If user U1 on server C1 subscribes to user U2 on server C2, does that mean when U2 posts, C2 sends the post to C1, independent of wether U1 is online or not?
If C1 can only request the updates while U1 is online, then the Fediverse should scale just like RSS.
I never heard anybody saying that RSS is doomed because of scaling issues.
There's be some savings from users just never signing in & never pulling. But also, server c2 also has hundreds or thousands of other users too. If any of those subscribe to u1, that traffic would have been sent either way.
So the hypothetical unread messages advantage of pull rapidly erodes at any kind of scale. I'm not sure what the gain might be.
One nice thing about rss is that it's relatively cacheable. CDNs are well optimized. Honestly I think it's mostly just bad implementations, that figuring out what messages to pushing data out ought be fairly optimizable & not terrible to scale out effectively. But being efficient & smart about it does take some though.
I mean it only worked for email since the dawn of the internet. :P
But ActivityPub has no concept of "online" for actors, the activities that have an actor in their recipients list will end up in their inbox no matter what.
[1] https://justingarrison.com/blog/2022-12-06-mastodon-files-in...