Re: Configuring synchronous replication

On Tue, 2010-09-21 at 16:58 +0900, Fujii Masao wrote:
> On Sat, Sep 18, 2010 at 4:36 AM, Dimitri Fontaine
>  wrote:
> > Simon Riggs  writes:
> >> On Fri, 2010-09-17 at 21:20 +0900, Fujii Masao wrote:
> >>> What synchronization level does each combination of sync_replication
> >>> and sync_replication_service lead to?
> >>
> >> There are only 4 possible outcomes. There is no combination, so we don't
> >> need a table like that above.
> >>
> >> The "service" specifies the highest request type available from that
> >> specific standby. If someone requests a higher service than is currently
> >> offered by this standby, they will either
> >> a) get that service from another standby that does offer that level
> >> b) automatically downgrade the sync rep mode to the highest available.
> >
> > I like the a) part, I can't say the same about the b) part. There's no
> > reason to accept to COMMIT a transaction when the requested durability
> > is known not to have been reached, unless the user said so.

Hmm, no reason? The reason is that the alternative is that the session
would hang until a standby arrived that offered that level of service.
Why would you want that behaviour? Would you really request that option?

> Yep, I can imagine that some people want to ensure that *all* the
> transactions are synchronously replicated to the synchronous standby,
> without regard to sync_replication. So I'm not sure if automatic
> downgrade/upgrade of the mode makes sense. We should introduce new
> parameter specifying whether to allow automatic degrade/upgrade or not?
> It seems complicated though.

I agree, but I'm not against any additional parameter if people say they
really want them *after* the consequences of those choices have been
highlighted.

IMHO we should focus on the parameters that deliver key use cases.

-- Simon Riggs           www.2ndQuadrant.comPostgreSQL Development, 24x7 Support, Training and Services