Subject: Re: standard interfaces for replication providers
From: Hannu Krosing <hannu@skype.net>
To: "Jim C. Nasby" <jnasby@pervasive.com>
Cc: Markus Schiltknecht <markus@bluegap.ch>,
	alfranio correia junior <alfranio@lsd.di.uminho.pt>,
	Christopher Browne <cbbrowne@acm.org>, pgsql-hackers@postgresql.org
In-Reply-To: <20060809185609.GK40481@pervasive.com>
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Date: Wed, 09 Aug 2006 22:57:47 +0300
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Ühel kenal päeval, K, 2006-08-09 kell 13:56, kirjutas Jim C. Nasby:
> On Wed, Aug 09, 2006 at 07:33:35AM +0000, Markus Schiltknecht wrote:
> > Hello Alfranio,
> > 
> > alfranio correia junior wrote:
> > >Of course not...
> > >It is impossible to build a replication system entirely by only using
> > >triggers...
> > 
> > Hm, I don't think it's impossible, just unpractical. Anyway, if you say 
> > so yourself, I really doubt the use of GAPI. If you need to create your 
> > own hooks anyway beside GAPI, why use GAPI at all? Better have one kind 
> > of 'hooking' (i.e. with a dynamically loaded library).
>  
> Why reinvent the wheel for everything if there was an interface that
> offered some of the needed functionality? Maybe PostgreSQL-R is simply
> too deep in the database for any of this to be useful, but I'm 99%
> certain that Slony could make use of some of this stuff, such as a hook
> on tuples being written out.

Slonys problems ar *not* in getting access to tuples written out. Why
fix something that aint broken ?

> Likewise, if there were hooks for WAL
> records and a way to inject WAL info into a backend it probably wouldn't
> be too hard to build WAL-based replication on top of that. Heck, PITR
> could probably be refactored to use such hooks.

What would it buy us ?

> One of the great things about Oracle is that they expose a hell of a lot
> of the technology they use to build features like replication; ie: take
> a look at DBMS_*.

Being an OSS project, postgresql exposes *everything* it has ;)

BTW, do you know how I can get something equivalent to postgresqls
snapshot from oracle ? 

I know one case when people had to do a really lot of unefficient work
to find out which transactions had committed during an interval in
oracle. 

> > >But definitely, there is a set of common requirements among a variety of
> > >replication systems. Moreover, such requirements are also useful to
> > >other systems as well.
> > 
> > So far, they are only partly usable for replication. I never looked into 
> > materialized views or anything else that would benefit from such triggers.
> > 
> > Regards
> > 
> > Markus
> > 
> > ---------------------------(end of broadcast)---------------------------
> > TIP 2: Don't 'kill -9' the postmaster
> > 
> 
-- 
----------------
Hannu Krosing
Database Architect
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