ActivityPub

ActivityPub is an open, decentralized protocol for building interoperable social web services. It is a World Wide Web Consortium (W3C) standard that enables users on independent servers to interact, follow, and share content across federated networks. ActivityPub relies on the Activity Streams 2.0 vocabulary to describe actions and objects, and supports both client-to-server and server-to-server communication.

The protocol defines two main layers: ActivityPub C2S for client-to-server interactions and ActivityPub S2S for federation between servers. A user on a server hosts an Actor (Person, Group, or Organization). Clients publish Activities to the Actor’s Outbox; other servers fetch or receive them via the Actor’s Inbox. When a user follows another, a Follow activity is sent to the target’s Inbox, and the target may accept with an Accept activity. Content is subsequently delivered to the followers’ feeds via their Outboxes.

Activity types include Create, Update, Delete, Like, Announce, and Follow. Objects can be Notes, Articles, Images, or Videos. Actors include Person, Group, Organization, and Application. Collections and OrderedCollections manage lists such as followers, following, and inbox/outbox items. The model emphasizes linked data and URL-based identification, with JSON-based payloads that reference actors, objects, and contexts.

Adoption and implementations cover a range of platforms, most prominently Mastodon, but also PeerTube, WriteFreely, GNUSocial, and others. ActivityPub enables interoperation between disparate services, which distinguishes it from centralized social networks and from older protocols like OStatus.

Security and governance considerations include actor discovery via WebFinger, reliance on HTTPS/TLS, and moderation challenges inherent to federated networks, where policy decisions at individual instances affect the broader federation.