Date: Wed, 08 Apr 2009 19:15:28 +1200
From: Mark Kirkwood <markir@paradise.net.nz>
Subject: Re: Best replication solution?
In-reply-to: <20090407171803.GQ14950@shinkuro.com>
To: Andrew Sullivan <ajs@crankycanuck.ca>, pgsql-performance@postgresql.org
Message-id: <49DC4F10.6090302@paradise.net.nz>
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Andrew Sullivan wrote:
>
> I should have stated that differently.  First, you're right that if
> you don't know where to look or what to look for, you can easily be
> unaware of nodes being out of sync.  What's not a problem with Slony
> is that the nodes can get out of internally consistent sync state: if
> you have a node that is badly lagged, at least it represents, for
> sure, an actual point in time of the origin set's history.  Some of
> the replication systems aren't as careful about this, and it's
> possible to get the replica into a state that never happened on the
> origin.  That's much worse, in my view.
>
> In addition, it is not possible that Slony's system tables report the
> replica as being up to date without them actually being so, because
> the system tables are updated in the same transaction as the data is
> sent.  It's hard to read those tables, however, because you have to
> check every node and understand all the states.
>
>   
>
Yes, and nicely explained!

> (on Londiste DDL + slave chaining)...
> Well, those particular features -- which are indeed the source of much
> of the complexity in Slony -- were planned in from the beginning.
> Londiste aimed to be simpler, so it would be interesting to see
> whether those features could be incorporated without the same
> complication.
>
>
>
>   
Yeah, that's the challenge!

Personally I would like DDL to be possible without any special wrappers 
or precautions, as the usual (accidental) breakage I end up looking at 
in Slony is because someone (or an app's upgrade script) has performed 
an ALTER TABLE directly on the master schema...

Cheers

Mark