Re: POC: enable logical decoding when wal_level = 'replica' without a server restart

Hi,

On Fri, Feb 14, 2025 at 12:17:48AM -0800, Masahiko Sawada wrote:
> On Tue, Feb 11, 2025 at 11:44 PM Bertrand Drouvot
> <bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:

> Looking at the latest custodian worker patch, the basic architecture
> is to have a single custodian worker and processes can ask it for some
> work such as removing logical decoding related files. The online
> wal_level change will be the one of the tasks that processes (eps.
> checkpointer) can ask for it. On the other hand, one point that I
> think might not fit this wal_level work well is that while the
> custodian worker is a long-lived worker process,

That was the case initialy but it looks like it would not have been the case
at the end. See, Tom's comment in [1]:

"
I wonder if a single long-lived custodian task is the right model at all.
At least for RemovePgTempFiles, it'd make more sense to write it as a
background worker that spawns, does its work, and then exits,
independently of anything else
"

> it's sufficient for
> the online wal_level change work to have a bgworker that does its work
> and then exits.

Fully agree and I did not think about changing this behavior.

> IOW, from the perspective of this work, I prefer the
> idea of having one short-lived worker for one task over having one
> long-lived worker for multiple tasks.

Yeah, or one short-lived worker for multiple tasks could work too. It just
starts when it has something to do and then exit.

> Reading that thread, while we
> need to resolve the XID wraparound issue for the work of removing
> logical decoding related files, the work of removing temporary files
> seems to fit a short-lived worker style. So I thought as one of the
> directions, it might be worth considering to have an infrastructure
> where we can launch a bgworker just for one task, and we implement the
> online wal_level change and temporary files removal on top of it.

Yeap, that was exactly my point when I mentioned the custodian thread (taking
into account Tom's comment quoted above).

Regards,

--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com