On Sat, Jul 22, 2006 at 04:17:47PM +0200, Florian G. Pflug wrote:
> Andrew Sullivan wrote:
> >Are you sure this will be an improvement?  It might just be a
> >foot-gun of a different calibre.
> I'm quite sure that it would be an improvement for at least
> my usecase of slony1. 

I don't like to be mean, but "this will help me" is not a reason to
implement, if it makes things worse for others.  The question is not
merely whether it will work for some cases, but whether it improves
the system overall for users.  If the tradeoff is that it makes
things better for some, but makes certain other failure cases way
more troublesome, that may be a trade-off we don't want to make.

> The worst that could happen is that you get some
> transaction stuck at prepared state, and need to manually roll them back
> on some nodes. Currently, it's quite easy to destroy your whole cluster
> by messing up a schema change.

The "on some nodes" thing is part of what is making me uneasy here.
What this says to me is that, to fix the issue that currently it is
easy for someone who hasn't carefully tested a DDL EXECUTE SCRIPT (or
who hasn't read the documentation) to break things, we're going to
introduce a failure mode whereby the DBA may need to intervent
manually on some nodes.  That seems to me like a step backwards.  If
the problem is that people are doing things which break stuff, then I
suspect we need to improve the interface such that it is harder to
break stuff, rather than introducing a new set of manual-intervention
steps.

Note that I'm not saying "don't do this".  I'm saying instead that a
2PC and a non-2PC approach in the same version of Slony at least
seems a bad idea to me -- it's too complicated.  Better to drop
support for non-2PC-capable versions.  Moreover, I'm saying that
you'd better have a pretty clean design and a nice set of
administration tools to handle the failure modes, or all you do is
move the pain around to some new place.  I can't see the point of
doing a lot of work to get beaten up by people complaining about some
other failure mode.

A 

-- 
Andrew Sullivan  | ajs at crankycanuck.ca
I remember when computers were frustrating because they *did* exactly what 
you told them to.  That actually seems sort of quaint now.
		--J.D. Baldwin