Re: Skipping logical replication transactions on subscriber side

On Wed, Jan 26, 2022 at 12:14 AM David G. Johnston
<david.g.johnston@gmail.com> wrote:
>
>
> On Tue, Jan 25, 2022 at 8:09 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>>
>> On Tue, Jan 25, 2022 at 11:58 PM David G. Johnston
>> <david.g.johnston@gmail.com> wrote:
>> >
>> > On Tue, Jan 25, 2022 at 7:47 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>> >>
>> >> Yeah, I think it's a good idea to clear the subskipxid after the first
>> >> transaction regardless of whether the worker skipped it.
>> >>
>> >
>> > So basically instead of stopping the worker with an error you suggest having the worker continue applying changes
(afterresetting subskipxid, and - arguably - the ?_error_* fields).  Log the transaction xid mis-match as a warning in
thelog file as opposed to an error. 
>>
>> Agreed, I think it's better to log a warning than to raise an error.
>> In the case where the user specified the wrong XID, the worker should
>> fail again due to the same error.
>>
>
> If it remains possible for the system to accept a wrongly specified XID I would agree that this behavior is
preferable. At least when the user wonders why the skip didn't work and they are seeing the same error again they will
havea log entry warning telling them their XID choice was incorrect. 

Yes.

>  I would prefer that the system not accept a wrongly specified XID and the user be told directly and sooner that
theirXID choice was incorrect. 

Given that we cannot use rely on the pg_stat_subscription_workers view
for this purpose, we would need either a new sub-system that tracks
each logical replication status so the system can set the error XID to
subskipxid, or to wait for shared-memory based stats collector. While
agreeing that ideally, we need such a sub-system I'm concerned that
everyone will agree to add complexity for this feature. That having
been said, if there is a significant need for it, we can implement it
as an improvement.



Regards,

--
Masahiko Sawada
EDB:  https://www.enterprisedb.com/