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ActivityPub
ActivityPub is a federated protocol that defines a set of interoperable social network interactions through specific APIs. Any server that implements this protocol can communicate with the rest of the network. It reached W3C recommendation status in 2018. It is one of several related specs produced by the Social Web Working Group.
ActivityPub consists of two layers: A server to server federation protocol, and a client to server protocol.
Mastodon is a popular federated alternative to Twitter built on ActivityPub.
Identity
User identities in ActivityPub are conceptualized as actor objects. To be spec compliant, each actor must have an "inbox" and an "outbox". They also should have "following" and "followers". They may have "liked" collections, and many other predefined possibilities. These are endpoints, URLs which are accessible on the server.
Networking/Message Passing
ActivityPub servers do not proactively network with each other, so they are unaware of each other's presence until a user finds and follows someone on another server. Servers maintain a list of remote accounts its users follow and subscribe to their posts.
ActivityPub messages are objects wrapped in an "activity", indicating what it is.
ActivityPub messages are not limited to HTTP only. This allows it to potentially be extended in more p2p directions.
Data Storage/Message Persistence
ActivityPub is not opinionated about how messages are persisted on the server as long as each server follows the protocol message requirements.
Moderation/Reputation
Moderation is primarily handled by server implementations. ActivityPub defines a "block" activity to help users control their experience.
The use of server-level bans to block content can lead to the isolation of an ActivityPub server instance if it is banned by many other servers, limiting its users to communication within its instance.
Social/Discovery
Messages are addressed to a user at their home server. Normal DNS and IP address routing are used to find the server addressed. Users can push messages to the special 'public' group which makes them available to all interested users. Servers may accept delivery of messages addressed as 'public' to a shared inbox available to all on the server, but are not required to. "Like"s and "Follow"s may be used by servers to determine which public messages to accept/retrieve.
There is no global search capability, as each server monitors a different set of messages. Searching for the same keyword on different instances yields different results. The federated timeline shows public posts that the user's server knows about. Essentially, users have access to posts of people followed by people on their instance.
User experience
ActivityPub is most widely used in Twitter-like applications (Mastodon, Pleroma). Some other applications that federate using ActivityPub include PixelFed and PeerTube.
Privacy & Access Control
Server to server federation is authenticated using HTTP signatures in conjunction with the signing key from the actor's publicKey field. To verify object integrity, linked data signatures are used to sign the object with hte publicKey of the actor who authored it.
A 2017 discussion of how to do encrypted messaging in ActivityPub.
Interoperability
Any service that implements the ActivityPub protocol can interoperate with the ecosystem. A service like Twitter would need to add Webfinger and JSON-LD representations of users and tweets.
Diaspora, another federated social network, chose not to adopt ActivityPub. A Diaspora developer's reasoning for the decision is detailed in this blog post.
Scalability
The ActivityPub ecosystem scales up by adding more server capacity to the network. This study on the Mastodon ecosystem analyzes the emergence of points of centralization as the network scales up.
Metrics
fediverse.network maintains statistics of the known oStatus/ActivityPub fediverse.
Implementations
Mastodon (the largest federated network built on ActivityPub) has 2699 nodes and 2.6M users as of 5/2020 (Mastodon home page asserts 4.4M, a bit more than what the-federation.info stats provide; maybe some servers are not counted)
Pleroma According to stats at the-federation.info, Pleroma has 620 nodes with 35K users as of 5/2020.
Related
ActivityPub inherits from a few other protocols that will not be covered in full, but are briefly summarized here.
ActivityPub was based on pump.io and ActivityStreams. Conceptually, these were preceded by OStatus. Pump.io is an activity streams social networking server. OStatus is an open standard for federated microblogging, and describes how a suite of protocols can be used together.
The IndieWeb protocols and community are also related to ActivityPub through a shared vision of social federation developed around the same time period. The IndieWeb was inspired by the Federated Social Web Summit in 2010, and formed around the idea of interconnecting individual websites rather than federating social platforms. A co-founder described the vision as, "someone should be able to take their web site and be able to use their web site to participate in the same distributed social network — federated social network."
Links
W3C ActivityPub Spec Social Web Working Group SocialHub, ActivityPub discussion forum