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

Hi,

On Tuesday, June 19, 2012 08:03:04 AM Tom Lane wrote:
> Andres Freund <andres@2ndquadrant.com> writes:
> > On Monday, June 18, 2012 11:51:27 PM Daniel Farina wrote:
> >> What's the cost of going a lot higher?  Because if one makes enough
> >> numerical space available, one can assign node identities without a
> >> coordinator, a massive decrease in complexity.
> > 
> > It would increase the size of every wal record. We just have 16bit left
> > there by chance...
> 
> "Every WAL record"?  Why in heck would you attach it to every record?
> Surely putting it in WAL page headers would be sufficient.  We could
> easily afford to burn a page switch (if not a whole segment switch)
> when changing masters.
The idea is that you can have cascading, circular and whatever replication 
topologies if you include the "logical origin" of a wal causing action into 
it.
That is, if you have nodes A(1) and B(2) and a insert happens on A the wal 
records generated by that will get an xl_origin_id = 1 and when it will be 
decoded and replayed on B it will *also* get the id 1. Only when a change 
originally is generated on Bit will get xl_origin_id = 2.
That way you can easily have circular or hierarchical replication topologies 
including diamonds.

> I'm against the idea of eating any spare space we have in WAL record
> headers for this purpose, anyway; there are likely to be more pressing
> needs in future.
Every other solution to allowing this seems to be far more complicated than 
this, thats why I arrived at the conclusion that its a good idea.

Greetings,

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