AS2 links

ActivityStreams 1.0 largely defines the precedence of using the "url"
property as a link to a "user viewable" version of the object (e.g. a http
link to a web page which represents the object):


url 

JSON <http://activitystrea.ms/specs/json/1.0/#RFC4627>  [RFC4627] String

An IRI [RFC3987] <http://activitystrea.ms/specs/json/1.0/#RFC3987>
identifying a resource providing an HTML representation of the object. An
object MAY contain a url property 

(The use HTML here is probably overly restrictive, but the intent is clear)

 

It makes some missteps:

*         For media links, url is the IRI of the media resource (e.g. an
image)

*         For collections, url is the IRI of a the collection in JSON form.
This is contradictory to the above, as collections are objects

 

These are best interpreted as bugs in the spec, IMO.

 

The AS2 ED defines url as follows:


url

Link Value
<http://jasnell.github.io/w3c-socialwg-activitystreams/activitystreams2.html
#linkValue> 

A Link Value describing a resource providing a representation of the object.
An object MAY contain a url
<http://jasnell.github.io/w3c-socialwg-activitystreams/activitystreams2.html
#url-attribute>  property. 

 

This provides no context for what the "url" property actually points to.
The examples in the spec then use the "url" property to refer to multiple
different resources: a web page representing a user, the actual image in the
case of an "image" object, etc. The "url" property provides no useful meta
data with regards to what its' pointing to. This is useless.

 

I suggest that we split the two conflated roles into two separate
properties:

*         url: The IRI of a resource which represents a human readable
representation (for example, a HTML document) of this object

*         href? src?: A link value containing one or more machine
processable representations of this resource. For example, for an object of
type image, this could provide multiple resolution images of the object

*         We might also codify self, The IRI at which the JSON form of this
resource can be found. This would simply be codifying use of the already
defined link relation for this purpose.

 

We might add language to the spec as follows:

*         The id of an object SHOULD be the IRI of a resource which can be
dereferenced to return an up to date description of the object in this
format.

*         In cases where the id does not correspond to an IRI dereferencable
as above, then the object SHOULD contain the self property, set to an IRI
which may be dereferenced for an AS2 representation of the object. 

*         In the absence of values for the url and href properties,
processors MAY attempt to dereference the IRI that is the object's id for
the resources to which these properties would normally point. This means
that you can just put the URI of an image in an Object's image property and
things will still work; no need to actually construct an image object.
Obviously in this case it may not really be possible for clients to interact
with the image as a discrete entity.

 

This actually has brought me back around to thinking that the AS1 concept of
a media link was correct, just incorrectly applied. I feel that they're
actually very useful, but were expressed incorrectly in AS1. The importance
of such objects is in sufficiently expressing the semantics of the data: if
all the resolution variations of an image become independent objects, then
what you have is not a meaningful object graph, but rather an object soup

 

AS1 example:

{

       "objectType": "image",

       "id": "https://example.com/api/image/sdashdkj",
       "url": "https://example.com/person/image/sdashdkj",
       "image": { "src": "https://example.com/api/image/sdashdkj/image.jpg"
}

}

 

Proposed AS2 example:

{

"objectType": "image",

"id": "https://example.com/api/image/sdashdkj",
"url": "https://example.com/person/image/sdashdkj",
"href": [{ 

"src": "https://example.com/api/image/sdashdkj/thumb.jpg",

"mimeType": "image/jpeg",

"width": 128, "height": 128

}, { 

"src": "https://example.com/api/image/sdashdkj/image.jpg",

"mimeType": "image/jpeg",

"width": 1024, "height": 1024

}]

}