Re: CommitFest 2009-07: Yay, Kevin! Thanks, reviewers!

On 19/08/10 04:46, Robert Haas wrote:
> At any rate, we should definitely NOT wait another
> month to start thinking about Sync Rep again.

Agreed. EnterpriseDB is interested in having that feature, so I'm on the
hook to spend time on it regardless of commitfests.

> I haven't actually
> looked at any of the Sync Rep code AT ALL but IIRC Heikki expressed
> the view that the biggest thing standing in the way of a halfway
> decent Sync Rep implementation was a number of polling loops that
> needed to be replaced with something that wouldn't introduce
> up-to-100ms delays.

Well, that's the only uncontroversial thing about it that doesn't
require any fighting over the UI or desired behavior. That's why I've
focused on that first, and also because it's useful regardless of
synchronous replication. But once that's done, we'll have to nail down
how synchronous replication is supposed to behave, and how to configure it.

> And so far we haven't seen a patch for that.
> Somebody write one. And then let's get it reviewed and committed RSN.

Fujii is on vacation, but I've started working on it. The two issues
with Fujii's latest patch are that it would not respond promptly on
platforms where signals don't interrupt sleep, and it suffers the
classic race condition that pselect() was invented for. I'm going to
replace pg_usleep() with select(), and use the so called "self-pipe
trick" to get over the race condition. I have that written up but I want
to do some testing and cleanup before posting the patch.

> It may seem like we're early in the release cycle yet, but for a
> feature of this magnitude we are not. We committed way too much big
> stuff at the very end of the last release cycle; Hot Standby was still
> being cleaned up in May after commit in November. We'll be lucky to
> commit sync rep that early.

Agreed. We need to decide the scope and minimum set of features real
soon to get something concrete finished.

BTW, on what platforms signals don't interrupt sleep? Although that
issue has been discussed many times before, I couldn't find any
reference to a real platform in the archives.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com