Saved this comment. It claims that the Lemmy frontend and backend are stateless and can be scaled arbitrarily, as can the web server. The media server (pict-rs) and Postgres database are the limitations to scaling. I'm working to deploy Lemmy with external object storage to solve media storage scaling and there's probably some database experts figuring out Postgres optimization and scaling as well. None of the instances are big enough to run into serious issues with vertical scaling yet, so this won't be a problem for a while.

I'm not sure if there really is issues, I think it's just new ground since most lemmy instances have been able to run on a single node due to the low populations. It seems most large public instances are just adding bigger servers to deal with the problem short term.

From what I can tell (I am not an expert in this field), it seems most of the architecture would spread horizontally without much issue. I haven't seen anywhere this is done yet, but I could be missing the obvious.

The lemmy backend api just takes HTTP requests (and at the present websockets, but this is changing in 0.18 to only HTTP requests), and it uses postgres as the backend storage. Using a kubernetes postgres operator to scale the database and then running multiple lemmy backend api instances (and frontend as needed) seems like it would work, or would require minimal work to get running.