Re: [PATCH 8/8] Introduce wal decoding via catalog timetravel

On Thu, Oct 11, 2012 at 03:16:39AM +0100, Greg Stark wrote:
> On Thu, Oct 11, 2012 at 2:40 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> >> I think I've mentioned it before, but in the interest of not being
> >> seen to critique the bikeshed only after it's been painted: this
> >> design gives up something very important that exists in our current
> >> built-in replication solution, namely pipelining.
> >
> > Isn't there an even more serious problem, namely that this assumes
> > *all* transactions are serializable?  What happens when they aren't?
> > Or even just that the effective commit order is not XID order?
> 
> Firstly, I haven't read the code but I'm confident it doesn't make the
> elementary error of assuming commit order == xid order. I assume it's
> applying the reassembled transactions in commit order.
> 
> I don't think it assumes the transactions are serializable because
> it's only concerned with writes, not reads. So the transaction it's
> replaying may or may not have been able to view the data written by
> other transactions that commited earlier but it doesn't matter when
> trying to reproduce the effects using constants. The data written by
> this transaction is either written or not when the commit happens and
> it's all written or not at that time. Even in non-serializable mode
> updates take row locks and nobody can see the data or modify it until
> the transaction commits.

How does Slony write its changes without causing serialization replay
conflicts?

-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + It's impossible for everything to be true. +