Re: Libpq support to connect to standby server as priority

From: Tatsuo Ishii <ishii(at)sraoss(dot)co(dot)jp>
To: tsunakawa(dot)takay(at)jp(dot)fujitsu(dot)com
Cc: ishii(at)sraoss(dot)co(dot)jp, pg(at)fastcrypt(dot)com, tgl(at)sss(dot)pgh(dot)pa(dot)us, robertmhaas(at)gmail(dot)com, laurenz(dot)albe(at)cybertec(dot)at, kommi(dot)haribabu(at)gmail(dot)com, jingwangian(at)gmail(dot)com, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Libpq support to connect to standby server as priority
Date: 2019-01-16 06:02:36
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> 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.

Best regards,
Tatsuo Ishii
SRA OSS, Inc. Japan

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