Re: [PATCH v1] Add and report the new "in_hot_standby" GUC pseudo-variable.

From: Elvis Pranskevichus <elprans(at)gmail(dot)com>
To: pgsql-hackers(at)postgresql(dot)org
Cc: "Tsunakawa, Takayuki" <tsunakawa(dot)takay(at)jp(dot)fujitsu(dot)com>, 'Tom Lane' <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Robert Haas <robertmhaas(at)gmail(dot)com>, Craig Ringer <craig(at)2ndquadrant(dot)com>, Magnus Hagander <magnus(at)hagander(dot)net>, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, Peter Eisentraut <peter(dot)eisentraut(at)2ndquadrant(dot)com>, Michael Banck <michael(dot)banck(at)credativ(dot)de>
Subject: Re: [PATCH v1] Add and report the new "in_hot_standby" GUC pseudo-variable.
Date: 2017-09-12 15:48:28
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On Wednesday, May 24, 2017 9:38:36 PM EDT Tsunakawa, Takayuki wrote:
> The clients will know the change of session_read_only when they do
> something that calls RecoveryInProgress(). Currently,
> RecoveryInProgress() seems to be the only place where the sessions
> notice the promotion, so I set session_read_only to the value of
> default_transaction_read_only there. I think that there is room for
> discussion here. It would be ideal for the sessions to notice the
> server promotion promptly and notify the clients of the change. I
> have no idea to do that well.

My original patch did that via the new SendSignalToAllBackends() helper,
which is called whenever the postmaster exits hot stanby.

I incorporated those bits into your patch and rebased in onto master.
Please see attached.

FWIW, I think that mixing the standby status and the default
transaction writability is suboptimal. They are related, yes, but not
the same thing. It is possible to have a master cluster in the
read-only mode, and with this patch it would be impossible to
distinguish from a hot-standby replica without also polling
pg_is_in_recovery(), which defeats the purpose of having to do no
database roundtrips.


Attachment Content-Type Size
session-read-only.patch text/x-patch 21.2 KB

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