Date: Fri, 22 Sep 2000 10:08:06 -0600
From: Bruce Guenter <bruceg@em.ca>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-general@postgresql.org
Subject: Re: replication
Message-ID: <20000922100806.A1676@em.ca>
References: <001d01c023ca$e37d9cc0$330a0a0a@Adam>
	<39CB1120.DBEC91AB@haneke.de> <20000922093102.A1715@em.ca>
	<29326.969638593@sss.pgh.pa.us>
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	from tgl@sss.pgh.pa.us on Fri, Sep 22, 2000 at 12:03:13PM -0400


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On Fri, Sep 22, 2000 at 12:03:13PM -0400, Tom Lane wrote:
> Bruce Guenter <bruceg@em.ca> writes:
> > I've been thinking that one way to emulate replication would be to run a
> > pgsql "shim" that would redirect queries to two back-ends
> > simultaneously, creating transactions for update commands, and aborting
> > when necessary.  The shim would be a long-running gateway process, that
> > could check for data consistency when it started up.  This is all off
> > the top of my head, so is this at all a reasonable idea?
>=20
> Awhile back, someone reported that they were successfully using exactly
> this idea.  Check the PG list archives (sorry, I forget which list).

Cool!  I'll take a look.

> I'm not convinced that this scales real well to multiple concurrent
> clients ... at the very least, you'd need a single gateway that
> serializes all the requests.

Or multiple gateways, one "near" each database server, that try to keep
some synchronization with each other.

> You might still have problems with the
> two databases not executing requests in exactly the same order.

> Also
> it'd be really dangerous to use OIDs as a way of identifying rows in
> application queries.

Oh, definitely agreed.  OIDs go out the window, unless both servers have
have identical queries delivered to them at all times.  This is rather
difficult to guarantee.
--=20
Bruce Guenter <bruceg@em.ca>                       http://em.ca/~bruceg/

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