Re: Re: PostgreSql - access modified rows in prepare transaction command

On Wednesday, February 20, 2013 5:10 PM pierpaolo.cincilla wrote:
> Thank you Heikki for your reply. As you suggest, I will explain better
> what
> I'm trying to accomplish.
>
> What I'm writing a ditributed two-phase-commit termination protocol
> that
> work in this manner:
>
> 1) Each site has a replica of the database. A site A perform a
> transaction
> t1 and prepare it (PREPARE TRANSACTION 't1'). Then it atomic broadcast
> a
> certification request for the transaction t1 *along with its writeset*
> (values updated by t1) to other sites.
>
> 2) When a site receive the certification request for transaction t1
> does the
> certification (check that there are no concurrent conflicting
> transactions).
> If the certification succeed then
> 2a) if the transaction is local (i.e. originated at that site) it
> commit the
> transaction (COMMMIT PREPARED 't1').
> 2b) If the transaction is remote (i.e. prepared at another site) *it
> apply
> locally the writeset of transaction t1* to reflect modifications to its
> local replica of the database (UPDATE command).
>
> The problem is that if I can't fetch the writeset of a transaction in
> phase
> 1 (before the commit request) then when I certify the transaction at
> another
> site I can't apply the updates performed by the remote transaction
> right
> away but I have to wait the originating site to commit the transaction
> and
> send back its writeset (now visible). This will be very bad because it
> adds
> an extra round to the algorithm.

I think the one possible way to get the transaction data at the point you
need will be through
WAL, but that will also not be straightforward, you need to decode and find
in WAL corresponding data.

With Regards,
Amit Kapila.