The social graph¶
ActivityStreams: The social graph¶
This social graph is, from an information science perspective, a directed graph, consisting of triple statements describing the relations between its nodes. The concept, as well as the technical implementation employed by ActivityPub, is anything but new: it is the well-known RDF (Resource Description Framework) graph model, which is also the foundation of some established standards and tools on the web:
RSS and Atom for blog and podcast feeds
OpenGraph meta-data for websites
Ontological databases like WikiData
…and many more…
In ActivityStreams, this graph structure is used to model relationships between Actors (users, groups, anyone who does something), Activities (create, update, delete, follow,…) and Objects (notes, attachments, events,…), all of which have unique IRIs (web addresses, colloquially speaking).
If we got hold of all information from all instances on the Fediverse at once, it could be put together in one big, consistend graph.
ActivityPub: Sub-graphs and federation¶
As the name suggests, the Fediverse is a federated system, which means that it is comprised of several instances that handle parts of it.
Relating this to the concept of the social graph, the implication is that every instance handles a sub-graph of the global social graph.
The role of ActivityPub is to ensure that the sub-graph an insatance sees includes all nodes that are relevant for the actors on the instance. For that purpose, objects (actors and activities) can be pulled from other instances (using an HTTP GET request to the URI of the desired node), and pushed (using an HTTP POST request to a special node (an inbox) on another instance).
In every pull and push, an even smaller sub-graph is transferred between instances, containing exactly the nodes and statements relevant to merge the desired object with the other instance’s sub-graph (technically, what is transferred is the CBD (Concise Bounded Description) of the object).
To conclude, ActivityPub servers keep pushing and pulling sub-graphs, so-called Concise Bounded Descriptions, of objects that are relevant for their users.