Re: logical decoding and replication of sequences, take 2

On 7/31/23 11:25, Amit Kapila wrote:
> On Sat, Jul 29, 2023 at 5:53 PM Tomas Vondra
> <tomas(dot)vondra(at)enterprisedb(dot)com> wrote:
>>
>> On 7/28/23 14:44, Ashutosh Bapat wrote:
>>> On Wed, Jul 26, 2023 at 8:48 PM Tomas Vondra
>>> <tomas(dot)vondra(at)enterprisedb(dot)com> wrote:
>>>>
>>>> Anyway, I was thinking about this a bit more, and it seems it's not as
>>>> difficult to use the page LSN to ensure sequences don't go backwards.
>>>> The 0005 change does that, by:
>>>>
>>>> 1) adding pg_sequence_state, that returns both the sequence state and
>>>> the page LSN
>>>>
>>>> 2) copy_sequence returns the page LSN
>>>>
>>>> 3) tablesync then sets this LSN as origin_startpos (which for tables is
>>>> just the LSN of the replication slot)
>>>>
>>>> AFAICS this makes it work - we start decoding at the page LSN, so that
>>>> we skip the increments that could lead to the sequence going backwards.
>>>>
>>>
>>> I like this design very much. It makes things simpler than complex.
>>> Thanks for doing this.
>>>
>>
>> I agree it seems simpler. It'd be good to try testing / reviewing it a
>> bit more, so that it doesn't misbehave in some way.
>>
>
> Yeah, I also think this needs a review. This is a sort of new concept
> where we don't use the LSN of the slot (for cases where copy returned
> a larger value of LSN) or a full_snapshot created corresponding to the
> sync slot by Walsender. For the case of the table, we build a full
> snapshot because we use that for copying the table but why do we need
> to build that for copying the sequence especially when we directly
> copy it from the sequence relation without caring for any snapshot?
>

We need the slot to decode/apply changes during catchup. The main
subscription may get ahead, and we need to ensure the WAL is not
discarded or something like that. This applies even if the initial sync
step does not use the slot/snapshot directly.

regards

--
Tomas Vondra
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