Re: [Patch] pg_rewind: options to use restore_command from recovery.conf or command line

On Mon, Oct 22, 2018 at 02:19:07PM -0300, Alvaro Herrera wrote:
> Hmm, I remember we had a project to have a new postmaster option that
> would report the value of some GUC option, so instead of parsing the
> file in the frontend, you'd invoke the backend to do the parsing. But I
> don't know what became of that ... I don't see it in the postmaster
> --help output.

I did not recall this one.

> Of course, recovery.conf options are not GUCs either ... that's another
> pending patch.

But I do recall this one. Still isn't it q bit different? Because in
the case of pg_rewind let's remember that the origin cluster can be
offline or online, and that the target has to be offline. I am also not
sure that we would want to use the same command restore_command with
pg_rewind and an instance in recovery.

Something that we could think about is directly to provide a command to
pg_rewind via command line. Another possibility would be to have a
separate tool which scans a data folder and fetches by itself a range of
WAL segments wanted. I have implemented something like that for an
internal solution, able to handle as well timeline jumps across
segments with a server-side and a client-side implementation (that was
to allow a passive to catch up using a set of WAL segments, if the
active does not have a replication slot, and segments were fetched
through a Postgres instance which has a local archive).
--
Michael