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The ActivityPub protocol is a social networking protocol based upon the ActivityStreams 2.0 data format. It is based upon experience gained from implementing and working with the OStatus and Pump.io protocols.
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The ActivityPub protocol is broadly based on the distribution of activities, described using [ActivityStreams]; these activities are produced in response to a user performing an activity. Most activities are purely responsive in nature - they are produced as a response to a user having done something. In addition, certain activities posted by a user to their outbox may trigger certain operations on the user's behalf. Finally, activities may have side effects as they propogate throughout the social graph. As an example of this:
like activity to their outbox. This activity causes the user's server to perform an activity on behalf of the user; in particular, it adds the specified object to the collection of objects the user has liked.This specification defines two closely related and interacting protocols:
All servers claiming conformance to this specification are required to implement the former protocol, and may optionally implement the latter. This gives three conformance classes:
It is called out whenever a portion of the specification only applies to implementation of the federation protocol. In addition, whenever requirements are specified, it is called out whether they apply to the client or server (for the client-to-server protocol) or which of a pair of servers in the server-to-server protocol.
Objects are the core concept around which both [ActivityStreams] and ActivityPub are built. Objects are often wrapped in Activities and are contained in streams of Collections, which are themselves subclasses of Objects. See the ActivityStreams Vocabulary document, particularly the Core Classes, to get a sense of how this lays out; ActivityPub follows the mapping of this vocabulary very closely.
Servers SHOULD validate the content they receive to avoid content spoofing attacks. In particular, servers SHOULD NOT trust client submitted content, and federated servers also SHOULD NOT trust content received from a server other than the content's origin without some form of verification. The general mecahism that is specified for this at this time is for a recipient of some referenced activitystreams object to verify the presence of that object on the original server. Other mechanisms such as verifying signatures are left out of scope of this document. (However, if other generally agreed upon mechanisms for handling verification become available in the future, servers are welcome to use those methods, in the future.)
{
"type": "Like",
"actor": "https://example.net/~mallory",
"to": ["https://hatchat.example/sarah/",
"https://example.com/peeps/john/"],
"object": {
"id": "https://example.org/~alice/note/23",
"type": "Note",
"author": "https://example.org/~alice",
"content": "I'm a goat"
}
}id both to ensure that it exists and is a valid object, and that it is not misrepresenting the object. (In this example, Mallory could be spoofing an object allegedly posted by Alice.)
All Objects in [ActivityStreams] should have unique global identifiers. ActivityPub extends this requirement; all objects distributed by the ActivityPub protocol MUST have unique global identifiers; these identifiers must fall into one of the following groups:
null object, which implies an anonymous object (a part of its parent context)
Identifiers MUST be provided for activities posted in server to server communication. However, for client to server communication, a server receiving an object with no specified id should allocate an object ID in the user's namespace and attach it to the posted object.
All objects must have the following properties:
There are several common operations upon social objects, which are expressed using standard HTTP methods applied to the Object's dereferencable URI. To avoid conflict with existing standards (and hence support "multiprotocol" server implemnetations), the scope of requests which servers are required to honor as defined in this specification is limited as follows:
DELETE method will always be honored as specified in 3.2.3 The DELETE methodapplication/activity+json, or where explicitly permitted the multipart/related media type with a root part of type application/activity+jsonAccept header MUST be specified, and the behavior defined by this specification ONLY applies if content negotion results in the selection of the
application/activity+json media type. For requests with a body, the client MAY omit the Accept header, in which case the server MUST provide successful responses as defined by this specification
Servers MAY implement other behavior for requests which do not comply with the above requirements. (For example, servers may implement additional legacy protocols, or may use the same URI for both HTML and ActivityStreams representations of a resource)
Servers MAY require authorization as specified in 5. Authorization, and may additionally implement their own authentication rules. Servers SHOULD fail requests which do not pass their authorization or authentication checks with the appropriate HTTP error code, or the 404 Not Found error code where the existence of the object is considered private.
GET methodThe GET method shall return an up-to-date version of the object.
PUT methodThis method requires user authorization
The PUT method may be used by a client to update the version of the object stored on the server.
The server MUST insert an activity with the type Update and this object as its object into the actor's
feed.
DELETE methodThis method requires user authorization
The DELETE method may be used by a client to delete the object. If successful, all requests to the URI which would have previously been successful SHOULD return the 410 Gone response code.
Actors in ActivityPub are represented by HTTP URI. When entered directly into a user interface (for example on a login form), it is desirable to support simplified naming. For this purpose cases, ID normalization SHOULD be performed as follows:
Once the actor's URI has been identified, it should be dereferenced.
Actor objects are ActivityStreams objects which are subclasses of the
actor type. Actor objects MUST have, in addition to the properties mandated by 3.1 Object Identifiers, the following properties:
Implementations SHOULD, in addition, provide the following properties:
id{
"@context": [
"http://www.w3.org/ns/activitystreams",
"http://www.w3.org/ns/activitypub"
],
"type": "Person",
"id": "https://johnsmith.example.com/",
"following": "https://johnsmith.example.com/following.json",
"followers": "https://johnsmith.example.com/followers.json",
"inbox": "https://johnsmith.example.com/inbox.json",
"outbox": "https://johnsmith.example.com/feed.json",
"preferredUsername": "johnsmith",
"name": "John Smith",
"summary": "Just an example guy",
"icon": [
"https://johnsmith.example.com/image/165987aklre4"
]
}Implementations MAY, in addition, provide the following properties:
When a user dereferences an actor's ID the page MUST contain HTML script element with the type attribute set to
application/ld+json.
[ActivityStreams] defines the collection concept; ActivityPub defines several collections with special behavior. Additionally, it defines a subset of collections termed streams.
Every actor MUST have a followers collection. This is a list of everyone who has sent a Follow activity for the user. This is where one would find a list of all the users that are following the actor.
The follow activity generally is a request to see the objects a user creates, this would make the "followers" collection a good default collection to provide to the user when selecting the audience targetting.
Every actor MAY have a following collection. This is a list of everybody that the actor has followed.
Every actor MAY have a likes collection. This is a list of every object that the actor has marked as having liked. (See the Like activity.)
Streams are collections of ActivityStreams objects, generally Activities, which are always presented in reverse chronological order.
The outbox stream contains objects the user has published, subject to the ability of the requestor to retrieve the object (that is, the contents of the outbox are filtered by the permissions of the person reading it). If a user submits a request without
Authorization the server should respond with all of the Public posts. This could potentially be all relevant objects published by the user, though the number of available items is left to the descretion of those implementing and deploying the server.
A client may submit an [ActivityStreams] activity to the server using a HTTP POST request to the dereferencable URI of the outbox collection. In this case, the request's Content-Type MUST be
application/activity+json, and the request MUST be authenticated with credentials authorized to act on behalf of the user to whom the outbox belongs. Such submitted activities will be added to the front of the feed retrieved by GET requests to the same endpoint.
The inbox stream contains all objects received by the user. The inbox MUST be filtered according to the requester's permission; this
MUST include allowing read access only to the owning user
A server may submit a HTTP POST to the dereferencable URI of the
inbox endpoint. This request MUST be authenticated with the credentials of the user identified by the actor stanza of the activity being delivered. The request MUST be of the [ActivityStreams] content type; it
MUST be a single activity. The incoming activity MUST have a valid URI as its id. The server MUST ensure the validity of the posted and any enclosed objects (generally by dereferencing the object on the originating server.)
The server MUST perform de-duplication of activities, whereby it discards repeats of already delivered activities (this can occur if an activity is addressed both to a user's followers, and a specific user which happens to follow said user, and the server has failed to de-duplicate the recipients list, for example). Such deduplication MUST be performed by comparing the id of the activities and dropping any activities already seen.
Notification and Delivery are two closely related acts. Notifications are sent to objects and are used to perform updates on the social graph, whilst delivery is the act of inserting an activity into a recipient agent's inbox
Activity distribution is per the audience targetting properties from the [ActivityStreams] specification. For each entry in the addressing properties of an object, the server shall
type) to discover its activity-inbox addressFor federated servers performing delivery to a 3rd party server, delivery SHOULD be performed asynchronously to completion of the original activity submission request, and SHOULD additionally retry delivery to recipients if it fails due to network error
followers collectionIn addition to delivery to recipients, the server MUST also perform notification as specified by 8.3 Notification
What to do when there are no recipients specified is not defined, however it's recomended that if no recipients are specified the object remains completely private and access controls restrict the access to object. If the object is just sent to the "public" collection the object is not federated to any users but is publically viewable in the person's outbox.
Access control is used to enforce permissions on objects; see 5. Authorization.
Access control is largely informed by addressing and 8.1 Delivery. (Implementations MAY provide additional methods of expansion or redaction of access beyond that of delivery.) Most implementations will resolve this via internally maintained access control lists. Implementations SHOULD provide initial access to recipients specified for delivery on an activity (recipients on a wrapped object however SHOULD be ignored).
A recipient receiving access through addressing to special collections of actors MAY be later removed from this group, and thus MAY also be removed from future access to the activity (and potentially its enclosing object).
When a user creates an activity (by submitting it to their outbox), the server should perform notification. Notification is the process of propogating awareness of the new activity to the activity's origin server such that the origin can act upon the activity (for example, notification that a user has liked an object is used to update the object's like count) and distribute the activity to ensure that it is propogated to the whole social graph which received the original object.
Notifications shall be sent to each of the following objects:
An object notified of an activity MUST deliver that activity to all recipients of the object.
In addition to [ActivityStreams] collections and objects, Activities may additionally be addressed to the special "public" audience denoted by the object
{
"@context": "http://www.w3.org/ns/activitystreams",
"id": "http://activityschema.org/collection/public",
"type": "Collection"
}Activities addressed to this special URI shall be accessible to all users, without authentication.
For the purposes of delivery, the server shall treat the public collection as if it contains the same entries as the user's followers collection.
TODO: New URI? Can we get as:public in the schema or such?
There is the ability to submit binary data such as images, video or other binary data you wish to use in activitypub. This is done by means of submitting the binary data to the user's file endpoint. The request MUST contain the HTTP headers Content-Type and Content-Length. Upon successful submission a response containing the object with an ID is returned that can be posted to the users outbox.
An example response for an image I might upload is:
{
"@context": "",
"id": "http://example.com/~erik/photos/1",
"type": "Image",
"to": ["http://example.com/~erik/public",
"http://example.org/~sarah/"],
"cc": ["http://example.com/~erik/followers"],
"url": [
{
"type": "Link",
"href": "http://example.com/~erik/photos/1.png",
"mediaType": "image/png"
}
]
}There are objects which have a content property, the value of which is provided by the user. These can be for example note, blog, image, etc.
The core of any [ActivityStreams] based protocol is activities within. Users post activities to their outbox, from which they are distributed to recipients' inboxes. ActivityPub places no restrictions on the activities which may be distributed; however, it defines certain activities with special behaviors
The Create activity is used to when posting a new object. This has the side effect that of the object that they are posting is being created.
For client to server posting, it is possible to create a new object without a surrounding activity. The server MUST also accept a valid [ActivityStreams] object that isn't a subtype of the "Activity" object being posted via a HTTP POST to the user's outbox in order to create it. The server then MUST create a wrapping Create Activity for the object. This activity then should be used in all server to server federation and throughout the Social API.
The audience is specified on the object which MUST be copied over to the newly created activity upon creation and removed as properties from the submitted object.
{
"@context": "http://www.w3.org/ns/activitystreams",
"type": "Note",
"content": "This is a note"
"to": ["https://example.org/~john/"],
"cc": ["http://example.com/~erik/followers"]
}{
"@context": "http://www.w3.org/ns/activitystreams",
"type": "Activity"
"actor": "https://example.net/~mallory",
"object": {
"id": "https://example.com/~alice/note/72",
"type": "Note",
"author": "https://example.net/~mallory",
"content": "This is a note",
"published": "2015-02-10T15:04:55Z"
},
"published": "2015-02-10T15:04:55Z",
"to": ["https://example.org/~john/"],
"cc": ["http://example.com/~erik/followers"]
}The Update activity is used when updating an already existing object. The object should be modified to reflect the new structure in defined in the update activity.
The Delete activity is used to delete an already existing object. This
MUST leave a shell of the object that will be displayed in activities which reference the deleted object. If the deleted object is requested the server should respond with the HTTP 410 Gone status code. A deleted object:
{
"@context": "http://www.w3.org/ns/activitystreams",
"id": "https://example.com/~alice/note/72",
"type": "Note",
"published": "2015-02-10T15:04:55Z",
"updated": "2015-02-10T15:04:55Z",
"deleted": "2015-02-10T15:04:55Z",
}The Follow activity is used to subscribe to the activities of another user. Once the user has followed a user, activities shared with the Follows of that user
SHOULD be added to the actors's inbox.
The Add activity is used to add the object to the collection specified in the
target property of the activity.
The Remove activity is used to remove the object from the collection specified in the
target property of the activity.
The Block activity is used to indicate that the actor does not want a user (defined in the object property) to be able to interact with objects posted by the user. The server SHOULD prevent the user from interacting with any object posted by the actor.
The Undo activity is used to undo a previous activity. For example, Undo may be used to undo a previous Like or Follow. The undo activity and the activity being undone MUST both have the same author. Side effects (such as adding activities to a user's inbox after a Follow) should be undone, to the extent possible.
There are some exceptions where there is an existing and explicit "inverse activity" which should be used instead.
Create based activities should instead use Delete, and Add activities should use Remove.
This section is non-normative.