On Wednesday, June 20, 2012 10:12:46 PM Aidan Van Dyk wrote:
> On Wed, Jun 20, 2012 at 3:49 PM, Andres Freund <andres@2ndquadrant.com>
wrote:
> > On Wednesday, June 20, 2012 09:41:03 PM Aidan Van Dyk wrote:
> >> On Wed, Jun 20, 2012 at 3:27 PM, Andres Freund <andres@2ndquadrant.com>
> >
> > wrote:
> >> >> OK, so in this case, I still don't see how the "origin_id" is even
> >> >> enough.
> >> >>
> >> >> C applies the change originally from A (routed through B, because
> >> >> it's faster). But when it get's the change directly from A, how
> >> >> does it know to *not* apply it again?
> >> >
> >> > The lsn of the change.
> >>
> >> So why isn't the LSN good enough for when C propagates the change back
> >> to A?
> >
> > Because every node has individual progress in the wal so the lsn doesn't
> > mean anything unless you know from which node it originally is.
> >
> >> Why does A need more information than C?
> >
> > Didn't I explain that two mails up?
>
> Probably, but that didn't mean I understood it... I'm trying to keep up
> here ;-)
Heh. Yes. This developed into a huge mess already ;)
> So the origin_id isn't strictly for the origin node to know filter an
> LCR it's applied already, but it is also to correlate the LSN's
> because the LSN's of the re-generated LCR's are meant to contain the
> originating nodes's LSN, and every every node applying LCRs needs to
> be able to know where it is in every node's LSN progress.
There are multiple use-cases for it, this is one of them.
> I had assumed any LCR's generated on a node woudl be relative to the
> LSN sequencing of that node.
We have the original lsn in the commit record. Thats needed to support crash
recovery because you need to know where to restart applying changes again
from.
> > Now imagine a scenario where #1 and #2 are in Europe and #3 and #4 in
> > north america.
> > #1 wants changes from #3 and #4 when talking to #3 but not those from #2
> > and itself (because that would be cheaper to get locally).
>
> Right, but if the link between #1 and #2 ever "slows down", changes
> from #3 and #4 may very well already have #2's changes, and even
> require them.
>
> #1 has to apply them, or is it going to stop applying LCR's from #3
> when it see's LCRs from #3 coming in that originate on #2 and have
> LSNs greater than what it's so far received from #2?
We will see ;). There are several solutions possible, possibly even depending
on the use-case. This patch just want's to provide the very basic information
required to implement such solutions...
Greetings,
Andres
-- Andres Freund http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
Services
