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

On Sat, Sep 6, 2025 at 3:58 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> On Tue, Sep 2, 2025 at 5:12 AM Shlok Kyal <shlok(dot)kyal(dot)oss(at)gmail(dot)com> wrote:
> >
> >
> > I tested the behaviour with HEAD and with Patch. And I confirmed the
> > change in behaviour between HEAD and Patch
> >
> > Suppose we have a primary and a standby with wal_level = logical and
> > guc parameters to enable slot sync worker are set accordingly. A slot
> > sync worker will be running.
> > Now we change the value of wal_level for primary to replica. And
> > restart the primary server
> >
> > With HEAD, during restart the existing sync_slot_worker will exit with:
> > 2025-09-02 11:49:08.846 IST [3877882] ERROR: synchronization worker
> > "" could not connect to the primary server: connection to server at
> > "localhost" (127.0.0.1), port 5432 failed: Connection refused
> > Is the server running on that host and accepting TCP/IP connections?
> > 2025-09-02 11:49:11.380 IST [3877885] FATAL: streaming replication
> > receiver "walreceiver" could not connect to the primary server:
> > connection to server at "localhost" (127.0.0.1), port 5432 failed:
> > Connection refused
> > Is the server running on that host and accepting TCP/IP connections?
> >
> > and after the restart of the primary server, slot sync worker will
> > restart and it is able to connect to the primary.
> >
> > With Patch, during restart the existing sync_slot_worker will exit.
> > But after the restart of the primary server, slot sync worker cannot
> > start and we can see following log:
> > 2025-09-02 12:44:51.497 IST [3947520] LOG: replication slot
> > synchronization worker is shutting down on receiving SIGINT
> > 2025-09-02 12:44:51.498 IST [3943504] LOG: replication slot
> > synchronization requires logical decoding to be enabled
> > 2025-09-02 12:44:51.498 IST [3943504] HINT: To enable logical
> > decoding on primary, set "wal_level" >= "logical" or create at least
> > one logical slot when "wal_level" = "replica".
> > 2025-09-02 12:45:51.537 IST [3943504] LOG: replication slot
> > synchronization requires logical decoding to be enabled
> > 2025-09-02 12:45:51.537 IST [3943504] HINT: To enable logical
> > decoding on primary, set "wal_level" >= "logical" or create at least
> > one logical slot when "wal_level" = "replica".
> >
> > So, with HEAD, after we restart the primary server with 'wal_level =
> > replica', the slot sync worker can restart and connect to the primary
> > but with patch it cannot start after restart due to the check in
> > ValidateSlotSyncParams.
>
> But the slotsync worker is launched again once logical decoding is
> enabled, no? I'm not sure that we want to launch the slotsync worker
> also when we know logical decoding is not enabled.
>

Why in the first place the logical_decoding enabled check has failed
because IIUC, the wal_level on standby is still 'logical'?

--
With Regards,
Amit Kapila.