That would implicitly make each actor it's own namespace (in the sense that DNS is a namespace), and allow easy(-ier) movement of actors between federated instances, by greatly decreasing potential URL breakage.
It could also lead to increased compatibility with future p2p uses of ActivityPub.
That would make URLs non-permanent, isn't that a bad thing?
On the other hand, how is it going to "know" which user or instance to namespace things under? At some point, that information is going to be added, and I think it's always going to be a bit of an issue when moving.
Maybe there should be a complete "this user has moved" type of activity, which triggers instances to "re-author" all activities they have from that user to point to the new URL. Would result in some messages being out of sync, at least temporarily.
@doenietzomoeilijk if the posts are stored in a format where the instance uses internal ids pointing to actors (which can be taken care of almost for free when receiving the post), then you wouldn't have to change anything in the posts when the actor changes location. The internal actor-id should ideally be unrelated to any other specific attribute of the actor.
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