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From: Laurenz Albe <laurenz.albe@cybertec.at>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Haribabu Kommi <kommi.haribabu@gmail.com>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: Jing Wang <jingwangian@gmail.com>
Cc: Pg Hackers <pgsql-hackers@postgresql.org>
Cc: Tsunakawa, Takayuki <tsunakawa.takay@jp.fujitsu.com>
Subject: Re: Libpq support to connect to standby server as priority
Date: Fri, 16 Nov 2018 18:56:11 +0100
Message-ID: <84d7a264fb5ad46873f6e40e10c3ee92983c55ca.camel@cybertec.at> (raw)
In-Reply-To: <28777.1542386147@sss.pgh.pa.us>
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	<28777.1542386147@sss.pgh.pa.us>

Tom Lane wrote:
> > As it is now, the patch doesn't keep two connections open.  It remembers
> > the index of the host of the first successful writable connection, but
> > closes the connection, and opens another one to that host if no read-only
> > host can be found.
> 
> Oh!  The reason I assumed it wasn't doing that is that such a behavior
> seems completely insane.  If the point is to keep down the load on your
> master server, then connecting only to immediately disconnect is not
> a friendly way to do that --- even without counting the fact that you
> might later come back and connect again.

That's why I had argued initially to keep the session open, but you
seem to dislike that idea as well.

> If that's the best we can do, we should forget the whole feature and
> just recommend putting slave servers first in your hosts list when
> you want prefer-slave.

If you know which is which, certainly.
But in a setup with automated failover you cannot be certain which is which.
That's what the proposed feature targets.

Yours,
Laurenz Albe





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