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

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?

Perhaps Chris' phrasing explains the basic idea better:

On Wednesday, June 20, 2012 07:06:28 PM Christopher Browne wrote:
> The case where it would be needful is if you are in the process of
> assembling together updates coming in from multiple masters, and need
> to know:
>    - This INSERT was replicated from node #1, so should be ignored
> downstream - That INSERT was replicated from node #2, so should be ignored
> downstream - This UPDATE came from the local node, so needs to be passed
> to downstream users

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).

Andres

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