From: Tom Lane <tgl@sss.pgh.pa.us>
To: Peter Eisentraut <peter.eisentraut@2ndquadrant.com>
cc: Robert Haas <robertmhaas@gmail.com>,
        pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: base backup client as auxiliary backend process
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Comments: In-reply-to Peter Eisentraut <peter.eisentraut@2ndquadrant.com>
	message dated "Thu, 11 Jul 2019 22:56:31 +0200"
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Date: Thu, 11 Jul 2019 17:28:27 -0400
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Peter Eisentraut <peter.eisentraut@2ndquadrant.com> writes:
> On 2019-07-11 22:20, Robert Haas wrote:
>> On Thu, Jul 11, 2019 at 4:10 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>> How's the postmaster to know that it's supposed to run pg_basebackup
>>> rather than start normally?  Where will it get the connection information?
>>> Seem to need configuration data *somewhere*.
>> 
>> Maybe just:
>> 
>> ./postgres --replica='connstr' -D createme

> What you are describing is of course theoretically possible, but it
> doesn't really fit with how existing tooling normally deals with this,
> which is one of the problems I want to address.

I don't care for Robert's suggestion for a different reason: it presumes
that all data that can possibly be needed to set up a new replica is
feasible to cram onto the postmaster command line, and always will be.

An immediate counterexample is that's not where you want to be specifying
the password for a replication connection.  But even without that sort of
security issue, this approach won't scale.  It also does not work even a
little bit nicely for tooling in which the postmaster is not supposed to
be started directly by the user.  (Which is to say, all postgres-service
tooling everywhere.)

			regards, tom lane