On Mon, 2010-09-06 at 23:07 +0100, Greg Stark wrote:
> On Mon, Sep 6, 2010 at 10:02 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
> > Then I respectfully suggest that you're releasing locks too early.
> >
> > Your proposal would allow a 2nd user to see the results of the 1st
> > user's transaction before the 1st user knew about whether it had
> > committed or not.
> >
> > I know why you want that, but I don't think its right.
>
> Well that's always possible. The 1st user might just not wake up
> before the 2nd user gets the response back.
>
> The question is what happens if the server crashes and is failed over
> to the slave. The 2nd user with the async transaction might have seen
> data commited by the 1st user with his sync transaction but was
> subsequently lost. Is the user expecting that making his transaction
> synchronously replicated guarantees that *nobody* can see this data
> unless the transaction is guaranteed to have been replicated or is he
> only expecting it to guarantee that *he* can't see the commit until it
> can be trusted to be replicated?
>
> For that matter I'm not entirely clear I understand how the timing
> here works at all. If transactions can't be considered to be committed
> before they're acknowledged by the replica what happens if the master
> crashes after the WAL is written and then comes back without a
> failover. Then the transaction would be immediately visible even if it
> still hasn't been replicated yet.
>
> I think there's no way with our current infrastructure to guarantee
> that other transactions can't see your data before it's been
> replicated, So making any promise otherwise for some cases is only
> going to be a lie.
>
> To guarantee synchronous replication doesn't show data until it's been
> replicated we would have to some kind of 2-phase commit where we send
> the commit record to the slave and wait until the slave has received
> it and confirmed it has written it (but it doesn't replay it unless
> there's a failover) then write the master's commit record and send the
> message to the slave that it's safe to replay those records.
Just to add that this part of the discussion has nothing at all to do
with my proposal for master controlled replication. Zoltan is simply
discussing when the wait should occur with sync replication. I have no
proposal to vary that myself, wherever we eventually decide the wait
should occur.
-- Simon Riggs www.2ndQuadrant.comPostgreSQL Development, 24x7 Support, Training and Services
