From W3C Editor’s Draft

The ActivityPub protocol is a decentralized social networking protocol.

In ActivityPub, a user is represented by “actors” via the user’s accounts on servers. User’s accounts on different servers correspond to different actors.

Used to be called OStatus which was the basis for Mastadon

Based on ActivityStreams, a social data syntax.

Each actor has:

  • An inbox: How they get messages from the world
  • An outbox: How they send messages to others

Here’s how sending and reading messages work

  • You can POST to someone’s inbox to send them a message (server-to-server / federation only… this is federation!)
  • You can GET from your inbox to read your latest messages (client-to-server; this is like reading your social network stream)
  • You can POST to your outbox to send messages to the world (client-to-server)
  • You can GET from someone’s outbox to see what messages they’ve posted (or at least the ones you’re authorized to see). (client-to-server and/or server-to-server)

Messages made by clients get posted to their own server’s outbox and the server then posts that to the receiver’s inbox.