Re: Support for N synchronous standby servers

On 08/28/2014 10:10 AM, Michael Paquier wrote:
> + #synchronous_standby_num = -1 # number of standbys servers using sync rep

To be honest, that's a horrible name for the GUC. Back when synchronous
replication was implemented, we had looong discussions on this feature.
It was called "quorum commit" back then. I'd suggest using the "quorum"
term in this patch, too, that's a fairly well-known term in distributed
computing for this.

When synchronous replication was added, quorum was left out to keep
things simple; the current feature set was the most we could all agree
on to be useful. If you search the archives for "quorum commit" you'll
see what I mean. There was a lot of confusion on what is possible and
what is useful, but regarding this particular patch: people wanted to be
able to describe more complicated scenarios. For example, imagine that
you have a master and two standbys in one the primary data center, and
two more standbys in a different data center. It should be possible to
specify that you must get acknowledgment from at least on standby in
both data centers. Maybe you could hack that by giving the standbys in
the same data center the same name, but it gets ugly, and it still won't
scale to even more complex scenarios.

Maybe that's OK - we don't necessarily need to solve all scenarios at
once. But it's worth considering.

BTW, how does this patch behave if there are multiple standbys connected
with the same name?

- Heikki