Re: Parallel Apply

On Sat, Sep 13, 2025 at 9:49 PM Abhi Mehta <abhi15.mehta@gmail.com> wrote:
>
> Hi Amit,
>
>
> Really interesting proposal! I've been thinking through some of the implementation challenges:
>
>
> On the memory side: That hash table tracking RelationId and ReplicaIdentity could get pretty hefty under load. Maybe
bloomfilters could help with the initial screening? Also wondering 
>
> about size caps with some kind of LRU cleanup when things get tight.
>

Yeah, this is an interesting thought and we should test, if we really
hit this case and if we could improve it with your suggestion.

>
> Worker bottleneck: This is the tricky part - hundreds of active transactions but only a handful of workers. Seems
likewe'll hit serialization anyway when workers are maxed out. What 
>
> about spawning workers dynamically (within limits) or having some smart queuing for when we're worker-starved?
>

Yeah, we would have a GUC or subscription-option max parallel workers.
We can consider smart-queuing or any advanced techniques for such
cases after the first version is committed as making that work in
itself is a big undertaking.

>
>
> Alternative approach(if it can be consider): Rather than full parallelization, break transaction processing into
overlappingstages: 
>
>
> • Stage 1: Parse WAL records
>

Hmm, this is already performed by the publisher.

> • Stage 2: Analyze dependencies
>
> • Stage 3: Execute changes
>
> • Stage 4: Commit and track progress
>
>
> This creates a pipeline where Transaction A executes changes while Transaction B analyzes dependencies
>

I don't know how to make this work in the current framework of apply.
But feel free to propose this with some more details as to how it will
work?

--
With Regards,
Amit Kapila.