Re: pg_receivewal starting position

At Wed, 1 Sep 2021 10:30:05 +0530, Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com> wrote in 
> On Mon, Aug 30, 2021 at 3:26 PM Ronan Dunklau <ronan.dunklau@aiven.io> wrote:
> > > 7) How about we let pg_receivewal use READ_REPLICATION_SLOT as an option?
> >
> > From my point of view, I already expected it to use something like that when
> > using a replication slot. Maybe an option to turn it off could be useful ?
> 
> IMO, pg_receivewal should have a way to turn off/on using
> READ_REPLICATION_SLOT. Imagine if the postgres server doesn't support
> READ_REPLICATION_SLOT (a lower version) but for some reasons the
> pg_receivewal(running separately) is upgraded to the version that uses
> READ_REPLICATION_SLOT, knowing that the server doesn't support
> READ_REPLICATION_SLOT why should user let pg_receivewal run an
> unnecessary code?

If I read the patch correctly the situation above is warned by the
following message then continue to the next step giving up to identify
start position from slot data.

> "server does not suport fetching the slot's position, resuming from the current server position instead"

(The message looks a bit too long, though..)

However, if the operator doesn't know the server is old, pg_receivewal
starts streaming from unexpected position, which is a kind of
disaster. So I'm inclined to agree to Bharath, but rather I imagine of
an option to explicitly specify how to determine the start position.

--start-source=[server,wal,slot]  specify starting-LSN source, default is
                     trying all of them in the order of wal, slot, server. 

I don't think the option doesn't need to accept multiple values at once.

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center