Re: [HACKERS] Multiple synchronous_standby_names rules

On Thu, 12 Jan 2017 at 12:06, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>
wrote:

> On Thu, Jan 12, 2017 at 9:53 AM, James Sewell <james(dot)sewell(at)jirotech(dot)com>
> wrote:
> > What is needed to support this is the ability to configure Px with
> something like:
> >
> > 1 (P1, P2, P3), 1 (D1, D2, D3)
> >
> > Would there be any appetite for this - or would it be seen as over
> complication of the current rules?
>
> There have been discussions about being able to do that and there are
> really use cases where that would be useful. As lately quorum commit
> has been committed, we have a better idea of the grammar to use
> (yeah!), though there are a couple of things remaining regarding the
> design of node subsets:
> - How to define group names? Making them mandatory would likely be the
> way to go.
> - How to represent that intuitively in pg_stat_replication? Perhaps
> the answer here is an extra column in this system view.
>

I'm coming back round to this as it's becoming increasingly discussed as we
look into systems with an RPO of (close to) 0 spanning multiple locations.

Before I start hacking - does anyone have any preference for syntax?

To me the best approach seems to be a list of items like:

ANY 2 (a,b) AS SiteA, ANY 2(c,d) AS SiteB
FIRST 2 (a,b,c) AS SiteA, d AS SiteB, e AS SiteC

You need a group name for the replication view as Michael noted, I suppose
you'd need to also allow no group name (one would be autogenerated??) for
backwards compatibility OR enforce group names when there are more than one
elements in the list.

I don't want to do embedded groups like:

ANY 2 (ANY 2 (a,b,c) AS SiteA, d as SiteB, c as SiteB)

Because that just seems like it's overcomplicating the issue.

thoughts?

James

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