Re: Switching XLog source from archive to streaming when primary available

On Sun, Oct 09, 2022 at 02:39:47PM +0530, Bharath Rupireddy wrote:
> We can give it a chance to restore from pg_wal before switching to
> streaming to not change any behaviour of the state machine. But, not
> definitely by setting currentSource to XLOG_FROM_WAL, we basically
> never explicitly set currentSource to XLOG_FROM_WAL, other than when
> not in archive recovery i.e. InArchiveRecovery is false. Also, see the
> comment [1].
>
> Instead, the simplest would be to just pass XLOG_FROM_WAL to
> XLogFileReadAnyTLI() when we're about to switch the source to stream
> mode. This doesn't change the existing behaviour.

It might be more consistent with existing behavior, but one thing I hadn't
considered is that it might make your proposed feature ineffective when
users are copying files straight into pg_wal. IIUC as long as the files
are present in pg_wal, the source-switch logic won't kick in.

> Unrelated to this patch, the fact that the standby polls pg_wal is not
> documented or recommended, is not true, it is actually documented [2].
> Whether or not we change the docs to be something like [3], is a
> separate discussion.

I wonder if it would be better to simply remove this extra polling of
pg_wal as a prerequisite to your patch. The existing commentary leads me
to think there might not be a strong reason for this behavior, so it could
be a nice way to simplify your patch.

--
Nathan Bossart
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