Re: Time delayed LR (WAS Re: logical replication restrictions)

On Tue, Feb 28, 2023 at 9:21 PM Hayato Kuroda (Fujitsu)
<kuroda(dot)hayato(at)fujitsu(dot)com> wrote:
>
> > 1.
> > + /*
> > + * If we've requested to shut down, exit the process.
> > + *
> > + * Note that WalSndDone() cannot be used here because the delaying
> > + * changes will be sent in the function.
> > + */
> > + if (got_STOPPING)
> > + {
> > + QueryCompletion qc;
> > +
> > + /* Inform the standby that XLOG streaming is done */
> > + SetQueryCompletion(&qc, CMDTAG_COPY, 0);
> > + EndCommand(&qc, DestRemote, false);
> > + pq_flush();
> >
> > Do we really need to do anything except for breaking the loop and let
> > the exit handling happen in the main loop when 'got_STOPPING' is set?
> > AFAICS, this is what we are doing in some other palces (See
> > WalSndWaitForWal). Won't that work? It seems that will help us sending
> > all the pending WAL.
>
> If we exit the loop after got_STOPPING is set, as you said, the walsender will
> send delaying changes and then exit. The behavior is same as the case that WalSndDone()
> is called. But I think it is not suitable for the motivation of the feature.
> If users notice the miss operation like TRUNCATE, they must shut down the publisher
> once and then recovery from back up or old subscriber. If the walsender sends all
> pending changes, miss operations will be also propagated to subscriber and data
> cannot be protected. So currently I want to keep the style.
> FYI - In case of physical replication, received WALs are not applied when the
> secondary is shutted down.
>

Fair point but I think the current comment should explain why we are
doing something different here. How about extending the existing
comments to something like: "If we've requested to shut down, exit the
process. This is unlike handling at other places where we allow
complete WAL to be sent before shutdown because we don't want the
delayed transactions to be applied downstream. This will allow one to
use the data from downstream in case of some unwanted operations on
the current node."

--
With Regards,
Amit Kapila.