Hey, I am currently trying to understand a bit better what the Activity Pub allows and what it doesn't as I am preparing a presentation on that topic.

On Lemmy I can join other servers without having to recreate an account. Does that also work for across Activity Pub supporting software? Could I join a PeerTube page and then post there? Could I upload videos? Could I join Mastodon or Pixelfed and post images?

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[–] psycotica0@lemmy.ca 11 points 2 days ago* (last edited 2 days ago) (13 children)

Quick clarification, because I can't tell from your words if you're confused about the concept of federation or not 😅

If by "join" other servers you mean use their site as if logged in, or like you have an account there, then that is not federation. That's single-sign-on (SSO) and is not a feature of the fediverse (Mastodon, Lemmy, Peertube, etc). That would be like the "login with Facebook" or Google buttons around, where by having this account on site A, you can instantly signup on site B without making a new password or anything. That's not how federation works.

Federation is like email. I can have an email with GMail, you can have one with Proton, and someone else can have Yahoo, and I can send an email to you anyway. It doesn't mean I can "join" Proton with my GMail account, it doesn't mean I have a Yahoo account, it means I don't need a Yahoo account to communicate with Yahoo users.

But, if by "join" you meant "join a community" as in subscribe to updates from a group on another server, then most other people's answers apply. I wouldn't call that "joining a server", though, because servers host many communities and you're not joining all of them.

Joining a community works like joining a mailing list. Activity Pub allows accounts on different servers to communicate without an account on their own server, so my account would send a message to your account saying "I'd like to subscribe to this community, send me a message whenever something happens on it", and then the other server says "okay, will do", and then after that will periodically send my server messages saying "hey, here's that update you asked for". And when I comment, like right now, it's like an email being sent from my server to yours, and then your server puts it into the history.

This allows my server to present the community from your server to me, without me having account on your server. Without me having to "join" your server, I'd say.

[–] psycotica0@lemmy.ca 2 points 2 days ago (12 children)

And so I'd say this also answers your broader questions. Since Activity Pub doesn't allow me to join other servers, it also doesn't allow me to join other sites.

So a Lemmy post may be compatible with a Mastodon message sent to a group or something, that's only because the messages Activity Pub sends are similar. But PeerTube is different software with different buttons, and the existence of those buttons on PeerTube doesn't change anything about what buttons Lemmy has, and I can't "login" to a PeerTube server with my Lemmy account, so I don't gain any special abilities outside of what Lemmy can do.

The only way it would be possible is if Lemmy added a feature for uploading videos that sent the same kinds of messages to other servers that PeerTube sends. Then, if they did that, someone on a PeerTube instance could see these messages coming from Lemmy and interpret them on their server as a PeerTube video or something.

But all that Activity Pub allows is exchanging of information between sites. For them to interoperate in a way that makes sense, they need to exchange the same kind of information.

[–] VoxAliorum@lemmy.ml 2 points 2 days ago* (last edited 2 days ago) (11 children)

Good response! You were right. I thought that it would work similarly to SSO i.e. your instance works as an identity provider and your content is then generated on the other page, not send there.

Alright, so following the mail metaphor: lemmy is unable to send a video attachment so it can't allow you to upload videos to peertube; however, activity pub could allow for that.

I think I have to read up on the exact definition of "group".

Maybe I should create a new post for this but has there ever been discussions regarding SSO? So you have one identity across all fediverse services?

[–] Snoopy@feddit.fr 1 points 2 days ago* (last edited 2 days ago) (1 children)

Alright, so following the mail metaphor: lemmy is unable to send a video attachment so it can't allow you to upload videos to peertube; however, activity pub could allow for that

I'm not sure that would be the correct metaphore. Lemmy can send video attachement. But your lemmy account is not allowed to upload video to peertube so you can't send any video.

There is also an economical reason : a video is ressource hungry.

So it is limited to watching peertube video. Let's imagine 1000 lemmy users uploading video to peertube...

[–] VoxAliorum@lemmy.ml 1 points 2 days ago (1 children)

But this is a peertube limitation, right? Activity Pub itself provides the video information and peertube - due to whatever reasons (most likely the economical reasons you mention) - decides to not use that information / reject "the mail". Is a message sent is only stored on the receiving end or also on the node I am sending it from?

[–] Ludrol@szmer.info 4 points 2 days ago* (last edited 2 days ago) (1 children)

Images and videos on Activity Pub are sent as direct links to the host.

So you can send text comments from Lemmy to peertube that will be replicated, and watch the video by "following" direct links to the host (it is done automagically in your browser or app).

[–] VoxAliorum@lemmy.ml 1 points 2 days ago

This explanation on how images and videos are embedded while text messages are copied is really helpful!

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