Re: replication_origin and replication_origin_lsn usage on subscriber

Hi,

On 14/07/2020 10:29, Amit Kapila wrote:
> On Tue, Jul 14, 2020 at 12:05 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
>>
>> On Tue, Jul 14, 2020 at 11:14 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>>>
>>> On Tue, Jul 14, 2020 at 11:00 AM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
>>>>
>>>> On Thu, Jul 9, 2020 at 6:55 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>>>>>
>>>>> On Thu, Jul 9, 2020 at 6:14 PM Petr Jelinek <petr(at)2ndquadrant(dot)com> wrote:
>>>>>>
>>>>>>
>>>>>> If we were to support the origin forwarding, then strictly speaking we
>>>>>> need everything only at commit time from correctness perspective,
>>>>>>
>>>>>
>>>>> Okay. Anyway streaming mode is optional, so in such cases, we can keep it 'off'
>>>>>
>>>>>> but
>>>>>> ideally origin_id would be best sent with first message as it can be
>>>>>> used to filter out changes at decoding stage rather than while we
>>>>>> process the commit so having it set early improves performance of decoding.
>>>>>>
>>>>>
>>>>> Yeah, makes sense. So, we will just send origin_id (with first
>>>>> streaming start message) and leave others.
>>>>
>>>> So IIUC, currently we are sending the latest origin_id which is set
>>>> during the commit time. So in our case, while we start streaming we
>>>> will send the origin_id of the latest change in the current stream
>>>> right?
>>>>
>>>
>>> It has to be sent only once with the first start message not with
>>> consecutive start messages.
>>
>> Okay, so do you mean to say that with the first start message we send
>> the origin_id of the latest change?
>>
>
> Yes.
>
>> because during the transaction
>> lifetime, the origin id can be changed.
>>
>
> Yeah, it could be changed but if we have to send again apart from with
> the first message then it should be sent with each message. So, I
> think it is better to just send it once during the transaction as we
> do it now (send with begin message).
>
>

I am not sure if I can follow the discussion here very well, but if I
understand correctly I'd like to clarify two things:
- origin id does not change mid transaction as you can only have one per xid
- until we have origin forwarding feature, the origin id is always same
for a given subscription

--
Petr Jelinek
2ndQuadrant - PostgreSQL Solutions for the Enterprise
https://www.2ndQuadrant.com/