Re: Libpq support to connect to standby server as priority

On Thu, 17 Jan 2019 at 18:03, Tatsuo Ishii <ishii(at)sraoss(dot)co(dot)jp> wrote:

> > On Wed, 16 Jan 2019 at 01:02, Tatsuo Ishii <ishii(at)sraoss(dot)co(dot)jp> wrote:
> >
> >> > From: Tatsuo Ishii [mailto:ishii(at)sraoss(dot)co(dot)jp]
> >> >> But pg_is_in_recovery() returns true even for a promoting standby. So
> >> >> you have to wait and retry to send pg_is_in_recovery() until it
> >> >> finishes the promotion to find out it is now a primary. I am not sure
> >> >> if backend out to be responsible for this process. If not, libpq
> would
> >> >> need to handle it but I doubt it would be possible.
> >> >
> >> > Yes, the application needs to retry connection attempts until success.
> >> That's not different from PgJDBC and other DBMSs.
> >>
> >> I don't know what PgJDBC is doing, however I think libpq needs to do
> >> more than just retrying.
> >>
> >> 1) Try to find a node on which pg_is_in_recovery() returns false. If
> >> found, then we assume that is the primary. We also assume that
> >> other nodes are standbys. done.
> >>
> >> 2) If there's no node on which pg_is_in_recovery() returns false, then
> >> we need to retry until we find it. To not retry forever, there
> >> should be a timeout counter parameter.
> >>
> >>
> > IIRC this is essentially what pgJDBC does.
>
> Thanks for clarifying that. Pgpool-II also does that too. Seems like a
> common technique to find out a primary node.
>
>
Checking the code I see we actually use show transaction_read_only.

Sorry for the confusion

Dave Cramer

davec(at)postgresintl(dot)com
www.postgresintl.com

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