Re: Libpq support to connect to standby server as priority

>> >> >> > 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.

Best regards,
--
Tatsuo Ishii
SRA OSS, Inc. Japan
English: http://www.sraoss.co.jp/index_en.php
Japanese:http://www.sraoss.co.jp