Re: Handle infinite recursion in logical replication setup

On Wed, Aug 17, 2022 at 8:48 AM houzj(dot)fnst(at)fujitsu(dot)com
<houzj(dot)fnst(at)fujitsu(dot)com> wrote:
>
> On Tuesday, August 2, 2022 8:00 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > On Tue, Jul 26, 2022 at 9:07 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> Thanks for the summary.
>
> I think it's fine to make the user use the copy_data option more carefully to
> prevent duplicate copies by reporting an ERROR.
>
> But I also have similar concern with Sawada-san as it's possible for user to
> receive an ERROR in some unexpected cases.
>
> For example I want to build bi-directional setup between two nodes:
>
> Node A: TABLE test (has actual data)
> Node B: TABLE test (empty)
>
> Step 1:
> CREATE PUBLICATION on both Node A and B.
>
> Step 2:
> CREATE SUBSCRIPTION on Node A with (copy_data = on)
> -- this is fine as there is no data on Node B
>
> Step 3:
> CREATE SUBSCRIPTION on Node B with (copy_data = on)
> -- this should be fine as user needs to copy data from Node A to Node B,
> -- but we still report an error for this case.
>
> It looks a bit strict to report an ERROR in this case and it seems not easy to
> avoid this. So, personally, I think it might be better to document the correct
> steps to build the bi-directional replication and probably also docuemnt the
> steps to recover if user accidently did duplicate initial copy if not
> documented yet.
>
> In addition, we could also LOG some additional information about the ORIGIN and
> initial copy which might help user to analyze if needed.
>

But why LOG instead of WARNING? I feel in this case there is a chance
of inconsistent data so a WARNING like "publication "pub1" could have
data from multiple origins" can be given when the user has specified
options: "copy_data = on, origin = NONE" while creating a
subscription. We give a WARNING during subscription creation when the
corresponding publication doesn't exist, eg.

postgres=# create subscription sub1 connection 'dbname = postgres'
publication pub1;
WARNING: publication "pub1" does not exist in the publisher

Then, we can explain in docs how users can avoid data inconsistencies
while setting up replication.

--
With Regards,
Amit Kapila.