Re: Libpq support to connect to standby server as priority

> On Thu, 17 Jan 2019 at 18:03, Tatsuo Ishii <ishii@sraoss.co.jp> wrote:
> 
>> > On Wed, 16 Jan 2019 at 01:02, Tatsuo Ishii <ishii@sraoss.co.jp> wrote:
>> >
>> >> > From: Tatsuo Ishii [mailto:ishii@sraoss.co.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?

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