Re: Libpq support to connect to standby server as priority

On Thu, 17 Jan 2019 at 19:38, 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
> >>
> >> So if all PostgreSQL servers returns transaction_read_only = on, how
> >> does pgJDBC find the primary node?
> >>
> >> well preferSecondary would return a connection.
>
> This is not my message :-)
>
> > I'm curious; under what circumstances would the above occur?
>
> Former primary goes down and one of standbys is promoting but it is
> not promoted to new primary yet.
>

seems like JDBC might have some work to do...Thanks

I'm going to wait to implement until we resolve this discussion

Dave

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