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

On Wednesday, June 20, 2012 09:24:29 PM Aidan Van Dyk wrote:
> On Wed, Jun 20, 2012 at 3:15 PM, Andres Freund  
wrote:
> > To recap why we think origin_id is a sensible design choice:
> > 
> > There are many sensible replication topologies where it does make sense
> > that you want to receive changes (on node C) from one node (say B) that
> > originated from some other node (say A).
> > Reasons include:
> > * the order of applying changes should be as similar as possible on all
> > nodes. That means when applying a change on C that originated on B and
> > if changes replicated faster from A->B than from A->C you want to be at
> > least as far with the replication from A as B was. Otherwise the
> > conflict ratio will increase. If you can recreate the stream from the
> > wal of every node and still detect where an individual change
> > originated, thats easy.
> 
> 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.

Andres
-- Andres Freund	                   http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services