Re: Re: [COMMITTERS] pgsql: Efficient transaction-controlled synchronous replication.

On Wed, Mar 23, 2011 at 3:27 AM, Markus Wanner <markus@bluegap.ch> wrote:
> On 03/22/2011 09:33 PM, Robert Haas wrote:
>> We might have a version of synchronous replication that works this way
>> some day, but it's not the version were shipping with 9.1.  The slave
>> acknowledges the WAL records when they hit the disk (i.e. fsync) not
>> when they are applied; WAL apply can lag arbitrarily.  The point is to
>> guarantee clients that the WAL is on disk somewhere and that it will
>> be replayed in the event of a failover.  Despite the fact that this
>> doesn't work as you're describing, it's a useful feature in its own
>> right.
>
> In that sense, our approach may be more synchronous than most others,
> because after the ACK is sent from the slave, the slave still needs to
> apply the transaction data from WAL before it gets visible, while the
> master needs to wait for the ACK to arrive at its side, before making it
> visible there.
>
> Ideally, these two latencies (disk seek and network induced) are just
> about equal.  But of course, there's no such guarantee.  So whenever one
> of the two is off by an order of magnitude or two (by use case or due to
> a temporary overload), either the master or the slave may lag behind the
> other machine.
>
> What pleases me is that the guarantee from the slave is somewhat similar
> to Postgres-R's: with its ACK, the receiving node doesn't guarantee the
> transaction *is* applied locally, it just guarantees that it *will* be
> able to do so sometime in the future.  Kind of a mind twister, though...

Yes.  What this won't do is let you build a big load-balancing network
(at least not without great caution about what you assume).  What it
will do is make it really, really hard to lose committed transactions.Both good things, but different.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company