Re: allow specifying action when standby encounters incompatible parameter settings

Thanks for taking a look!

On Thu, Apr 14, 2022 at 11:36:11AM +0900, Kyotaro Horiguchi wrote:
> At Wed, 13 Apr 2022 14:35:21 -0700, Nathan Bossart <nathandbossart(at)gmail(dot)com> wrote in
>> I initially set out to see if it was possible to automatically adjust these
>> parameters on a standby, but that is considerably more difficult. It isn't
>> enough to just hook into the restart_after_crash functionality since it
>> doesn't go back far enough in the postmaster logic. IIUC we'd need to
>> reload preloaded libraries (which there is presently no support for),
>> recalculate MaxBackends, etc. Another option I considered was to
>
> Sure.
>
>> automatically adjust the parameters during startup so that you just need to
>> restart the server. However, we need to know for sure that the server is
>> going to be a hot standby, and I don't believe we have that information
>> where such GUC changes would need to occur (I could be wrong about this).
>
> Conldn't we use AlterSystemSetConfigFile for this purpose in
> CheckRequiredParameterValues?

That's an interesting idea. When an incompatible parameter change is
detected, the server would automatically run the equivalent of an ALTER
SYSTEM SET command before shutting down or pausing replay. I might draft
up a proof-of-concept to see what this looks like and how well it works.

>> Anyway, for now I'm just proposing the modest change described above, but
>> I'd welcome any discussion about improving matters further in this area.
>>
>> [0] https://postgr.es/m/4ad69a4c-cc9b-0dfe-0352-8b1b0cd36c7b%402ndquadrant.com
>
> Is the reason for the enum the extensibility to add a new choice like
> "auto-adjust"?

Yes.

--
Nathan Bossart
Amazon Web Services: https://aws.amazon.com