ActivityPub: activities across decentralized servers
Learn how ActivityPub uses ActivityStreams objects and server-to-server federation to move social activities between independent services.
Why this matters
ActivityPub helps explain a different style of federation from Matrix or AT Protocol. Comparing them shows why 'federated' describes an architectural property, not one universal message format.
Learn it
ActivityPub is a W3C Recommendation built on ActivityStreams 2.0. It defines client-to-server and server-to-server behavior for decentralized social applications. Actors have inboxes and outboxes; activities such as Create can carry objects such as Note. Implementations can support federation so activities travel between servers. Security and verification are important because accepting remote content blindly can enable spoofing or abuse.
Work through an example
A Note object can contain text content. A Create activity can wrap that Note and deliver it to another actor's inbox.
{
"@context": "https://www.w3.org/ns/activitystreams",
"type": "Note",
"content": "Observation received"
}Practice
- 1Identify actor, activity, object, inbox, and outbox in a simple ActivityPub flow.
- 2Compare a Note with a Matrix event and an ATProto record without pretending they are interchangeable.
- 3Explain which part is federation behavior and which part is application content.
What to watch for
Do not collapse Mastodon and ActivityPub into the same thing. Mastodon is an application that implements ActivityPub; the protocol is broader than one product.
Where this takes you
This prepares users to understand Mastodon/ActivityPub connections in Atlas Crossband and to reason about open-network interoperability.