ActivityPub Won by Being Boring

At FediForum 2026, Evan Prodromou delivered a talk titled "How To Bullshit Your Way Through a Conversation about ActivityPub." As one of the five authors of the W3C ActivityPub Recommendation, Prodromou provided an honest architecture talk explaining why the Fediverse functions. He explained that every term in the Fediverse can be treated as a portmanteau.

For instance, Fediverse is a combination of federated and universe, ActivityPub is a blend of activity and publishing, and RESTful, in his humorous version, is a fusion of rest and full. Protocol developers, too, have hobbies.

Prodromou emphasized that ActivityPub works because its core is intentionally ordinary. It functions in the same way a well-designed loading dock operates. Trucks arrive, and boxes move without generating poetic descriptions of forklifts. Any competent web engineer from the mid-2010s could review ActivityPub and believe it was a straightforward design. The primer for understanding ActivityPub can be found in "What Is ActivityPub?" This article focuses on the importance of its familiar components.

Federation within the Fediverse starts with a simple agreement. Independent networks agree to exchange activity data, without any agreement on database, moderation model, admission policy, user interface, ranking system, or product category. This minimum agreement is crucial. Different servers, such as Mastodon on C.IM, PeerTube on P.LU, and Lemmy on R.NF, maintain their own shape while sharing a social graph without becoming a single product.

The ActivityPub specification consists of two layers: a client-to-server API and a server-to-server federation protocol. The server-to-server part is where most Fediverse operators encounter the machinery. Actors have inboxes and outboxes. Servers deliver activities to remote inboxes, and other servers fetch actor documents, objects, and collections using URLs.

This model is plain and aggressive in its simplicity. An activity is a sentence with a subject, verb, and object. For example, Alice liked an article, Bob followed Alice, or someone announced a post. The information transmitted over the wire is typically a sentence about the content, not just the content itself. This distinction makes ActivityPub social.

The activity concept traces back to 1930s Soviet activity theory, associated with psychologist Alexei Leontiev. It then permeated user-experience research before finding its way into Activity Streams. The Fediverse operates using repurposed Marxist psychology, which might sound like satire to some. Protocol concepts often originate from previous generations' tools, which are dragged in as needed.

The transport in ActivityPub is even less advanced. It uses HTTP, with an actor's URL providing information about the actor. Posting an activity to an inbox delivers it. The specification's overview clearly states that inboxes and outboxes are URLs, and federation typically occurs by servers posting messages to other servers' inboxes. Even the familiar user@domain handle is not inherently part of ActivityPub. It emerges through WebFinger, a modernization of older Internet identity lookup habits.