this post was submitted on 12 Jun 2023
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Ensuring there's no data leakage in those cached calls can be tricky, especially if any api calls return anything sensitive (login tokens, authentication information, etc) but I can see caching all read-only endpoints that return the same data regardless of permissions for a second or two being helpful for the larger servers.
It's also worth noting that postgres does its own query-level caching, quite aggressively too. I've worked in some places where we had to add a
SELECT RANDOM()to a query to ensure it was pulling the latest data.In my experience, the best benefits gained from caching are done before the backend and are stored in RAM, so the query never even reaches those services at all. I've used varnish for this (which is also what the big CDN providers use). In Lemmy, I imagine that would be the ngnix proxy that sits in-front of the backend.
I haven't heard admins discussing web-proxy caching, which may have something to do with the fact that the Lemmy API is currently pretty much entirely over websockets. I'm not an expert in web-sockets, and I don't want to say that websockets API responses absolutely can't be cached... but it's not like caching a restful API. They are working on moving away from websockets, btw... but it's not there yet.
The comments from Lemmy devs in https://github.com/LemmyNet/lemmy/issues/2877 make me think that there's a lot of database query optimization low-hanging fruit to be had, and that admins are frequently focusing on app configs like worker counts and db configs to maximize the effectiveness of db-level caches, indexes, and other optimizations.
Which isn't to say there aren't gains in the direction your suggesting, but I haven't seen evidence that anyone's secret sauce is in effective web-proxy caches.
Yeah, that's exactly why I'm asking this question. All the effort seems to be going into the DB -- but you can have a horribly shitty DB and backend but still have a massively performant webserver by just caching away the reads to RAM.
I didn't see any tickets about this on the GitHub, which is why I'm asking around to see if there's actually some very low-hanging-fruit for improving all the instances with a frontend RAM cache.
Much of your post seemed to focus on the techniques employed by
lemmy.world, caching websocket responses in the web-proxy does not seem to prominently feature among those techniques.If you're interested in advancing the state of the discussion around web-proxy caching, I'd consider standing up an instance to experiment with it and report your own findings. You wouldn't necessarily have to take on the ongoing expense and moderation headache of a public instance, you could set up with new user registrations closed, create your own test users, and write a small load generator powered by https://join-lemmy.org/api/ to investigate the effect of caching common API queries.