Re: Clustering features for upcoming developer meeting -- please claim yours!

On 5/10/10, Jan Wieck  wrote:
> On 5/10/2010 1:39 PM, Josh Berkus wrote:
> > > Aside from that list, I'd like to get into a little more detail on DDL
> > > triggers. This seems to be something I could actually work on in the
> > > future.
> > >
> >
> > Is this the same thing as the general modification trigger?
> >
>
>  To my understanding, the general modification triggers are meant to unify
> the "data" queue mechanisms, both Londiste and Slony are based on, under one
> new, built in mechanism with the intention to cut down the overhead
> associated with them.
>
>  There is certainly a big need to coordinate this project with any attempts
> made in the direction of DDL triggers. I think it is obvious that I would
> later on like to make use of them within Slony to replicate schema changes.
> This of course requires that such schema changes get applied on the
> replica's at the correct place inside the data stream. For example, if you
> "ALTER TABLE ADD COLUMN", you want to replicate all DML changes, that
> happened before that ALTER TABLE grabbed its exclusive lock, before that
> ALTER TABLE itself. And it would be quite disastrous to attempt to apply any
> INSERT that happened on the master with that new column before the ALTER
> TABLE happened on the replica.

AFAICS the "agreeable order" should take care of positioning:

  http://wiki.postgresql.org/wiki/ModificationTriggerGDQ#Suggestions_for_Implementation

This combined with DML triggers that react to invalidate events (like
PgQ ones) should already work fine?

Are there situations where such setup fails?

-- 
marko