ActivityStreams Vocabulary defines a Follow activity, and ActivityPub defines its side effects of manipulating a followers collection, but ActivityPub does not specify a full algorithm for how to follow something. This FEP aims to provide guidance on which objects can be followed:
followers collection present.inbox, then you MAY recurse upwards through attributedTo until you find a resource with an inbox. The maximum recursion depth SHOULD be 1.A Follow activity can then be constructed for that object and delivered to the discovered inbox. Additional requirements for the structure of the Follow activity are out-of-scope.
In order to follow an object, we use a Follow activity. But in order to use this Follow activity, we need to know the following:
1) What can be followed?
2) Where do we send the Follow for that object?
At the time of writing, current software practices within the fediverse enforce a limit on what can be followed. For example, Mastodon currently limits follows to actors that can be mapped to their internal concept of “accounts”, and these “accounts” are in turn limited to being any of the five “actor types” described in AS2-Vocab: Person, Group, Organization, Application, Service.
Consequently, other software which does not have such a conceptual limit is forced to declare their actors as one of the five types, or else Mastodon users will not be able to follow their actors. ActivityPub says that there is no specified mapping between “users” and “actors”, but Mastodon’s interpretation of “actors” as “users” de facto disallows following anything but a strict subset of resources.
We may therefore express Mastodon’s requirements as follows:
type of any of Person, Group, Organization, Application, Service.inbox where the Follow will be sent.A proposed modified requirement and algorithm are as follows:
followers collection present. (If it does not, then it is unknown whether it is followable.)inbox, then you MAY recurse upwards through attributedTo until you find a resource with an inbox. The maximum recursion depth SHOULD be 1.Given an object O and a recursion limit L, a general algorithm for following it can be expressed like so:
Inputs:
Outputs:
(1) Initialize a variable INBOX.
(2) If O.followers is not present, optionally return an error OBJECT_HAS_UNKNOWN_FOLLOWERS_COLLECTION.
(3) If O.inbox is present, set INBOX to the referenced IRI.
(4) If INBOX is unset, then initialize a variable R whose initial value is O.
(5) While INBOX is unset:
(5.1) Check that L > 0. If false, return an error MAX_RECURSION_LIMIT.
(5.2) Dereference R.attributedTo and set the variable R to this referenced resource.
(5.3) If R.inbox is present, set INBOX to the referenced IRI.
(5.4) Set the variable L to the value L - 1.
(6) Initialize a document ACTIVITY:
(6.1) …whose actor is yourself
(6.2) …whose type is Follow
(6.3) …whose object is O
(7) Make an HTTP POST request whose target is INBOX and whose body is ACTIVITY.
Additional requirements for the ACTIVITY can be defined at a protocol level, but the exact protocol and those requirements are out-of-scope for this FEP. The scope of this FEP is limited to determining which objects can be followed, and where to send the Follow.
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To the extent possible under law, the authors of this Fediverse Enhancement Proposal have waived all copyright and related or neighboring rights to this work.