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Re: Synchronous Standalone Master Redoux

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Jose Ildefonso Camargo Tolosa <ildefonso(dot)camargo(at)gmail(dot)com>
Cc: Hampus Wessman <hampus(at)hampuswessman(dot)se>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Synchronous Standalone Master Redoux
Date: 2012-07-25 21:43:23
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Lists: pgsql-hackers
On Fri, Jul 13, 2012 at 08:08:59PM -0430, Jose Ildefonso Camargo Tolosa wrote:
> On Fri, Jul 13, 2012 at 10:22 AM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> > On Fri, Jul 13, 2012 at 09:12:56AM +0200, Hampus Wessman wrote:
> >> How you decide what to do with the servers on failures isn't that
> >> important here, really. You can probably run e.g. Pacemaker on 3+
> >> machines and have it check for quorums to accomplish this. That's a
> >> good approach at least. You can still have only 2 database servers
> >> (for cost reasons), if you want. PostgreSQL could have all this
> >> built-in, but I don't think it sounds overly useful to only be able
> >> to disable synchronous replication on the primary after a timeout.
> >> Then you can never safely do a failover to the secondary, because
> >> you can't be sure synchronous replication was active on the failed
> >> primary...
> >
> > So how about this for a Postgres TODO:
> >
> >         Add configuration variable to allow Postgres to disable synchronous
> >         replication after a specified timeout, and add variable to alert
> >         administrators of the change.
> I agree we need a TODO for this, but... I think timeout-only is not
> the best choice, there should be a maximum timeout (as a last
> resource: the maximum time we are willing to wait for standby, this
> have to have the option of "forever"), but certainly PostgreSQL have
> to detect the *complete* disconnection of the standby (or all standbys
> on the synchronous_standby_names), if it detects that no standbys are
> eligible for sync standby AND the option to do fallback to async is
> enabled = it will go into standalone mode (as if
> synchronous_standby_names were empty), otherwise (if option is
> disabled) it will just continue to wait for ever (the "last resource"
> timeout is ignored if the fallback option is disabled).... I would
> call this "soft_synchronous_standby", and
> "soft_synchronous_standby_timeout" (in seconds, 0=forever, a sane
> value would be ~5 seconds) or something like that (I'm quite bad at
> picking names :( ).

TODO added:

	Allow synchronous_standby_names to be disabled after communication
	failure with all synchronous standby servers exceeds some timeout
	    This also requires successful execution of a synchronous
	notification command.

  Bruce Momjian  <bruce(at)momjian(dot)us>

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

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