ActivityPub

I'm sure you could do that but, thus far, it seems like algorithmic feeds are rejected wholesale by the fediverse communities.

Centralized algorithms for feeds are generally rejected by the community so far. Some projects such as Mastodon provide users the ability define filters of their raw chronological feed. People can also use and follow hashtags.

It's funny but I definitely wouldn't have put YT on that list. Even after attempting to "train" it by removing from the feed stuff I'm not interested in, it's still very random and comically irrelevant at times.

In ActivityPub "feeds" are not really a thing. A "feed" is the content of an Actor's Inbox and it will contain all the Activities that have been received by that Actor in chronological order.

That being said, you can overlay some semblance of algorithmic feed in the client that presents the Inbox contents to the end-user, but that is not related to the specification in any way.

This solutionism is kind of a little a lot problematic. Expecting a single pat answer that utterly projects & reduces all possible wins into a single minded "this is how it will work" confines imagination & potential (just by the pointed ask alone), reduces a complex place we can keep exploring & winning & growing, into an overbearing consumerist expectation that all answers are known, all is worked out, and that there is some total answer thats gonna drop in our laps.

Synthetic feeds of recommendations is an open topic. How much data one is willing to send upstream, as inputs to one's recommender engine, is as much of a topic as how such things would work, what kind of activitypub feeds the engine would emit.

Right now there's nearly nothing & (while I dont agree) a vast amount of the userbase considers this lack to be a feature: a distinguishing characteristic that makes this network human and not aggressively eye-ball stealing machine & greedy. Human connection & old fashioned following people you run into is a win for organic matter & innate connection over pre-programmed ways to insure more eyeball-time.

That philosophy battle aside: there's infinite possibilities here. The way the question is phrased expects there is a singular clear way. You probably should not sign up for a fediverse instance! You dont seem like a fit.

I don't think there's a firehose alternative for Mastodon. There's the generic stream API (https://docs.joinmastodon.org/methods/timelines/streaming/) which should contain all public posts but that will only reflect the local timeline.

The general public API also has a "fetch posts since ID <x>" call but in practice you'll only be able to fetch 400 posts at most based on the docs I can find.

There are tools like this: https://github.com/cscape/mastodon-firehose which you can use to combine multiple stream endpoints. That tool probably comes closest if you provide it with enough servers to monitor.

Mastodon requires more than simply ActivityPub to be able to interact with it. Its user discovery is based on WebFinger and if you don't have that working, your service won't be able to be interact with Mastodon servers.

Nextcloud, PeerTube, PixelFed, and others.

https://en.wikipedia.org/wiki/ActivityPub - has a list, almost certainly incomplete but seems to cover the major ones.

Check out this article from the other day. It gives a nice info-graphic showing a lot of the major players in Activity-pub. No shortage of options :)

https://news.ycombinator.com/item?id=33472622

The post must either be retrieved via https, or signed with a key pair retrieved from the same origin.

I use Mastodon over the web-interface. Where would signatures of sent content be generated or signed? How does the receiving end verify the signature?

With XMPP+OMEMO I have verified the fingerprints of my friends by scanning their QR code.