Pluriverse Interop

[erlend-sh]

Matrix and ActivityPub are two different internet protocols which are both federated, yet when we netizens talk about the fediverse it refers only to the ActivityPub network and its associated applications. Interoperability between Matrix and ActivityPub means embracing an even grander vision of a pluriverse wherein multiple fundamental internet protocols are able to talk to each other.

Chatty Threads

There is some notable overlap between the Matrix protocol and the ActivityPub protocol. Though there's nothing about the Matrix protocol that is inherently chat-specific, that is by far its most mature use case, most evident in its flagship app Element.

ActivityPub on the other hand is not suited for realtime messaging, but is innately threaded. While Mastodon encourages a free-form, chatty kind of threading, Lemmy is very strictly threaded.

Another way to look at the two protocols is across a Private ↔ Public spectrum:

No such model will be a perfect representation, as there are always exceptions. Gitter for instance, which now runs on the Matrix protocol, is a fully public chat. Conversely there are also a handful of completely or partially private Mastodon/Fedi spaces.

Connection vs Broadcast

We find it most useful to think of Matrix as the ideal protocol for message sending & storage, whereas ActivityPub is a protocol for message distribution:

• Matrix is telephoning 📞
• ActivityPub is radio 📻

Commune starts with Matrix because interpersonal messaging (1-to-1) necessarily precedes mass-messaging (one to many). Cozy spaces start small.

The path from one to another can be gleaned from the two articles Transitioning /r/rust to the Threadiverse and Group Convergence.

Informed by these developments, our path towards ActivityPub compatibility is twofold:

Group Following

Our primary interoperability target in the fediverse is Lemmy, since it neatly overlaps with Commune's notion of threads.

By supporting FEP-1b12 Group Federation, Commune channels will be able to opt into apub federation for its threads, thus connecting it to the 'threadiverse'.

By supporting FEP-d36d: Sharing Content Across Federated Forums, federated Commune channels will be able to follow and syndicate with existing sub-spaces in the threadiverse.

ActivityPub Groups combined with Matrix' own federation features sets the stage for an incredibly dynamic kind of webrings 2.0.

OAuth2/OIDC

OAuth logins are already standard in Mastodon and will likely arrive in Lemmy. Supporting logins via Mastodon et.al. will enable seamless onboarding of existing fediverse users into Commune.