Re: Libpq support to connect to standby server as priority

> 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.

Best regards,
--
Tatsuo Ishii
SRA OSS, Inc. Japan
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