Re: Logical Replication of sequences

On Tue, Jun 4, 2024 at 7:40 PM Ashutosh Bapat
<ashutosh(dot)bapat(dot)oss(at)gmail(dot)com> wrote:
>
> On Tue, Jun 4, 2024 at 4:27 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>>
>>
>> 3. Replicate published sequences via walsender at the time of shutdown
>> or incrementally while decoding checkpoint record. The two ways to
>> achieve this are: (a) WAL log a special NOOP record just before
>> shutting down checkpointer. Then allow the WALsender to read the
>> sequence data and send it to the subscriber while decoding the new
>> NOOP record. (b) Similar to the previous idea but instead of WAL
>> logging a new record directly invokes a decoding callback after
>> walsender receives a request to shutdown which will allow pgoutput to
>> read and send required sequences. This approach has a drawback that we
>> are adding more work at the time of shutdown but note that we already
>> waits for all the WAL records to be decoded and sent before shutting
>> down the walsender during shutdown of the node.
>>
>> Any other ideas?
>>
>
> In case of primary crash the sequence won't get replicated. That is true even with the previous approach in case walsender is shut down because of a crash, but it is more serious with this approach.
>

Right, but if we just want to support a major version upgrade scenario
then this should be fine because upgrades require a clean shutdown.

>
How about periodically sending this information?
>

Now, if we want to support some sort of failover then probably this
will help. Do you have that use case in mind? If we want to send
periodically then we can do it when decoding checkpoint
(XLOG_CHECKPOINT_ONLINE) or some other periodic WAL record like
running_xacts (XLOG_RUNNING_XACTS).

--
With Regards,
Amit Kapila.