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One big server with a flat text file DB on NVME drives AFAIK.

I believe the application code is single threaded due to its interpreter too.

it does run pretty slow under load though, and they have acknowledged it's due to this

HN has mutable data though. That's a much harder problem than indexing a large amount of static data like a search engine.

Not only that, but I don't think the custom datastore handles concurrent writes.

One thread FTW. :)