Re: Conflict Detection and Resolution - Mailing list pgsql-hackers

On Fri, Jun 7, 2024 at 5:39 PM Ashutosh Bapat
<ashutosh.bapat.oss@gmail.com> wrote:
>
> On Thu, Jun 6, 2024 at 5:16 PM Nisha Moond <nisha.moond412@gmail.com> wrote:
>>
>> >
>>
>> Here are more use cases of the "earliest_timestamp_wins" resolution method:
>> 1) Applications where the record of first occurrence of an event is
>> important. For example, sensor based applications like earthquake
>> detection systems, capturing the first seismic wave's time is crucial.
>> 2) Scheduling systems, like appointment booking, prioritize the
>> earliest request when handling concurrent ones.
>> 3) In contexts where maintaining chronological order is important -
>>   a) Social media platforms display comments ensuring that the
>> earliest ones are visible first.
>>   b) Finance transaction processing systems rely on timestamps to
>> prioritize the processing of transactions, ensuring that the earliest
>> transaction is handled first
>
>
> Thanks for sharing examples. However, these scenarios would be handled by the application and not during replication. What we are discussing here is the timestamp when a row was updated/inserted/deleted (or rather when the transaction that updated row committed/became visible) and not a DML on column which is of type timestamp. Some implementations use a hidden timestamp column but that's different from a user column which captures timestamp of (say) an event. The conflict resolution will be based on the timestamp when that column's value was recorded in the database which may be different from the value of the column itself.
>

It depends on how these operations are performed. For example, the
appointment booking system could be prioritized via a transaction
updating a row with columns emp_name, emp_id, reserved, time_slot.
Now, if two employees at different geographical locations try to book
the calendar, the earlier transaction will win.

> If we use the transaction commit timestamp as basis for resolution, a transaction where multiple rows conflict may end up with different rows affected by that transaction being resolved differently. Say three transactions T1, T2 and T3 on separate origins with timestamps t1, t2, and t3 respectively changed rows r1, r2 and r2, r3 and r1, r4 respectively. Changes to r1 and r2 will conflict. Let's say T2 and T3 are applied first and then T1 is applied. If t2 < t1 < t3, r1 will end up with version of T3 and r2 will end up with version of T1 after applying all the three transactions.
>

Are you telling the results based on latest_timestamp_wins? If so,
then it is correct. OTOH, if the user has configured
"earliest_timestamp_wins" resolution method, then we should end up
with a version of r1 from T1 because t1 < t3. Also, due to the same
reason, we should have version r2 from T2.

>
 Would that introduce an inconsistency between r1 and r2?
>

As per my understanding, this shouldn't be an inconsistency. Won't it
be true even when the transactions are performed on a single node with
the same timing?