Horton

Re: Libpq support to connect to standby server as priority

Haribabu Kommi <kommi.haribabu@gmail.com>

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From: Haribabu Kommi <kommi.haribabu@gmail.com>
To: "Tsunakawa, Takayuki" <tsunakawa.takay@jp.fujitsu.com>
Cc: Laurenz Albe <laurenz.albe@cybertec.at>, Tom Lane <tgl@sss.pgh.pa.us>, Dave Cramer <pg@fastcrypt.com>, Robert Haas <robertmhaas@gmail.com>, Jing Wang <jingwangian@gmail.com>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2019-01-20T23:16:14Z
Lists: pgsql-hackers 
On Fri, Jan 18, 2019 at 5:33 PM Tsunakawa, Takayuki <
tsunakawa.takay@jp.fujitsu.com> wrote:

> From: Tsunakawa, Takayuki [mailto:tsunakawa.takay@jp.fujitsu.com]
> > As for prefer-standby/prefer-read, if host parameter specifies
> host1,host2
> > in this order, and host1 is the primary with
> > default_transaction_read_only=true, does the app get a connection to
> host1?
> > I want to connect to host2 (standby) only if host1 is down.
>
> Oops, reverse -- I wanted to say "I want to connect to host1 (primary)
> only if host2 is down."
>

Thanks for finding out the problem, how about the following way of checking
for prefer-read/prefer-standby.

1. (default_transaction_read_only = true and recovery = true)
2. If none of the host satisfies the above scenario, then recovery = true
3. Last check is for default_transaction_read_only = true

Regards,
Haribabu Kommi
Fujitsu Australia

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.