Horton

Re: Libpq support to connect to standby server as priority

Dave Cramer <pg@fastcrypt.com>

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From: Dave Cramer <pg@fastcrypt.com>
To: Tatsuo Ishii <ishii@sraoss.co.jp>
Cc: "Tsunakawa, Takayuki" <tsunakawa.takay@jp.fujitsu.com>, Tom Lane <tgl@sss.pgh.pa.us>, Robert Haas <robertmhaas@gmail.com>, laurenz.albe@cybertec.at, Haribabu Kommi <kommi.haribabu@gmail.com>, Jing Wang <jingwangian@gmail.com>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2019-01-17T13:36:53Z
Lists: pgsql-hackers 
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.


Dave Cramer

davec@postgresintl.com
www.postgresintl.com

>
>

Commits

  1. Make test_target_session_attrs more robust against connection failure.

  2. Extend the abilities of libpq's target_session_attrs parameter.

  3. Mark default_transaction_read_only as GUC_REPORT.

  4. Introduce a new GUC_REPORT setting "in_hot_standby".

  5. Avoid spamming the client with multiple ParameterStatus messages.

  6. Avoid harmless Valgrind no-buffer-pin errors.

  7. Restructure libpq code to remove some duplicity

  8. libpq: Add target_session_attrs parameter.