Re: Minimal logical decoding on standbys

On 2019-05-27 17:04:44 +0530, Amit Khandekar wrote:
> On Fri, 24 May 2019 at 21:00, Amit Khandekar <amitdkhan(dot)pg(at)gmail(dot)com> wrote:
> >
> > On Fri, 24 May 2019 at 19:26, Amit Khandekar <amitdkhan(dot)pg(at)gmail(dot)com> wrote:
> > > Working on the patch now ....
> >
> > Attached is an incremental WIP patch
> > handle_wal_level_changes_WIP.patch to be applied over the earlier main
> > patch logical-decoding-on-standby_v4_rebased.patch.
>
> I found an issue with these changes : When we change master wal_level
> from logical to hot_standby, and again back to logical, and then
> create a logical replication slot on slave, it gets created; but when
> I do pg_logical_slot_get_changes() with that slot, it seems to read
> records *before* I created the logical slot, so it encounters
> parameter-change(logical=>hot_standby) record, so returns an error as
> per the patch, because now in DecodeXLogOp() I error out when
> XLOG_PARAMETER_CHANGE is found :

> @@ -190,11 +190,23 @@ DecodeXLogOp(LogicalDecodingContext *ctx,
> XLogRecordBuffer *buf)
> * can restart from there.
> */
> break;
> + case XLOG_PARAMETER_CHANGE:
> + {
> + xl_parameter_change *xlrec =
> + (xl_parameter_change *) XLogRecGetData(buf->record);
> +
> + /* Cannot proceed if master itself does not have logical data */
> + if (xlrec->wal_level < WAL_LEVEL_LOGICAL)
> + ereport(ERROR,
> + (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
> + errmsg("logical decoding on standby requires "
> + "wal_level >= logical on master")));
> + break;
> + }
>
> I thought it won't read records *before* the slot was created. Am I
> missing something ?

That's why I had mentioned that you'd need to adapt
ReplicationSlotReserveWal(), to use the replay LSN or such.

Greetings,

Andres Freund