync after commit has been called to start ending the
> transaction.

That's not quite right and indeed the trouble we're seeing stems from 
Synchronizations that (indirectly) register other Synchronizations from 
within beforeCompletion i.e. at a point in time after commit() is called 
by the app or container and the transaction manager has thus begun tx 
termination processing, of which beforeCompletion calls are one of the 
early parts.

Since the tx is technically still active during beforeCompletion, it is 
legitimate for beforeCompletions to call registerSynchronization or 
registerInterposedSynchronization. Prior to the introduction of the 
latter method in JTA 1.1 that was the end of the story - newly 
registered Syncs just got tacked onto the end of the pending list and 
the beforeCompletion phase continued to run until no new ones were 
registered. Registration order determined call order, although that's 
not a JTA requirement, just an impl detail.

With JTA 1.1 there is now a spec defined partial order for 
Synchronizations which differs from registration order, so in some cases 
we can't simply tack newly registered ones onto the list - we're passed 
the point at which we would call them and so we reject the registration.

A beforeCompletion can only register a Sync only if it would be ordered 
after the one that's currently executing. Technically this is true even 
within Sync types, but it only manifests as a problem between types 
(interposed/non-interposed) as we take care of the ordering between 
instances of the same type internally in the tx code - you can't 
override that bit through the JTA api.

So, the rule for activity in beforeCompletion is:

  - a Sync registered via registerSynchronization may call either 
registerSynchronization or registerInterposedSynchronization.

  - a Sync registered via registerInterposedSynchronization may call 
only registerInterposedSynchronization.

Thus hibernate (interposed) can't call the JCA (non-interposed) to get a 
db connection. But it could call registerInterposedSynchronization, 
which is basically why there is now a more to convert everything that it 
may call to use interposed syncs.  That escalation may not necessarily 
be the best long term move, as it leaves you with a problem if you wind 
up needing a yet tighter defined ordering equivalent to 
registerInterposedInterposed...

Jonathan.

>
> Note from Jonathan Halliday about the same:
> "
> per the JTA spec, interposed Synchronizations must be called after
> non-interposed ones. Since hibernate is using interposed
> Synchronizations, the tx has already passed the point at which it can
> call non-interposed Synchronizations. Therefore it does not allow them
> to be registered, on the basis that there needs to be some way to spot
> that they are not going to get called, otherwise you'd spend hours
> scratching your head and wondering why the Synchronization was not
> getting invoked. There is a corner case where you don't give a toss
> about beforeCompletion and want the Synchronization only for
> afterCompletion, but there is no way in the API to indicate that so we
> play it safe and disallow uniformly.


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