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

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 ;-)

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.

I had assumed any LCR's generated on a node woudl be relative to the
LSN sequencing of that node.

> 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?


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