Hong Minhee is an open source developer and the creator of the Fedify ActivityPub server framework.

We talk about how applications like Mastodon and Misskey communicate with one another using ActivityPub. This includes discussions on built-in activites, extending the specification in a backwards compatible way, difficulties implementing JSON-LD, the inbox model, and his experience implementing the specification.

Hong Minhee:

Specifications:

ActivityPub implementations:

ActivityPub tools:

Transcript

You can help correct transcripts on GitHub.

What's ActivityPub?

[00:00:00] Jeremy: Today, I'm talking to Hong Minhee. He is the developer of Fedify. A TypeScript library for building ActivityPub server applications.

The first thing I think we should start with is defining ActivityPub. what is ActivityPub?

[00:00:16] Hong: ActivityPub is the protocol that lets social networks talk to each other and it's officially recommended by W3C. It's what powers this thing we call the Fediverse which is basically a way for different social media platforms to work together.

Users of ActivityPub

[00:00:39] Jeremy: Can you give some examples that people might have heard of -- either users of ActivityPub or things that are a part of this fediverse?

[00:00:50] Hong: Mastodon is probably the biggest one out there. And you know what's interesting?

Meta threads has actually started implementing ActivityPub this summer. So this still pretty much a one way street right now. In East Asia, especially Japan, there's this really popular microblogging platform called misskey.

It's got so many forks that people actually joke around and called them forkeys. but it's not just about Twitter style microblogging, there's Pixelfed which is kind of like Instagram, but for the fediverse. And those same folks recently launched loops. Which is basically doing what TikTok does, but in the Fediverse.

Then you've got stuff like Lemmy and which are doing the reddit thing up in the Fediverse.

[00:02:00] Jeremy: Oh like Reddit.

[00:02:01] Hong: Yeah. There are so much more out there that I haven't even mentioned. Um, most of it is open source, which is pretty cool.

[00:02:13] Jeremy: So the first few examples you gave, Mastodon and Meta's threads, they're very similar to, to Twitter, right? So that's what you were calling the, the Microblogging applications.

And I think what you had said, which is a little bit interesting is you had said Metas threads is only one way. So could you kind of describe like what you mean by that?

[00:02:37] Hong: Currently meta threads only can be followed by other ActivityPub applications but you cannot follow other people in the fediverse.

[00:02:55] Jeremy: People who are using another Microblogging platform like Mastodon can follow someone on Meta's Threads platform. But the other way is not true. If you're on threads, you can't follow someone on Mastodon.

[00:03:07] Hong: Yes, that's right.

[00:03:09] Jeremy: And that's not a limitation of the protocol itself. That's a design decision or a decision made by Meta.

[00:03:17] Hong: Yeah. They are slowly implementing ActivityPub and I hope they will implement complete ActivityPub in the future.

Interoperability through Activities

[00:03:27] Jeremy: And then the other examples you gave, one is I believe it was Pixel Fed is very similar to Instagram. And then the last examples you gave was I think it was Lemmy, you said it's similar to Reddit. Because you mentioned the term Fediverse before and you mentioned that these all use ActivityPub and since these seem like different kinds of applications, what does it mean for them to interact?

Because with Mastodon and Threads I can kind of understand because they're both similar to Twitter. So you're posting messages and replying, but, but what does it mean, for example, for someone on Mastodon to interact with someone on Lemmy which is like Reddit because they seem very different.

[00:04:16] Hong: People in Lemmy and Mastodon are called actors and can follow each other.

They have interactions between them called activities. And there are several types of activities like, create and follow and undo, like, and so on.

So, ActivityPub applications tend to, use these vocabulary to implement their features.

So, for example, Lemmy uses like activities for upvoting and like activities for down voting and it's translated to likes in Mastodon. So if you submit a post on Lemmy and it shows up on your Mastodon timeline. If you like that post (it) is upvoting in Lemmy.

[00:05:36] Jeremy: And probably similarly with Pixelfed, which you said is like Instagram, if you follow someone's Pixelfed account in Mastodon and they post a photo in Pixel Fed, they would see it as a post in Mastodon natively and they could give it a like there.

Adding activities or properties

[00:05:56] Jeremy: And these activities that you mentioned -- So the like and the dislike are those part of ActivityPub itself?

[00:06:05] Hong: Yes, and this vocabulary can be extended.

[00:06:10] Jeremy: So you can add, additional actions (activities) or are you adding information (properties) to the existing actions?

[00:06:37] Hong:

It is called activity vocabulary, and there are, things like accept, add, arrive, block, create lead, dislike, flag, follow, ignore invite, join, and so on.

So, basically, almost everything you need to build social media is already there in the vocabulary, but if you want to extend some more, you can define your, own vocabulary.

[00:06:56] Jeremy: Most of the things that an Instagram or a Twitter, or a Reddit would need is already there. But you're saying that you can have your own vocabulary. So if there's an action or an activity that is not covered by the specification, you can create one yourself.

[00:07:13] Hong: Yes. For example, Misskey and Pleroma defined emoji reactor to represent emoji reactions.

[00:07:25] Jeremy: Because the systems can extend the vocabulary. What are some other examples of cases where mastodon or any other of these systems has found that the existing vocabulary is not enough. What are some other examples of applications extending it?

[00:07:45] Hong: For example, uh, mastodon defined suspended -- suspended property. They are not activities, but they are properties in the activity.

ActivityPub consists of several types of objects and there are activities and normal objects like, article. they can have properties and there are several existing properties, but they can be also extended. So Mastodon extended some properties they need. So for example, they define suspended or discoverable.

[00:08:44] Hong: Suspended for to tell if an actor is suspended by moderators. Discoverable tells if an actor itself wants to be, searched and indexed, and there are much more properties. Mastodon extended.

Actors

[00:09:12] Jeremy: And these are, these are properties of the actor. These are properties of the user?

[00:09:19] Hong: Yes. Actors.

[00:09:21] Jeremy: Cause I think earlier you mentioned that. The concept of a user is an actor, and it sounds like what ...