Re: [PATCH 10/16] Introduce the concept that wal has a 'origin' node

On Wednesday, June 20, 2012 03:54:43 PM Robert Haas wrote:
> On Wed, Jun 20, 2012 at 9:43 AM, Andres Freund <andres(at)2ndquadrant(dot)com>
wrote:
> >> If you do that, then, yes,
> >> everything that you need to disentangle various network topologies
> >> must be present in WAL. But what I'm saying is: don't do it like
> >> that. Generate the LCRs just ONCE, at the origin node, and then pass
> >> them around the network, applying them at every node. Then, the
> >> information that is needed in WAL is confined to one bit: the
> >> knowledge of whether or not a particular transaction is local (and
> >> thus LCRs should be generated) or non-local (and thus they shouldn't,
> >> because the origin already generated them and thus we're just handing
> >> them around to apply everywhere).
> >
> > Sure, you can do it that way, but I don't think its a good idea. If you
> > do it my way you *guarantee* that when replaying changes from node B on
> > node C you have replayed changes from A at least as far as B has. Thats
> > a really nice property for MM.
> > You *can* get same with your solution but it starts to get complicated
> > rather fast. While my/our proposed solution is trivial to implement.
>
> That's an interesting point. I agree that's a useful property, and
> might be a reason not to just use a single-bit flag, but I still think
> you'd be better off handling that requirement via some other method,
> like logging the node ID once per transaction or something. That lets
> you have as much metadata as you end up needing, which is a lot more
> flexible than a 16-bit field, as Kevin, Heikki, and Tom have also
> said.
If it comes down to that I can definitely live with that. I still think making
the filtering trivial so it can be done without any logic on a low level is a
very desirable property but if not, so be it.

Andres
--
Andres Freund http://www.2ndQuadrant.com/
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