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Date: Thu, 3 Sep 2026 21:21:41 +0200
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Subject: Re: pg_rewind does not rewind diverging timelines
To: Andrey Borodin <x4mmm@yandex-team.ru>
Cc: Japin Li <japinli@hotmail.com>,
 pgsql-hackers mailing list <pgsql-hackers@lists.postgresql.org>
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From: Mats Kindahl <mats.kindahl@gmail.com>
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Hi Andrey, and thank you for the comments.

On 8/30/26 19:35, Andrey Borodin wrote:

> Hi Mats,
>
> On Tue, Aug 12, 2026, Mats Kindahl wrote:
>> Even with a shared archive, it is impossible to ensure such things. It
>> is a normal consensus problem.
> This race came up again in an open-space discussion here. What do you
> think about allowing a promotion request to supply the new TimelineID?

> In managed HA setups, the tool that decides which node may promote
> normally already has a DCS. It could allocate the TLI through the DCS
> while selecting the new primary, then pass it to PostgreSQL. PostgreSQL
> would still validate that it is greater than the current TLI and does
> not conflict with any history file it can see. This would prevent the
> collision when such a coordinator exists, while the UUID could remain a
> last-resort check for uncoordinated promotions.

If we have some sort of API or callback for assigning a new TLI that 
would be excellent. Would be useful for a bunch of DCS as a way to 
ensure that you do not pick a conflicting TLI. Should probably be 
configurable though, but a normal hook callback would probably be 
sufficient... and yes, the UUID is good to have as a last-resort check 
to ensure that you don't break things.

>
> archive_command and restore_command are file-transfer interfaces. They
> cannot express an atomic allocation, so they seem like the wrong place
> to solve the distributed problem of choosing an identifier.

Yup, agree.

>
> At least until PostgreSQL gets built-in Paxos. Every joke contains a
> grain of joke, though; Kostya Osipov's related built-in consensus thread
> is [0].

Would love to see that. Kostja would definitely know how to implement 
that. :)

>
> Would this fit the model you have in mind?

That would work fine. My main concern about adding UUIDs were to prevent 
problems when you accidentally allocate same TLI to timelines that are 
not the same. Integration hooks with a DCS would be icing on the cake. :)

Best wishes,
Mats Kindahl

>
> Thank you!
>
>
> Best regards, Andrey Borodin.
>
> [0]https://www.postgresql.org/message-id/flat/Z_1Cq7JvabsFYjQo%40ark
>
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    <p>Hi Andrey, and thank you for the comments.</p>
    <p>On 8/30/26 19:35, Andrey Borodin wrote:</p>
    <blockquote type="cite"
      cite="mid:80EEB51E-176C-4E57-B86F-5200F4348D02@yandex-team.ru">
      <pre wrap="" class="moz-quote-pre">Hi Mats,

On Tue, Aug 12, 2026, Mats Kindahl wrote:
</pre>
      <blockquote type="cite">
        <pre wrap="" class="moz-quote-pre">Even with a shared archive, it is impossible to ensure such things. It
is a normal consensus problem.
</pre>
      </blockquote>
      <pre wrap="" class="moz-quote-pre">
This race came up again in an open-space discussion here. What do you
think about allowing a promotion request to supply the new TimelineID?</pre>
    </blockquote>
    <p><span style="white-space: pre-wrap">
</span></p>
    <blockquote type="cite"
      cite="mid:80EEB51E-176C-4E57-B86F-5200F4348D02@yandex-team.ru">
      <pre wrap="" class="moz-quote-pre">In managed HA setups, the tool that decides which node may promote
normally already has a DCS. It could allocate the TLI through the DCS
while selecting the new primary, then pass it to PostgreSQL. PostgreSQL
would still validate that it is greater than the current TLI and does
not conflict with any history file it can see. This would prevent the
collision when such a coordinator exists, while the UUID could remain a
last-resort check for uncoordinated promotions.</pre>
    </blockquote>
    <p>If we have some sort of API or callback for assigning a new TLI
      that would be excellent. Would be useful for a bunch of DCS as a
      way to ensure that you do not pick a conflicting TLI. Should
      probably be configurable though, but a normal hook callback would
      probably be sufficient... and yes, the UUID is good to have as a
      last-resort check to ensure that you don't break things.</p>
    <blockquote type="cite"
      cite="mid:80EEB51E-176C-4E57-B86F-5200F4348D02@yandex-team.ru">
      <pre wrap="" class="moz-quote-pre">

archive_command and restore_command are file-transfer interfaces. They
cannot express an atomic allocation, so they seem like the wrong place
to solve the distributed problem of choosing an identifier.</pre>
    </blockquote>
    <p>Yup, agree.</p>
    <blockquote type="cite"
      cite="mid:80EEB51E-176C-4E57-B86F-5200F4348D02@yandex-team.ru">
      <pre wrap="" class="moz-quote-pre">

At least until PostgreSQL gets built-in Paxos. Every joke contains a
grain of joke, though; Kostya Osipov's related built-in consensus thread
is [0].</pre>
    </blockquote>
    <p>Would love to see that. Kostja would definitely know how to
      implement that. :)</p>
    <blockquote type="cite"
      cite="mid:80EEB51E-176C-4E57-B86F-5200F4348D02@yandex-team.ru">
      <pre wrap="" class="moz-quote-pre">

Would this fit the model you have in mind?</pre>
    </blockquote>
    <p>That would work fine. My main concern about adding UUIDs were to
      prevent problems when you accidentally allocate same TLI to
      timelines that are not the same. Integration hooks with a DCS
      would be icing on the cake. :)</p>
    <p>Best wishes,<br>
      Mats Kindahl</p>
    <blockquote type="cite"
      cite="mid:80EEB51E-176C-4E57-B86F-5200F4348D02@yandex-team.ru">
      <pre wrap="" class="moz-quote-pre">

Thank you!


Best regards, Andrey Borodin.

[0] <a class="moz-txt-link-freetext" href="https://www.postgresql.org/message-id/flat/Z_1Cq7JvabsFYjQo%40ark">https://www.postgresql.org/message-id/flat/Z_1Cq7JvabsFYjQo%40ark</a>

</pre>
    </blockquote>
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