RE: Libpq support to connect to standby server as priority

From: "Tsunakawa, Takayuki" <tsunakawa(dot)takay(at)jp(dot)fujitsu(dot)com>
To: 'Laurenz Albe' <laurenz(dot)albe(at)cybertec(dot)at>, 'Tom Lane' <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Dave Cramer <pg(at)fastcrypt(dot)com>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Haribabu Kommi <kommi(dot)haribabu(at)gmail(dot)com>, Jing Wang <jingwangian(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: RE: Libpq support to connect to standby server as priority
Date: 2019-01-21 06:33:05
Message-ID: 0A3221C70F24FB45833433255569204D1FB6C706@G01JPEXMBYT05
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From: Laurenz Albe [mailto:laurenz(dot)albe(at)cybertec(dot)at]
> Tsunakawa, Takayuki wrote:
> > I'm sorry to repeat myself, but anyway, I think we need a method to connect
> to a standby
> > as the original desire, because the primary instance may be read only
> by default while
> > only limited users update data. That's for reducing the burdon on the
> primary and
> > minimizing the impact on users who update data. For example,
> >
> > * run data reporting on the standby
> > * backup the database from the standby
> > * cascade replication from the standby
> I see.
> But then the new value should not be called "prefer-read", because that
> would be
> misleading. It would also not be related to the existing "read-write".
> For what you have in mind, there should be the options "primary-required"
> and
> "standby-preferred", however we implement them.

Yes, that's what I'm proposing and expecting with a new parameter whose naming follows PgJDBC's.

> Have there been a lot of complaints that the existing "read-write" is not
> good
> enough to detect replication primaries?

I haven't heard anything. I guess almost nobody uses default_transaction_read_only.

Before that, see the description of target_session_attr:

I'm afraid most users don't know whether they can connect to the primary/standby. Just searching "primary", "master" or "standby" in this page doesn't show anything relevant.

> One use case I can think of is logical replication (or other replication
> methods like
> Slony). You can use the feature by setting default_transaction_read_only
> = on
> on the standby.

I know that, but I suspect that's really a practical use case. Anyway, I'm OK with relying on target_session_attr to fulfill that need.

Takayuki Tsunakawa

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