Racket has green threads, which gives us non-blocking IO but that's about it. There's another concurrency mechanism that uses OS threads but Arc doesn't rely on it, and from the little I looked at, I don't think it would work with the HN software's design.

There are two parallelism approaches in Racket, both of which provide OS threads. The most useful one for web servers is called 'places', and is somewhat like Erlang -- you can only communicate immutable serializable data between different places, as well as OS resources like file handles. I know other people have used places to scale web applications, but I haven't looked at how HN's design would be affected.

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