replication hooks

On 5/29/08, Andrew Sullivan <ajs(at)commandprompt(dot)com> wrote:
> On Thu, May 29, 2008 at 12:05:18PM -0700, Robert Hodges wrote:
> > people are starting to get religion on this issue I would strongly
> > advocate a parallel effort to put in a change-set extraction API
> > that would allow construction of comprehensive master/slave
> > replication.
>
> You know, I gave a talk in Ottawa just last week about how the last
> effort to develop a comprehensive API for replication failed. I had
> some ideas about why, the main one of which is something like this:
> "Big features with a roadmap have not historically worked, so unless
> we're willing to change the way we work, we won't get that."
>
> I don't think an API is what's needed. It's clear proposals for
> particlar features that can be delivered in small pieces. That's what
> the current proposal offers. I think any kind of row-based approach
> such as what you're proposing would need that kind of proposal too.
>
> That isn't to say that I think an API is impossible or undesirable.
> It is to say that the last few times we tried, it went nowhere; and
> that I don't think the circumstances have changed.

I think the issue is simpler - API for synchronous replication is
undesirable - it would be too complex and hinder future development
(as I explained above).

And the API for asynchronous replication is already there - triggers,
txid functions for queueing.

There is this tiny matter of replicating schema changes asynchronously,
but I suspect nobody actually cares. Few random points about that:

- The task cannot even be clearly defined (on technical level - how
the events should be represented).
- Any schema changes need to be carefully prepared anyway. Whether
to apply them to one or more servers does not make much difference.
- Major plus of async replica is ability to actually have different
schema on slaves.
- People _do_ care about exact schema on single place - failover servers.
- But for failover server we want also synchronous replication.

So if we have synchronous WAL based replication for failover servers,
the interest in hooks to log schema changes will decrease even more.

--
marko