Re: Parallel Apply

Hi!

On Mon, 11 Aug 2025 at 09:46, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> Hi,
>
> Background and Motivation
> -------------------------------------
> In high-throughput systems, where hundreds of sessions generate data
> on the publisher, the subscriber's apply process often becomes a
> bottleneck due to the single apply worker model. While users can
> mitigate this by creating multiple publication-subscription pairs,
> this approach has scalability and usability limitations.
>
> Currently, PostgreSQL supports parallel apply only for large streaming
> transactions (streaming=parallel). This proposal aims to extend
> parallelism to non-streaming transactions, thereby improving
> replication performance in workloads dominated by smaller, frequent
> transactions.

Sure.

> Design Overview
> ------------------------
> To safely parallelize non-streaming transactions, we must ensure that
> transaction dependencies are respected to avoid failures and
> deadlocks. Consider the following scenarios to understand it better:
> (a) Transaction failures: Say, if we insert a row in the first
> transaction and update it in the second transaction on the publisher,
> then allowing the subscriber to apply both in parallel can lead to
> failure in the update; (b) Deadlocks - allowing transactions that
> update the same set of rows in a table in the opposite order in
> parallel can lead to deadlocks.
>

Build-in subsystem for transaction dependency tracking would be highly
beneficial for physical replication speedup projects like[0]

>
> Thoughts?

Surely we need to give it a try.

[0] https://github.com/koichi-szk/postgres

--
Best regards,
Kirill Reshke