Re: [PoC] pg_upgrade: allow to upgrade publisher node

On Thu, Oct 5, 2023 at 6:43 PM Bharath Rupireddy
<bharath(dot)rupireddyforpostgres(at)gmail(dot)com> wrote:
>
> On Thu, Oct 5, 2023 at 1:48 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
>
> > The other potential problem Andres pointed out is that during shutdown
> > if due to some reason, the walreceiver goes down, we won't be able to
> > send the required WAL and users won't be able to ensure that because
> > even after restart the same situation can happen. The ideal way is to
> > have something that puts the system in READ ONLY state during shutdown
> > and then we can probably allow walreceivers to reconnect and receive
> > the required WALs. As we don't have such functionality available and
> > it won't be easy to achieve the same, we can leave this for now.
> >
> > Thoughts?
>
> You mean walreceiver for streaming replication? Or the apply workers
> going down for logical replication?
>

Apply workers.

>
> If there's yet-to-be-sent-out WAL,
> pg_upgrade will fail no? How does the above scenario a problem for
> pg_upgrade of a cluster with just logical replication slots?
>

Even, if there is a WAL yet to be sent, the walsender will simply exit
as it will receive PqMsg_Terminate ('X') from standby. See
ProcessRepliesIfAny(). After that shutdown checkpoint will finish. So,
in this case upgrade can fail due to slots. But, I think the server
should be able to succeed in consecutive runs. Does this make sense?

--
With Regards,
Amit Kapila.