Re: replication hooks

On Thu, May 29, 2008 at 11:05:09PM +0300, Marko Kreen wrote:

> There is this tiny matter of replicating schema changes asynchronously,
> but I suspect nobody actually cares.

I know that Slony's users call this their number one irritant, so I
have my doubts nobody cares. But maybe nobody cares enough.

> - The task cannot even be clearly defined (on technical level - how
> the events should be represented).

Really? I've been in discussions where different people had clear
(but, alas, different) ideas of how to represent them.

> - Any schema changes need to be carefully prepared anyway. Whether
> to apply them to one or more servers does not make much difference.

One problem that designers of replication systems have is that they're
already thinking in the Serious Database Application world. But I
have recently had the pleasure of being reminded how many users of
database systems neither know nor care to know any of the details of
the underlying system. They already know how to make schema changes:
log into database, and start typing "ALTER TABLE. . ." You or I
agreeing that more careful preparation than that is important will not
change their mind. This is part of the reason MySQL looks so good:
you can "just do" these things. If it doesn't work out later, well,
you don't know that when your ALTER TABLE "just works".

> - Major plus of async replica is ability to actually have different
> schema on slaves.

I agree.

> - People _do_ care about exact schema on single place - failover servers.

Yeah, but not only there. One of the things I was hoping to have
nailed down in the "hooks" discussion was, in fact, the use cases.
Half the time, people have such a clear idea of what _they_ want from
their replication that they come to believe "replication" means that.

Another thing I like about the current proposal is that it is very
clear about what it is (and isn't) aiming for.

A

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