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

On Thu, Nov 27, 2025 at 11:23 PM vignesh C <vignesh21(at)gmail(dot)com> wrote:
>
>
> Few comments:
> 1) There is no validation after creation of temporary logical
> replication slot, can we see if "logical decoding is enabled upon
> creating a new logical replication slot" is logged as a validation for
> this:
> +# Create a temporary logical slot but exits without releasing it explicitly.
> +# This enables logical decoding but skips disabling it and delegates to the
> +# checkpointer.
> +$primary->safe_psql('postgres',
> + qq[select pg_create_logical_replication_slot('test_tmp_slot',
> 'test_decoding', true)]
> +);
> +
> +# Wait for the checkpointer to disable logical decoding.
> +wait_for_logical_decoding_disabled($primary);
>
> I had a look at the header file inclusions in the patch and found few
> issues with it:
> 2) Here logicalctl.h should be included before logicallauncher.h:
> #include "postmaster/interrupt.h"
> #include "replication/logicallauncher.h"
> +#include "replication/logicalctl.h"
> #include "replication/slotsync.h"
>
> 3) I was able to compile without inclusion of logicalctl.h, may be it
> is not required because we include origin.h which includes xlog.h
> which includes logicalctl.h already:
> diff --git a/src/backend/access/transam/xlog.c
> b/src/backend/access/transam/xlog.c
> index 22d0a2e8c3a..6f3f10af8fd 100644
> --- a/src/backend/access/transam/xlog.c
> +++ b/src/backend/access/transam/xlog.c
> @@ -79,7 +79,9 @@
> #include "postmaster/walsummarizer.h"
> #include "postmaster/walwriter.h"
> #include "replication/origin.h"
> +#include "replication/logicalctl.h"
>
> 4) Similarly it is not required in checkpointer.c, logical.c,
> logicalctl.c, slotsync.c, slot.c, slotfuncs.c, walsender.c, ipci.c,
> procsignal.c, standby.c and postinit.c
>
> 5) Few comments in the test file exceeds 80 chars, if possible we can
> move it to the next line:
> 5.a) +# Create and drop another logical slot, then check if
> effective_wal_level remains
> +# 'logical'.
> 5.b) +# Add other settings to test if we disable logical decoding when
> invalidating the last
> +# logical slot.
> 5.c) # Check if logical decoding is disabled after invalidating the
> last logical slot.
> 5.d) # Check if effective_wal_level is increased to 'logical' on the
> cascaded standby.
> 5.e) # Check that the logical decoding is not enabled on the standby4.
> Note that it still has
> # the invalidated logical slot.
> 5.f) # Restart the primary with setting wal_level = 'logical' and
> create a new logical
> # slot.
> 5.g) # Drop the logical slot, requesting to disable logical decoding
> to the checkpointer.
> 5.h) # Test the abort process of logical decoding activation. We drop
> the primary's
> # slot to decrease its effective_wal_level to 'replica'.

I've fixed the above points and run pgperltidy again. I'll submit the
updated patch soon.

Regards,

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