What do we need to federate with other ActivityPub based services? What is there to look out for?


Most AP services, including Mastodon, require all actors to have a webfinger handle, whose user part corresponds to the Actor's preferredUsername property.

The webfinger response must contain a link with rel: "self", type: "application/activity+json" and an href pointing to the URL of the actor endpoint.

Other optional webfinger data provided by Mastodon include:

"rel": "http://webfinger.net/rel/profile-page"
For ther user's HTML profile page
"rel": "http://ostatus.org/schema/1.0/subscribe"
To enable remote interactions. Hugh Rundle has a good blog post on this.
"rel": "http://webfinger.net/rel/avatar"
For the user's avatar


The ActivityPub sepecification requires all Objects to have an ID and a type, and Actors to have an inbox and an outbox. The ActivityStreams spec provides a small number of Actor types, at least one of which should be used as your Actor's type.

Furthermore, due to the reliance on webfinger Mastodon and other platforms require a preferredUsername.


According to the ActivityStreams spec, you must serialize your JSON output using the standard JSON-LD Compaction Alorithm using the default ActivityStreams context (as well as any other contexts you wish to use). This is actually quite convenient from a practical standpoint, since you don't need to run the JSON-LD expansion process on a document if you're only reading the base vocabulary (if you're using extensions you still _should_ expand the document, especially if it's a vocabulary that's less well-used in the fediverse in general).