Re: POC: enable logical decoding when wal_level = 'replica' without a server restart

On Thu, Sep 11, 2025 at 10:46 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> On Wed, Sep 10, 2025 at 11:32 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > On Sat, Sep 6, 2025 at 3:46 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > >
> > > I've attached the updated patch that incorporated all comments I got so far.
> > >
> >
> > *
> > + /*
> > + * While all processes are using the new status, there could be some
> > + * transactions that might have started with the old status. So wait
> > + * for the running transactions to complete so that logical decoding
> > + * doesn't include transactions that wrote WAL with insufficient
> > + * information.
> > + */
> > + running = GetRunningTransactionData();
> > + LWLockRelease(ProcArrayLock);
> > + LWLockRelease(XidGenLock);
> > +
> > + elog(DEBUG1, "waiting for %d transactions to complete", running->xcnt);
> > +
> > + for (int i = 0; i < running->xcnt; i++)
> > + {
> > + TransactionId xid = running->xids[i];
> > +
> > + if (TransactionIdIsCurrentTransactionId(xid))
> > + continue;
> > +
> > + XactLockTableWait(xid, NULL, NULL, XLTW_None);
> > + }
> >
> > When building a snapshot during the start of logical decoding, we
> > anyway wait for running transactions to finish via the snapbuild
> > machinery. So, why do we need it here? And if it is needed, can we
> > update the comments to explain why it is required in spite of
> > snapbuild machinery doing similar thing?
>
> Fair point. I don't see any reason we need to wait here. Will remove this step.
>
> > * Is it a good idea to enable/disable decoding for temporary logical
> > slots? The temporary slots are released during ERROR or at session
> > end, is that a good time to do the disable processing that even
> > requires WAL writing.
>
> I think the same is true for slots with RS_EPEMERAL state.

Just to be clear, I meant a case like where one logical slot is
already present and the slot is removed between when another newly
created logical slot is created with RS_EPHEMERAL state and removed
due to an error. In this case, the ephemeral slot is the last logical
replication slot to drop.

Regards,

--
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com