ActivityPub, as it turns out, follows the actor model of computation. Okay, many people implementing the fediverse don't know about the actor model aspect of ActivityPub but I am here to tell YOU, dear reader, that it is an important thing, not a detail
I'll take one more note about federation which is that often time the message passing mechanism of the fediverse is often called "federation", but theoretically another mechanism could exist, but I'm actually not so sure of that.
There's a reason the actor model and the lambda calculus are undying
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