Re: How should the primary behave when the sync standby goes away? Re: Sync Rep v17

On Fri, 2011-03-04 at 16:57 +0900, Fujii Masao wrote: 
> On Wed, Mar 2, 2011 at 11:30 PM, Fujii Masao <masao.fujii@gmail.com> wrote:
> > On Wed, Mar 2, 2011 at 8:22 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
> >> The WALSender deliberately does *not* wake waiting users if the standby
> >> disconnects. Doing so would break the whole reason for having sync rep
> >> in the first place. What we do is allow a potential standby to takeover
> >> the role of sync standby, if one is available. Or the failing standby
> >> can reconnect and then release waiters.
> >
> > If there is potential standby when synchronous standby has gone, I agree
> > that it's not good idea to release the waiting backends soon. In this case,
> > those backends should wait for next synchronous standby.
> >
> > On the other hand, if there is no potential standby, I think that the waiting
> > backends should not wait for the timeout and should wake up as soon as
> > synchronous standby has gone. Otherwise, those backends suspend for
> > a long time (i.e., until the timeout expires), which would decrease the
> > high-availability, I'm afraid.
> >
> > Keeping those backends waiting for the failed standby to reconnect is an
> > idea. But this looks like the behavior for "allow_standalone_primary = off".
> > If allow_standalone_primary = on, it looks more natural to make the
> > primary work alone without waiting the timeout.
> 
> Also I think that the waiting backends should be released as soon as the
> last synchronous standby switches to asynchronous mode. Since there is
> no standby which is planning to reconnect, obviously they no longer need
> to wait.

I've not done this, but we could.

It can't run in a WALSender, so this code would need to live in either
WALWriter or BgWriter.

-- Simon Riggs           http://www.2ndQuadrant.com/books/PostgreSQL Development, 24x7 Support, Training and Services