Re: [RFC][PATCH] wal decoding, attempt #2 - Design Documents (really attached)

On Monday, October 15, 2012 10:03:40 PM Tom Lane wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> > On Mon, Oct 15, 2012 at 3:18 PM, Peter Geoghegan <peter(at)2ndquadrant(dot)com>
wrote:
> >> On 15 October 2012 19:19, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> >>> I think Robert is right that if Slony can't use the API, it is unlikely
> >>> any other replication system could use it.
> >>
> >> I don't accept that. Clearly there is a circular dependency, and
> >> someone has to go first - why should the Slony guys invest in adopting
> >> this technology if it is going to necessitate using a forked Postgres
> >> with an uncertain future?
> >
> > Clearly, core needs to go first. However, before we commit, I would
> > like to hear the Slony guys say something like this: We read the
> > documentation that is part of this patch and if the feature behaves as
> > advertised, we believe we will be able to use it in place of the
> > change-capture mechanism that we have now, and that it will be at
> > least as good as what we have now if not a whole lot better.
> >
> > If they say something like "I'm not sure we have the right design for
> > this" or "this wouldn't be sufficient to replace this portion of what
> > we have now because it lacks critical feature X", I would be very
> > concerned about that.
>
> The other point here is that core code without any implemented use-cases
> is unlikely to be worth a tinker's damn. Regardless of what time-frame
> the Slony guys are able to work on, I think we need to see working code
> (of at least prototype quality) before we believe that we've got it
> right. Or if not code from them, code from some other replication
> project.

FWIW we (as in 2ndq), unsurprisingly, have a user of this which is in
development atm.

> A possibly-useful comparison is to the FDW APIs we've been slowly
> implementing over the past couple releases. Even though we *did* have
> some use-cases written right off the bat, we got it wrong and had to
> change it in 9.2, and I wouldn't bet against having to change it again
> in 9.3 (even without considering the need for extensions for non-SELECT
> operations). And, despite our very clear warnings that all that stuff
> was in flux, people have been griping because the APIs changed.

On the other hand, I don't think we would have FDWs today at all if it wouldn't
have been done that way. So I really cannot see that as an indication of not
working incrementally.
Obviously thats not an argument for not trying to get the API correct right off
the bat. I seriously hope the userlevel API continues to be simpler than what
FDWs need.

Regards,

Andres
--
Andres Freund http://www.2ndQuadrant.com/
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