Re: XA support (distributed transactions)

Danny Milosavljevic wrote:
> Hi,
> 
> We (Fabalabs) are looking into adding XA support (distributed
> transactions) to Postgresql.

Postgres already supports 2 phase commits, which is can basis
on which XA can be implemented I think. The "only" missing
part is an transaction manager, but that wouldn't have to
be integrated into postgres. As far as I understand things,
the only thing you really need the transaction manager for
is for automatic recovery if one member of a distributed
transaction fails while the transaction is still in progress.
In that case the transaction manager needs to either rollback
the transaction, if it wasn't already prepared on all nodes,
or commit it.

The only difficulty I see in this "transaction manager" is that
it will needto (persistenly) keep track of transactions, because
due to the design of 2-phase-commit, you cannot deduce the faith
of a transaction by just looking at the nodes.

Image you find a transaction that is prepared in 3 out of 5
participating nodes. It might be that it was originally
prepared on all 5 nodes, and already comitted on two of them,
or it might have been comitted on only 3 nodes before the
transaction manager itself crashed. In the first case you
should commit the transaction on those 3 nodes, while you
should roll it back in the second case.

I believe there are open-source implementations of such transaction 
managers, but I don't have any links at hand.

greetings, Florian Pflug