Re: Synchronous replication patch built on SR

Robert Haas írta:
> On Fri, May 14, 2010 at 9:33 AM, Boszormenyi Zoltan <zb(at)cybertec(dot)at> wrote:
>
>> If min_sync_replication_clients == 0, then the replication is async.
>> If min_sync_replication_clients == max_wal_senders then the
>> replication is fully synchronous.
>> If 0 < min_sync_replication_clients < max_wal_senders then
>> the replication is partially synchronous, i.e. the master can wait
>> only for say, 50% of the clients to report back before it's considered
>> synchronous and the relevant transactions get released from the wait.
>>
>
> That's an interesting design and in some ways pretty elegant, but it
> rules out some things that people might easily want to do - for
> example, synchronous replication to the other server in the same data
> center that acts as a backup for the master; and asynchronous
> replication to a reporting server located off-site.
>

No, it doesn't. :-) You didn't take into account the third knob
usable in recovery.conf:
synchronous_slave = on/off
The off-site reporting server can be an asynchronous standby,
while the on-site backup server can be synchronous. The only thing
you need to take into account is that min_sync_replication_clients
shouldn't ever exceed your actual number of synchronous standbys.
The setup these three knobs provide is pretty flexible I think.

> One of the things that I think we will probably need/want to change
> eventually is the fact that the master has no real knowledge of who
> the replication slaves are.

The changes I made in my patch partly changes that,
the server still doesn't know "who" the standbys are
but there's a call that returns the number of connected
_synchronous_ standbys.

> That might be something we want to change
> in order to be able to support more configurability. Inventing syntax
> out of whole cloth and leaving semantics to the imagination of the
> reader:
>
> CREATE REPLICATION SLAVE reporting_server (mode asynchronous, xid_feedback on);
> CREATE REPLICATION SLAVE failover_server (mode synchronous,
> xid_feedback off, break_synchrep_timeout 30);
>
>

--
Bible has answers for everything. Proof:
"But let your communication be, Yea, yea; Nay, nay: for whatsoever is more
than these cometh of evil." (Matthew 5:37) - basics of digital technology.
"May your kingdom come" - superficial description of plate tectonics

----------------------------------
Zoltán Böszörményi
Cybertec Schönig & Schönig GmbH
http://www.postgresql.at/