ActivityPub is an open network protocol for social media federation. It was developed as part of the World Wide Web Consortium's (W3C) Social Web project and became a W3C Recommendation in January 2018.
This protocol allows the servers of different social media platforms to communicate with each other. That is, users of one platform can interact with users of another platform without registering on the latter. In this way, ActivityPub supports the decentralization of social networks and reduces users' dependence on centralized platforms.
The operation of ActivityPub is based on the transmission of "activities", which are actions performed by users. These activities communicate between servers using the HTTP protocol.
Activities on ActivityPub can be of different types, such as posting a message, following another user, or replying to a message. Each activity is a JSON object that contains information about the type of activity, who performs it, and other relevant data.
Servers that implement ActivityPub can be "actors" that perform activities and/or "input boxes" that receive activities. For example, when a user posts a message, that user's server sends a "Create" activity to the input boxes of the other users who should receive that message.
The main advantages of ActivityPub are as follows:
Despite its advantages, ActivityPub also presents some challenges:
ActivityPub is used on a number of Fediverso platforms, including: