Note: in hindsight, half of this post is answering my own questions as I explore this rarer side of federation, but there are still some remaining questions which I have highlighted.
This post is created on lemmy.ml. The benefits of federating this post to other Lemmy instances is immediately obvious, since they can use most or all of the site features to read it as intended and interact (voting, replying, reporting, saving, cross-posting or browsing and subscribing to email@example.com).
There is also intuitive benefit in being able to federate with other link aggregators such as lotide and Prismo instances. All these sites have the same basic interface of link-posting, text-posting, voting, commenting and voting on comments. The base format is very compatible, even if extra features are not. I wouldn’t be surprised if Lemmy and lotide form a dynamic similar to Mastodon and Pleroma, two microblogging services which again have an intuitive base compatibility.
But what about different types?
What are the benefits of, for example, making Lemmy federate with Mastodon, Friendica or PeerTube?
One approach to answering that is asking what cross-interaction is already possible, like some posts in !feditolemmy which were posted from Friendica. This nerdica.net post which is also replicated on !fediverse shows a conversation in replies between a few Lemmy instances and a Friendica account, and demonstrates the clear analogue of our communities and their forums, and of our votes and their likes (it’s just a test ;) )
So Friendica posts federating to Lemmy makes reasonable sense. I’m not sure about the opposite. I guess their posts are analogous to our text posts or text & link posts, so it might be possible to render their forums as browsable communities here.
Question 1: Does my Lemmy account browsing and making new posts on Friendica forums make sense? Or will the federation only make sense for enabling Lemmy to aggregate Friendica posts and allowing cross-rating and cross-commenting?
Note: I found this Friendica forum on Lemmy, which was properly interpreted as a community instead of a user by Lemmy, but posts aren’t replicating yet. I’m guessing it’s a base for future completion to allow further cross-integration. Friendica does not appear to be able to browse Lemmy users or communities yet.
I also assume microblogging sites like Mastodon and Pleroma, along with the Prismo link aggregator, can use hashtags as an analogy for communities. While a post on those sites can belong to multiple tags, Lemmy can imitate this with crossposting in multiple communities. Is this reasonable?
PeerTube is where I get more confused, and I’m not alone. As a reply there mentioned, we can view a PeerTube user account, such as https://firstname.lastname@example.org and https://email@example.com , although it doesn’t seem to work for framatube.org.
However the interfaces of Lemmy and PeerTube are radically different, as PeerTube is foremost a video hosting site and Lemmy is a link aggregator. I think it’s fair to assert that a Lemmy post cannot be displayed on a PeerTube instance without hacks no-one wants, which leaves PeerTube->Lemmy posting, and mutual liking/commenting/reporting/etc… A PeerTube video can be adapted as a link post in Lemmy. I’m not certain how a PeerTube upload would signal which communities it should be posted to in Lemmy, but there are reasonable options like an extra field in the upload settings, or a link in the description.
Question 2: Is there a plan to have anything more than PeerTube creating link posts in Lemmy communities with federation between comment sections?
Trying to learn the current situation in order to ask good questions has taught me a lot, I was in a mindset that we had to be able to make posts on other sites in order to usefully federate, when that isn’t really our role as a link aggregator site. Media sites can usefully post to here with federated voting and comment sections.