Re: pgsql: walreceiver uses a temporary replication slot by default

On Mon, Feb 10, 2020 at 01:46:04PM -0800, Andres Freund wrote:
> I still architecturally don't find it attractive that the active
> configuration between walreceiver and startup process can diverge
> though. Imagine if we e.g. added the ability to receive WAL over
> multiple connections from one host, or from multiple hosts (e.g. to be
> able to get the bulk of the WAL from a cascading node, but also to
> provide syncrep acknowledgements directly to the primary), or to allow
> for logical replication without needing all WAL locally on a standby
> doing decoding. It seems not great if there's potentially diverging
> configuration (hot standby feedback, temporary slots, ... ) between
> those walreceivers, just depending on when they started. Here the model
> e.g. parallel workers use, which explicitly ensure that the GUC state is
> the same in workers and the leader, is considerably better, imo.

Yes, I still think that we should fix that inconsistency, mark the new
GUC wal_receiver_create_temp_slot as PGC_POSTMASTER, and add a note at
the top of RequestXLogStreaming() and walreceiver.c about the
assumptions we'd prefer rely to for the GUCs starting a WAL receiver.

> So I think adding more of these parameters affecting walreceivers
> without coordination is not going quite in the right direction.

Indeed. Adding more comments would be one way to prevent the
situation to happen here, I fear that others may forget this stuff in
the future.
--
Michael