Re: pgsql: Fix a couple of bugs in MultiXactId freezing

On Wed, Dec 4, 2013 at 8:43 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Magnus Hagander <magnus(at)hagander(dot)net> writes:
> > On Tue, Dec 3, 2013 at 7:20 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> >> I assume what would happen is the slave would PANIC upon seeing a WAL
> >> record code it didn't recognize.
>
> > I wonder if we should for the future have the START_REPLICATION command
> (or
> > the IDENTIFY_SYSTEM would probably make more sense - or even adding a new
> > command like IDENTIFY_CLIENT. The point is, something in the replication
> > protocol) have walreceiver include it's version sent to the master. That
> > way we could have the walsender identify a walreceiver that's too old and
> > disconnect it right away - with a much nicer error message than a PANIC.
>
> Meh. That only helps for the case of streaming replication, and not for
> the thirty-seven other ways that some WAL might arrive at something that
> wants to replay it.
>
> It might be worth doing anyway, but I can't get excited about it for this
> scenario.
>

It does, but I bet it's one of the by far most common cases. I'd say it's
that one and restore-from-backup that would cover a huge majority of all
cases. If we can cover those, we don't have to be perfect - so unless it
turns out to be ridiculously complicated, I think it would be worthwhile
having.

--
Magnus Hagander
Me: http://www.hagander.net/
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