W3C-ACTIVITYPUB

W3C ActivityPub Fediverse social federation.

Definition

ActivityPub 2 sides: (1) Client-to-Server (C2S): user app communicates with home server via Inbox + Outbox HTTP endpoints. (2) Server-to-Server (S2S): home servers federate with other servers via HTTP POST Activity JSON-LD AS2 to remote inbox. Actor objects: ~/actor.json (Person, Service, Application, Group, Organization) + properties inbox URL + outbox URL + followers + following + liked + streams. Key patterns: (a) Follow Activity: POST Follow activity to target actor inbox, target server responds Accept Activity to follower inbox. (b) Create Activity: POST Create activity (envelopes new Note, Article, Image, Video) to target inbox(es). (c) Like, Announce (boost/retweet), Block, Update, Delete activities. Signature: HTTP Signatures spec (draft-cavage-http-signatures) signs HTTP requests + JSON-LD signatures (Linked Data Proofs) data integrity. Mastodon largest implementation ~12M users 8000+ instances 2024. Threads Meta partial ActivityPub support 2024+.

Origin

ActivityPub specification draft 2014 by Christine Lemmer-Webber + Jessica Tallon W3C Social Web WG ; W3C Recommendation 23 January 2018 ; Mastodon adopts 2017 v1.6 (before W3C Rec final) ; Threads Meta partial 2024.

Example in context

Mastodon.social user @alice follows PeerTube channel @bob@video.example.com: POST Follow activity AS2 to bob inbox, video.example.com server processes, responds Accept Activity to alice inbox, alice now subscribed to bob video uploads federated cross-instance.

Related terms

• W3C AS2 — vocabulary base.