Re: recovery modules

On Tue, Dec 27, 2022 at 02:11:11PM -0800, Andres Freund wrote:
> On 2022-12-27 11:24:49 -0800, Nathan Bossart wrote:
>> I've attached a patch set that adds the restore_library,
>> archive_cleanup_library, and recovery_end_library parameters to allow
>> archive recovery via loadable modules. This is a follow-up to the
>> archive_library parameter added in v15 [0] [1].
>
> Why do we need N parameters for this? To me it seems more sensible to have one
> parameter that then allows a library to implement all these (potentially
> optionally).

The main reason is flexibility. Separate parameters allow using a library
for one thing and a command for another, or different libraries for
different things. If that isn't a use-case we wish to support, I don't
mind combining all three into a single recovery_library parameter.

>> * Unlike archive modules, recovery libraries cannot be changed at runtime.
>> There isn't a safe way to unload a library, and archive libraries work
>> around this restriction by restarting the archiver process. Since recovery
>> libraries are loaded via the startup and checkpointer processes (which
>> cannot be trivially restarted like the archiver), the same workaround is
>> not feasible.
>
> I don't think that's a convincing reason to not support configuration
> changes. Sure, libraries cannot be unloaded, but an unnecessarily loaded
> library is cheap. All that's needed is to redirect the relevant function
> calls.

This might leave some stuff around (e.g., GUCs, background workers), but if
that isn't a concern, I can adjust it to work as you describe.

>> * pg_rewind uses restore_command, but there isn't a straightforward path to
>> support restore_library. I haven't addressed this in the attached patches,
>> but perhaps this is a reason to allow specifying both restore_command and
>> restore_library at the same time. pg_rewind would use restore_command, and
>> the server would use restore_library.
>
> That seems problematic, leading to situations where one might not be able to
> use restore_command anymore, because it's not feasible to do
> segment-by-segment restoration.

I'm not following why this would make segment-by-segment restoration
infeasible. Would you mind elaborating?

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