On Mon, Jun 8, 2009 at 7:34 PM, Tom Lane<tgl@sss.pgh.pa.us> wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
>> I see that you've carefully not quoted Greg's remark about "mechanism
>> not policy" with which I completely agree.
>
> Mechanism should exist to support useful policy. I don't believe that
> the proposed switch has any real-world usefulness.
I guess I agree that it doesn't seem to make much sense to trigger
failover on a DB crash, as the OP suggested. The most likely cause of
a DB crash is probably a software bug, in which case failover isn't
going to help (won't you just trigger the same bug on the standby
server?). The case where you'd probably want to do failover is when
the whole server has gone down to a hardware or power failure, in
which case your hypothetical home-grown supervisor process won't be
able to run anyway.
But I'm still not 100% convinced that the proposed mechanism is
useless. There might be other reasons to want to get control in the
event of a crash. You might want to page the system administrator, or
trigger a filesystem snapshot so you can go back and do a post-mortem.(The former could arguably be done just as well
byscanning the log
file for the relevant log messages, I suppose, but the latter
certainly couldn't be, if your goal is to get a snapshot before
recovery is done.)
But maybe I'm all wet...
...Robert