What is ActivityPub and how does it work?

Question

What is ActivityPub and how does it work? (Fediverse, too)

I’m hearing a lot about ActivityPub and the Fediverse these days and I don’t totally understand the concepts (even after reading explainer articles). I understand what it means to joint Mastadoon, eg, or to be a subscriber to a Ghost publication. But they now interact now somehow? Or are automagically linked, as are other parts of the Interbang in ways that they weren’t before? Not a Tech Person, just an end user who wants to get this. One does not subscribe to ActivityPub? One can access it, or it’s a framework, or …??

Thanks!

In: Technology

Answer 1

ActivtyPub is a software protocol: it is a formalized structure and set of rules on how two web services can talk to each other and exchange information relating to social media functionality.

So, much in the same way you know the “script” for when you order pizza over the phone, a social media server that has implemented ActivityPub knows how to ask other ActivityPub services for their post, or information about specific users, or that a user on network A wants to send a message to someone on network B.

The Fediverse is largely built up around the ActivityPub protocol. It’s how all these different Mastodon instances know how to talk to each other. It’s how Mastodon users can post things on Lemmy threads, because both implemented ActivityPub and thus know how to speak with each other. The concepts of what “a post” means might be different on Mastodon and Lemmy, but they both follow the same rules and if Mastodon says “I want to post this on Lemmy” Lemmy will understand the intention because Mastodon sent the right message to the right point. What lemmy *actually* does after that is then not important, just like you don’t really care how precisely the Pizza got delivered to your door as a result of your phone call, just that it shows up and looks correct.

If you write a program that implements ActivityPub, what you have done is written a program that knows how to talk to other ActivityPub programs, and theoretically that means you can quite easily link your program together with their program – at least as far as the protocol goes.