Re: Support logical replication of DDLs

On Mon, Apr 11, 2022 at 6:16 PM Euler Taveira <euler(at)eulerto(dot)com> wrote:
>
> On Mon, Apr 11, 2022, at 2:00 AM, Amit Kapila wrote:
>
> On Thu, Apr 7, 2022 at 3:46 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > On Wed, Mar 23, 2022 at 10:39 AM Japin Li <japinli(at)hotmail(dot)com> wrote:
> >
> > 2. For DDL replication, do we need to wait for a consistent point of
> > snapshot? For DMLs, that point is a convenient point to initialize
> > replication from, which is why we export a snapshot at that point,
> > which is used to read normal data. Do we have any similar needs for
> > DDL replication?
> >
>
> I have thought a bit more about this and I think we need to build the
> snapshot for DML replication as we need to read catalog tables to
> decode the corresponding WAL but it is not clear to me if we have a
> similar requirement for DDL replication. If the catalog access is
> required then it makes sense to follow the current snapshot model,
> otherwise, we may need to think differently for DDL replication.
>
> One more related point is that for DML replication, we do ensure that
> we copy the entire data of the table (via initial sync) which exists
> even before the publication for that table exists, so do we want to do
> something similar for DDLs? How do we sync the schema of the table
> before the user has defined the publication? Say the table has been
> created before the publication is defined and after that, there are
> only Alter statements, so do we expect, users to create the table on
> the subscriber and then we can replicate the Alter statements? And
> even if we do that it won't be clear which Alter statements will be
> replicated after publication is defined especially if those Alters
> happened concurrently with defining publications?
>
> The *initial* DDL replication is a different problem than DDL replication. The
> former requires a snapshot to read the current catalog data and build a CREATE
> command as part of the subscription process. The subsequent DDLs in that object
> will be handled by a different approach that is being discussed here.
>

I think they are not completely independent because of the current way
to do initial sync followed by replication. The initial sync and
replication need some mechanism to ensure that one of those doesn't
overwrite the work done by the other. Now, the initial idea and patch
can be developed separately but I think both the patches have some
dependency.

--
With Regards,
Amit Kapila.