Re: [HACKERS] make async slave to wait for lsn to be replayed

On Thu, Mar 7, 2024 at 5:14 PM Kartyshov Ivan
<i(dot)kartyshov(at)postgrespro(dot)ru> wrote:
>
> Intro
> ==========
> The main purpose of the feature is to achieve
> read-your-writes-consistency, while using async replica for reads and
> primary for writes. In that case lsn of last modification is stored
> inside application. We cannot store this lsn inside database, since
> reads are distributed across all replicas and primary.
>
>
> Two implementations of one feature
> ==========
> We left two different solutions. Help me please to choose the best.
>
>
> 1) Classic (wait_classic_v7.patch)
> https://www.postgresql.org/message-id/3cc883048264c2e9af022033925ff8db%40postgrespro.ru
> Synopsis
> ==========
> advantages: multiple wait events, separate WAIT FOR statement
> disadvantages: new words in grammar
>
>
>
> WAIT FOR [ANY | ALL] event [, ...]
> BEGIN [ WORK | TRANSACTION ] [ transaction_mode [, ...] ]
> [ WAIT FOR [ANY | ALL] event [, ...]]
> event:
> LSN value
> TIMEOUT number_of_milliseconds
> timestamp
>
>
>
> 2) After style: Kyotaro and Freund (wait_after_within_v6.patch)
> https://www.postgresql.org/message-id/d3ff2e363af60b345f82396992595a03%40postgrespro.ru
> Synopsis
> ==========
> advantages: no new words in grammar
> disadvantages: a little harder to understand, fewer events to wait
>
>
>
> AFTER lsn_event [ WITHIN delay_milliseconds ] [, ...]
> BEGIN [ WORK | TRANSACTION ] [ transaction_mode [, ...] ]
> [ AFTER lsn_event [ WITHIN delay_milliseconds ]]
> START [ WORK | TRANSACTION ] [ transaction_mode [, ...] ]
> [ AFTER lsn_event [ WITHIN delay_milliseconds ]]
>

+1 for the second one not only because it avoids new words in grammar
but also sounds to convey the meaning. I think you can explain in docs
how this feature can be used basically how will one get the correct
LSN value to specify.

As suggested previously also pick one of the approaches (I would
advocate the second one) and keep an option for the second one by
mentioning it in the commit message. I hope to see more
reviews/discussions or usage like how will users get the LSN value to
be specified on the core logic of the feature at this stage. IF
possible, state, how real-world applications could leverage this
feature.

--
With Regards,
Amit Kapila.