Have you thought about load balancing across one or more nodes? You're going to have to at some point.
Haha, more than one node. That's pretty non-trivial for the way hubski is currently built: https://news.ycombinator.com/item?id=5032739 (Notice that pg corrected my more general comment, but making the changes he suggests to hubski is still non-trivial.) Update: you'll probably want to read the original story first, the programmer types among you: http://davidkendal.net/articles/2011/11/a-man-a-plan
So the issue is that the fnid links reference a function's closure, which is stored in a hashmap? So the more fnid links get served up, the more memory gets used? And then when it expires, you get the unknown or expired link sort of thing? Sounds like this wouldn't translate very well into multiple machines, unless figured out a way to store closures externally.
It also makes it sound like running behind a CDN (or any kind of proxy) won't work too well. That really sucks. I don't care too much for programming language pissing contests, but if that's really a limitation, well...that's a pretty severe one to say the least.
I'm not sure it is a programming language limitation. The link in the link above points out the Paul Gram pretty much pioneered lisp on the Web, and that most thing just emulate his ideas. That means that change could come, but that right now the "standard" implementation has limits. This is good, but requires a lot of work to make happen.
Also caching all the things. I'm not sure how arc would integrate with memcached though, for after some googling, it doesn't appear to have any bindings already written. Not surprising due to the tiny adoption of it. For a short term bandage, varnish doesn't really care what's upstream of it, I believe. Unfortunately, there are some serious issues that come from adopting an un-adopted language. And I think that's why reddit moved to python. Not saying it won't work, just saying it's a lot more work to make it work. It's certainly important for people to experiment with less widely used languages - that's how movements are formed.