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Re: draft RFC: concept for partial, wal-based replication

From: Andres Freund <andres(at)anarazel(dot)de>
To: " Hans-Jürgen =?iso-8859-1?q?_Sch=F6nig?=" <hs(at)cybertec(dot)at>
Cc: Stefan Kaltenbrunner <stefan(at)kaltenbrunner(dot)cc>, pgsql-hackers(at)postgresql(dot)org, Itagaki Takahiro <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp>, Boszormenyi Zoltan <zb(at)cybertec(dot)at>
Subject: Re: draft RFC: concept for partial, wal-based replication
Date: 2009-11-30 17:59:14
Message-ID: 200911301859.15114.andres@anarazel.de (view raw or flat)
Thread:
Lists: pgsql-hackers
On Monday 30 November 2009 17:46:45 Hans-Jürgen Schönig wrote:
> On Nov 30, 2009, at 10:32 AM, Stefan Kaltenbrunner wrote:
> > the question is if filtering on the sending side is actually the
> > "right thing" to do.
> > It increases the overhead and the complexity on the master,
> > especially if you think about different (partial) replication
> > agreements for different slaves and it might also be hard to
> > integrate with the planned sync/async modes.
> > On the other hand if you filter on the master you might be able to
> > avoid a lot of network traffic du to filtered wal records.
> > I think for a first step it might make more sense to look into doing
> > the filtering on the receiving side and look into actual integration
> > with SR at a later stage.
> one problem with not-filtering on the master is that you will end up
> with a lot of complexity if you start adding new tables to a replica
> because you just cannot add tables as easy as when you are doing stuff
> on the slave. the procedure seems ways more complex.
> in addition to that you are sending WAL which has to be discarded
> anyway.
> we thought about filtering "outside the master" a lot but to me it did
> not sound like good plan.
One possibility for the far future would be to allow filtering on a slave as 
well:

master ---- full replication ---> primary slave --- split ---> slaves

Possibly doing only catalog recovery on the primary slave. In my opinion thats 
heaps more complex and not better in all situation. So I would probably write 
it down as a nice idea but not more.

Andres

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