One document matched: draft-snell-activitystreams-04.xml
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<rfc category="std" ipr="trust200902" docName="draft-snell-activitystreams-04" >
<front>
<title abbrev="ActivityStreams">JSON Activity Streams 2.0</title>
<author fullname="James M Snell" initials="J." surname="Snell" role="editor">
<organization>IBM</organization>
<address>
<email>jasnell@gmail.com</email>
</address>
</author>
<date month="September" year="2013" />
<area>General</area>
<workgroup>Activity Streams (http://activitystrea.ms)</workgroup>
<keyword>JSON</keyword>
<keyword>Activity Streams</keyword>
<abstract>
<t>
This specification details a model for representing potential and
completed activities using the JSON format.
</t>
</abstract>
<note title="Author's Note">
<t>
This draft is heavily influenced by the original JSON
Activity Streams 1.0 specification that was originally co-authored
by Martin Atkins, Will Norris, Chris Messina, Monica Wilkinson,
Rob Dolin and James Snell. The author is very thankful for their
significant contributions and gladly stands on their shoulders.
Some portions of the original text of Activity Streams 1.0 are
used in this document.
</t>
<t>
The Activity Streams 1.0 and 2.0 specifications are works produced
by the Activity Streams Working Group (http://activitystrea.ms/)
operating independently of the IETF. Discussion and feedback about
this specification is invited and should be directed to the Activity
Streams Mailing List (see
https://groups.google.com/forum/#!forum/activity-streams).
</t>
</note>
</front>
<middle>
<section anchor="introduction" title="Introduction">
<t>
In the most basic sense, an "activity" is a semantic description
of potential or completed actions. In the former case,
the activity expresses what can be done with a particular object,
while in the latter case, it expresses what has already been
done.
</t>
<t>
It is the goal of this specification to provide a JSON-based syntax
that is sufficient to express metadata about activities in a rich,
human-friendly, machine-processable and extensible manner. This may
include constructing natural-language descriptions or visual
representations about the activity, associating actionable
information with various types of objects, communicating or recording
activity logs, or delegation of potential actions to other applications.
</t>
<t>
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in
<xref target="RFC2119" />.
</t>
<section title="Relationship to JSON Activity Streams 1.0" anchor="asms-1">
<t>
The <xref target="activitystreams-1.0">JSON Activity
Streams 1.0</xref> specification was published in May
of 2011 and provided a baseline extensible syntax for the expression
of completed activities. This specification builds upon that
initial foundation by incorporating lessons learned through
extensive implementation, community feedback and related work
being performed in other standards development communities.
</t>
<t>
While the syntax defined by this specification diverges somewhat
from that defined by JSON Activity Streams 1.0, the verbs, objectTypes,
extensions and fundamental model defined by that original
specification remain intact.
</t>
<t>
Refer to <xref target="deprecated-syntax" /> for more detail
about the differences between the 1.0 and 2.0 syntax and for
a listing of specific backwards compatibility requirements.
</t>
<t>
This specification incorporates several existing extensions
to the 1.0 syntax directly into the 2.0 model. These include
portions of the <xref target="base-schema">Activity Streams 1.0 Base Schema</xref>,
<xref target="audience">Audience Targeting</xref>,
<xref target="responses">Responses</xref>,
and <xref target="priority">Priority</xref>
extensions.
</t>
</section>
<section title="Relationship to JSON-LD 1.0" anchor="json-ld" >
<t>
The <xref target="W3C.WD-json-ld-20130411">JSON-based Serialization for
Linked Data (JSON-LD)</xref> describes a rich syntax for the
serialization of semantically-rich metadata using the JSON format.
While the updated Activity Streams representation provided by this
document is not defined as a "JSON-LD Vocabulary", the syntax is
designed to be closely compatible with JSON-LD.
</t>
<t>
There are a few differences between JSON-LD and the
serialization syntax described here, specifically:
<list style="symbols">
<t>
JSON-LD uses certain field names with a leading "@" character,
such as "@id" and "@language". In this specification, the
leading "@" is omitted.
</t>
<t>
While JSON-LD allows using relative IRI references
in the values of "id" properties, this specification
limits identifiers to absolute IRIs.
</t>
<t>
While it is possible to derive a JSON-LD "@context" description
for the Activity Streams 2.0 JSON syntax one is not normatively
provided by this specification.
</t>
</list>
</t>
<t>
When processing an Activity Streams document as JSON-LD, the
following rules apply:
<list style="symbols">
<t>
The "objectType" property MUST be treated as an alias of
JSON-LD "@type".
</t>
<t>
The "id" property MUST be treated as an alias of JSON-LD "@id".
</t>
<t>
The "language" property MUST be treated as an alias of JSON-LD
"@language".
</t>
<t>
A JSON array used to convey <xref target="link">Link</xref> values
MUST be treated as an unordered JSON-LD @set (@container = @set).
</t>
<t>
The JSON array value for the "items" property defined in
<xref target="collection" /> MUST be treated as an ordered
JSON-LD @list (@container = @list).
</t>
<t>
The "displayName", "title", "content" and "summary" properties
defined in <xref target="object" /> and <xref target="object-ext" />
MUST be treated as JSON-LD Language Maps (@container = @language).
</t>
</list>
</t>
</section>
<section anchor="serialization" title="Syntax Conventions">
<t>
This specification defines a <xref target="RFC4627">JSON-based</xref>
serialization syntax.
</t>
<t>
When serialized, absent properties are represented by either
(a) setting the property value to null, or (b) by omitting the
property declaration altogether at the option of the publisher;
these representations are semantically equivalent. If a
property has an array value, the absence of any items in that array
MUST be represented by omitting the property entirely or by setting
the value to null.
</t>
<t>
This specification uses IRIs <xref target="RFC3987"/>.
Every URI <xref target="RFC3986" /> is also an IRI, so a URI may be used
wherever an IRI is named. There are two special considerations:
(1) when an IRI that is not also a URI is given for dereferencing, it
MUST be mapped to a URI using the steps in Section 3.1 of
<xref target="RFC3987"/> and (2) when an IRI is serving as
an "id" value, it MUST NOT be so mapped.
</t>
<t>
Unless otherwise specified, all properties with date and
time values MUST conform to the "date-time" production in
<xref target="RFC3339" />, with an uppercase "T" character
used to separate date and time, and an uppercase "Z" character
in the absence of a numeric time zone offset. All such timestamps
SHOULD be represented relative to Coordinated Universal Time (UTC).
</t>
</section>
</section>
<section anchor="examples" title="Example Activities">
<t>
Following are three examples of activities with varying
degrees of detail.
</t>
<section title="Example 1: Minimal Activity">
<t>
Expresses the statement "'urn:example:person:martin' posted
'http://example.org/foo.jpg'". No additional detail is
given.
</t>
<figure>
<artwork><![CDATA[
{
"verb": "post",
"actor": "urn:example:person:martin",
"object": "http://example.org/foo.jpg"
}
]]></artwork>
</figure>
</section>
<section title="Example 2: Basic activity with some additional detail">
<t>
Expresses the statement "Martin Smith posted an article to
the blog 'Martin's Blog' at 3:04 PM GMT on February 2, 2011."
Some additional details about the article, actor and target
blog are given.</t>
<figure><artwork><![CDATA[
{
"verb": "post",
"published": "2011-02-10T15:04:55Z",
"language": "en",
"actor": {
"objectType": "person",
"id": "urn:example:person:martin",
"displayName": "Martin Smith",
"url": "http://example.org/martin",
"image": {
"url": "http://example.org/martin/image.jpg",
"mediaType": "image/jpeg",
"width": 250,
"height": 250
}
},
"object" : {
"objectType": "article",
"id": "urn:example:blog:abc123/xyz"
"url": "http://example.org/blog/2011/02/entry",
"displayName": "Why I love Activity Streams"
},
"target" : {
"objectType": "blog",
"id": "urn:example:blog:abc123",
"displayName": "Martin's Blog",
"url": "http://example.org/blog/"
}
}
]]></artwork></figure>
</section>
<section title="Example 3: An extended activity">
<t>
A more extensive, single-entry "Activity Stream"
follows. In addition to containing a number of required and optional
core properties, the example contains the additional, undefined extension
properties "foo" and "foo2" for illustrative purposes only.
</t>
<figure><artwork><![CDATA[
{
"totalItems": 1,
"items" : [
{
"verb": "post",
"language": "en",
"published": "2011-02-10T15:04:55Z",
"foo": "some extension property",
"generator": "http://example.org/activities-app",
"provider": "http://example.org/activity-stream",
"displayName": {
"en": "Martin posted a new video to his album.",
"ga": "Martin phost le físeán nua a albam."
},
"actor": {
"objectType": "person",
"id": "urn:example:person:martin",
"displayName": "Martin Smith",
"url": "http://example.org/martin",
"foo2": "some other extension property",
"image": {
"url": "http://example.org/martin/image",
"mediaType": "image/jpeg",
"width": 250,
"height": 250
}
},
"object" : {
"objectType": {
"id": "http://example.org/Photo",
"displayName": "Photo"
},
"id": "urn:example:album:abc123/my_fluffy_cat",
"url": "http://example.org/album/my_fluffy_cat.jpg",
"image": {
"url": "http://example.org/album/my_fluffy_cat_thumb.jpg",
"mediaType": "image/jpeg",
"width": 250,
"height": 250
}
},
"target": {
"objectType": {
"id": "http://example.org/PhotoAlbum",
"displayName": "Photo-Album"
},
"id": "urn:example.org:album:abc123",
"url": "http://example.org/album/",
"displayName": {
"en": "Martin's Photo Album",
"ga": "Grianghraif Máirtín"
},
"image": {
"url": "http://example.org/album/thumbnail.jpg",
"mediaType": "image/jpeg",
"width": 250,
"height": 250
}
}
}
]
}
]]></artwork></figure>
</section>
</section>
<section anchor="model" title="Object Model">
<section anchor="object" title="Object">
<t>
The following "core properties" apply to all JSON objects serialized
within an Activity Stream document.
</t>
<texttable>
<ttcol align="left" width="15%">Property</ttcol>
<ttcol align="left" width="21%">Value</ttcol>
<ttcol align="left">Description</ttcol>
<c>id</c>
<c>IRI</c>
<c>
Provides a permanent, universally unique identifier for the
object in the form of an absolute IRI <xref target="RFC3987"/>.
Objects SHOULD contain a single "id" property. If an object
does not contain an "id" property, consumers MAY use the value of
the "url" property as a less-reliable, non-unique identifier.
</c>
<c>objectType</c>
<c><xref target="type-value">Type value</xref></c>
<c>
Identifies the type of object. An object MAY contain a "objectType"
property whose value is a <xref target="type-value">Type value</xref>.
If no "objectType" property is specified, the object has no specific type.
</c>
<c>language</c>
<c><xref target="RFC5646"/> Language Tag</c>
<c>
Establishes the default language assumed for human-readable,
natural-language metadata values included in the object. An
object MAY contain a "language" property whose value MUST be a
<xref target="RFC5646"/> Language-Tag.
</c>
<c>displayName</c>
<c><xref target="lang">Natural Language value</xref></c>
<c>
A simple human-readable, plain-text name for the
object. HTML markup MUST NOT be included. An object MAY contain a
"displayName" property. If the object does not specify a "objectType"
property, the object SHOULD specify a "displayName".
</c>
<c>url</c>
<c><xref target="link">Link</xref> value</c>
<c>
A <xref target="link">Link</xref> value describing a resource
that provides a representation of the object. An object
MAY contain a "url" property.
</c>
</texttable>
</section>
<section anchor="lang" title="Natural Language Values">
<t>
Natural Language values represent human-readable character
sequences in one or more languages. They are expressed as
either (1) a single JSON string or (2) a JSON dictionary
mapping <xref target="RFC5646" /> Language-Tags to localized,
equivalent translations of the same string value.
</t>
<t>
For instance, the "displayName" property in all objects is a
Natural Language value.
</t>
<figure>
<preamble>A single String value using the default language:</preamble>
<artwork>
{
"language": "en",
"displayName": "This is the title"
}
</artwork>
</figure>
<figure>
<preamble>Multiple, language-specific values:</preamble>
<artwork>
{
"displayName": {
"en": "This is the title",
"fr": "C'est le titre",
"sp": "Este es el título"
}
}
</artwork>
</figure>
<t>
Each key in the JSON dictionary MUST be an <xref target="RFC5646" />
Language Tag. The associated values MUST be Strings.
</t>
</section>
<section anchor="type-value" title="Type Values">
<t>
Type values represent references to or descriptions of an abstract
type. They are expressed as either: (1) a String conforming to
either the "isegment-nz-nc" or "IRI" productions in
<xref target="RFC3987"/> or (2) an <xref target="object">Object</xref>.
When represented as a String, the use of relative references other
than a simple name is not allowed. When represented as an Object,
the "id" property MUST be specified.
</t>
<t>
Within the Activity Streams 2.0, Type values are used only
by the "objectType" and "verb" properties.
</t>
<figure><preamble>Object type as a simple name (isegment-nz-nc):</preamble><artwork>
{
"objectType": "person",
"displayName": "John"
}
</artwork></figure>
<figure><preamble>Object type as an absolute IRI:</preamble><artwork>
{
"objectType": "http://example.org/Person",
"displayName": "John"
}
</artwork></figure>
<figure><preamble>Object type as an object:</preamble><artwork>
{
"objectType": {
"id": "http://example.org/Person",
"displayName": "Person"
},
"displayName": "John"
}
</artwork></figure>
<t>
Because the second and third examples above each specify
"http://example.org/Person", the two examples are considered
to specify the same type.
</t>
<figure><preamble>Verb as a simple name (isegment-nz-nc):</preamble><artwork>
{
"verb": "post",
"actor": "acct:john.doe@example.org",
"object": "http://example.org/123"
}
</artwork></figure>
<figure><preamble>Verb as an absolute IRI:</preamble><artwork>
{
"verb": "http://example.com/Upload",
"actor": "acct:john.doe@example.org",
"object": "http://example.org/123"
}
</artwork></figure>
<figure><preamble>Verb as an object:</preamble><artwork>
{
"verb": {
"id": "http://example.com/Upload",
"displayName": "Upload"
},
"actor": "acct:john.doe@example.org",
"object": "http://example.org/123"
}
</artwork></figure>
<t>
Allowing verbs and object types to be represented as objects rather
than simple names or IRIs is intended to simplify the use of
extensions that an implementation might not have encountered
previously. The object properties provide additional information
and metadata about the new verb or object type.
</t>
<t>
It is important to note that because the "id" property is strictly
limited to absolute IRI values, the object representation cannot
be used to describe types with simple names.
</t>
</section>
<section anchor="link" title="Link Values">
<t>
Link values represent references to other objects and resources.
They are expressed as either: (1) a String containing an
absolute or relative IRI, (2) an <xref target="object">Object</xref>,
or (3) a JSON Array containing a mixture of IRIs or
<xref target="object">Objects</xref>. Link values are closely related
to the conceptual model of Links as established in
<xref target="RFC5988"/>.
</t>
<t>
For example, as defined previously, all
<xref target="object">objects</xref> can contain an
"image" property whose value describes a graphical
representation of the containing object. This property
will typically be used to provide the URL to a JPEG,
GIF or PNG type resource that can be displayed to
the user. Any given object might have multiple such visual
representations -- multiple screenshots, for instance, or
the same image at different resolutions. Using Link values,
there are essentially three ways of describing such
references.
</t>
<figure><preamble>To reference a single image without any
additional metadata, the link value can be expressed as
a simple JSON string containing an absolute or relative
IRI:</preamble><artwork>
{
"objectType": "application",
"id": "http://example.org/application/123",
"displayName": "My Application",
"image": "http://example.org/application/123.png"
}
</artwork></figure>
<figure><preamble>Alternatively, if additional metadata is required,
the link can be expressed as an object containing the url
property.</preamble>
<artwork>
{
"objectType": "application",
"id": "http://example.org/application/123",
"displayName": "My Application",
"image": {
"url": "http://example.org/application/123.png",
"mediaType": "image/png",
"height": 320,
"width": 320
}
}
</artwork>
</figure>
<figure><preamble>If more than one link value is to be expressed,
A JSON Array with a mix of string and object elements can be
used:</preamble><artwork>
{
"objectType": "application",
"id": "http://example.org/application/123",
"displayName": "My Application",
"image": [
"http://example.org/application/abc.gif",
{
"url": "http://example.org/application/123.png",
"mediaType": "image/png",
"height": 320,
"width": 320
}
]
}
</artwork></figure>
<t>
Individual items contained in such an array are independent
of the others and no significance is given to the ordering of
those items.
</t>
<t>
RFC 5988 defines that all Links have a "link relation"
that describes the contextual purpose of the link. Within
an <xref target="object">object</xref>, in the absence of a specific
"rel" property within the link object itself, the name of the
property whose value is a link serves as the "link relation".
Any valid link relation value, as defined by RFC 5988, can be used
as a property with a link value in any Activity Streams object, except
where the link relation might conflict with any other property
defined by this specification.
</t>
<figure><preamble>In the following example, two separate links are
provided. The link relation of the first is "image", while the
link relation of the second is "preview". Both links, however, can
be used as alternative visual representations of the
"application" object.</preamble><artwork><![CDATA[
{
"objectType": "application",
"image": [
"http://example.org/foo.jpg",
{
"url": "http://example.org/screens/1.jpg",
"rel": "preview",
"mediaType": "image/jpeg"
}
]
}
]]></artwork></figure>
<t>
When an <xref target="object">object</xref> is used to represent
a Link value, the following additional properties MAY be used:
</t>
<texttable>
<ttcol align="left" width="15%">Property</ttcol>
<ttcol align="left" width="21%">Value</ttcol>
<ttcol align="left">Description</ttcol>
<c>rel</c>
<c>RFC 5988 Link Relation</c>
<c>
The RFC 5988 Link Relation associated with this link value.
If absent, the name of the property is assumed to specify
the link relation.
</c>
<c>mediaType</c>
<c>MIME Media Type</c>
<c>
The MIME media type of the resource being referenced.
</c>
</texttable>
</section>
<section anchor="activity" title="Activity">
<t>
Activity objects are specializations of the base
<xref target="object">Object</xref> type that provide
metadata about potential or completed actions.
</t>
<t>
Within an Activity object, the "verb" property is used
to identify the type of activity. All existing verb definitions
used in JSON Activity Streams 1.0 implementations can continue
to be used and retain their existing semantics. If the "verb"
is not specified, the "objectType" property MAY be used as an
alternative means of determining the activity type.
</t>
<t>
Activity objects extend the core <xref target="object">object</xref>
definition with the following additional, optional properties:
</t>
<texttable>
<ttcol align="left" width="15%">Property</ttcol>
<ttcol align="left" width="21%">Value</ttcol>
<ttcol align="left">Description</ttcol>
<c>verb</c>
<c><xref target="type-value">Type value</xref></c>
<c>
Identifies the type of activity. An activity SHOULD contain a "verb"
property whose value is a <xref target="type-value">Type value</xref>.
If the "verb" property is not specified, the activity MUST contain a
"objectType" property.
</c>
<c>actor</c>
<c><xref target="link">Link</xref> value</c>
<c>
Describes one or more entities that either peformed or are
expected to perform the activity.
</c>
<c>object</c>
<c><xref target="link">Link</xref> value</c>
<c>
Describes the primary object of the activity. For instance,
in the activity, "John saved a movie to his wishlist", the object
of the activity is "movie". An activity SHOULD contain an
"object" property. If the "object" property is not contained, the
primary object of the activity MAY be implied by context.
</c>
<c>target</c>
<c><xref target="link">Link</xref> value</c>
<c>
Describes the target of the activity. The precise meaning of the
activity's target is dependent on the activities "verb",
but will often be the object the English preposition "to".
For instance, in the activity, "John saved a movie to his
wishlist", the target of the activity is "wishlist". The activity
target MUST NOT be used to identity an indirect object that is
not a target of the activity.
</c>
<c>result</c>
<c><xref target="link">Link</xref> value</c>
<c>
Describes the result of the activity. For instance, if a particular
action results in the creation of a new resource, the "result"
property can be used to describe that new resource.
</c>
<c>priority</c>
<c>Decimal Number between 0.00 and 1.00</c>
<c>
An indicator of the relative priority or importance that
the creator of an activity considers the it to
have. Represented as a numeric decimal between 0.00 and 1.00,
with two decimal places of precision. If the property is
omitted or set to null, the assumption is that a default priority
can be assumed. The value 0.00 represents the lowest possible
priority while 1.00 represents the highest.
</c>
</texttable>
<section title="Considerations on the use of "priority"">
<t>
The presence of the "priority" property does not impose any
specific processing or display requirements on the part of any
entity consuming the activity.
</t>
<t>
Expressing the value as a range of numeric decimal values is intended
to provide the greatest level of flexibility in the expression and
consumption of prioritization detail. It is expected that
implementors consuming activity objects containing "priority" will
utilize and expose the additional information in a number of
different ways depending on the unique requirements of each
application use case.
</t>
<t>
Many existing systems do not represent priority values as
numeric ranges. Such systems might use fixed, labeled brackets
such as "low", "normal" and "high" or "urgent". Similar mechanisms
can be established, by convention, when using the "priority"
property. In typical use, it is RECOMMENDED that implementations
wishing to work with such defined categories treat "priority"
property values in the range 0.00 to 0.25 as "low" priority; values
greater than 0.25 to 0.75 as "normal" priority; and values greater
than 0.75 to 1.00 as "high" priority. Specific implementations are
free to establish alternative conventions for the grouping of
priority values with the caveat that such conventions likely will
not be understood by all implementations.
</t>
</section>
<section anchor="audience-targeting" title="Audience Targeting Properties">
<t>
Every Activity has both a Primary and Secondary audience. The
Primary audience consists of those entities either directly
involved in the performance of the activity or who "own" the
objects involved. The Secondary audience consists of the
collection of entities sharing an interest in the activity but
who are not directly involved (e.g. "followers").
</t>
<t>
For instance, suppose a social network of three individuals:
Bob, Joe and Jane. Bob and Joe are each friends with Jane but
not friends with one another. Bob has chosen to "follow" activities
for which Jane is directly involved. Jane shares a file with Joe.
</t>
<t>
In this example, Jane and Joe are each directly involved in the
file sharing activity and together make up the Primary Audience
for that event. Bob, having an interest in activities involving
Jane, is the Secondary Audience. Knowing this, a system that
produces or consumes the activity can intelligently notify each
person of the event.
</t>
<t>
While there are means, based on the verb, actor, object and
target of the activity, to infer the primary audience for many
types of activities, those do not work in every case and do not
provide a means of identifying the secondary audience. The
"to", "cc", "bto" and "bcc" properties MAY be used within an
Activity to explicitly identify the Primary and Secondary
audiences.
</t>
<texttable>
<ttcol align="left" width="15%">Property</ttcol>
<ttcol align="left" width="21%">Value</ttcol>
<ttcol align="left">Description</ttcol>
<c>to</c>
<c><xref target="link">Link</xref> value</c>
<c>
Specifies the public primary audience.
</c>
<c>cc</c>
<c><xref target="link">Link</xref> value</c>
<c>
Specifies the public secondary audience.
</c>
<c>bto</c>
<c><xref target="link">Link</xref> value</c>
<c>
Specifies the private primary audience.
</c>
<c>bcc</c>
<c><xref target="link">Link</xref> value</c>
<c>
Specifies the private secondary audience.
</c>
</texttable>
<t>
The prototypical use case for an Activity containing these
properties is the publication and redistribution of Activities
through an intermediary. That is, an event source generates the
activity and publishes it to the intermediary which determines
a subset of events to display to specific individual users or
groups. Such a determination can be made, in part, by identifying
the Primary and Secondary Audiences for each activity.
</t>
<t>
When the event source generates the activity and specifies values
for the to and cc fields, the intermediary SHOULD redistribute
that event with the values of those fields intact, allowing any
processor to see who the activity has been targeted to. This is
precisely the same model used by the to and cc fields in email
systems.
</t>
<t>
There are situations, however, in which disclosing the identity
of specific members of the audience may be inappropriate. For
instance, a user may not wish to let other users know that they
are interested in various topics, individuals or types of events.
To support this option, an event source generating an activity
MAY use the "bto" and "bcc" properties to list entities to whom the
activity should be privately targeted. When an intermediary
receives an activity containing these properties, it MUST remove
those values prior to redistributing the activity. The intent is
that systems MUST consider entities listed within the "bto" and
"bcc" properties as part of the Primary and Second audience but
MUST NOT disclose that fact to any other party.
</t>
<t>
Audience targeting information included within an Activity only
describes the intent of the activity creator. With clear exception
given to the appropriate handling of "bto" and "bcc", this
specification leaves it up to implementations to determine how
the audience targeting information is used.
</t>
</section>
</section>
<section anchor="object-ext" title="Additional Object Properties">
<t>
The following "additional properties" MAY be used with any
JSON Object serialized within an Activity Stream document.
</t>
<texttable>
<ttcol align="left" width="15%">Property</ttcol>
<ttcol align="left" width="21%">Value</ttcol>
<ttcol align="left">Description</ttcol>
<c>alias</c>
<c>IRI</c>
<c>
Provides a contextually meaningful alternative label for the
object in addition to the "id". For instance, within some
systems, groups can be identified both by a unique global ID
and a more "human-friendly" label such as "@friends" or "@network".
The value of the "alias" property MUST match either the "isegment-nz-nc"
or the "IRI" production in <xref target="RFC3987"/>. The use of a
relative reference other than a simple name is not allowed.
</c>
<c>attachments</c>
<c><xref target="link">Link</xref> value</c>
<c>
A <xref target="link">Link</xref> value referencing one or more
objects associated with the containing object. These are similiar
in concept to files attached to an email message.
</c>
<c>author</c>
<c><xref target="link">Link</xref> value</c>
<c>
A <xref target="link">Link</xref> value referencing one or more
entity that created or authored the object.
</c>
<c>content</c>
<c><xref target="lang">Natural Language value</xref></c>
<c>
A Natural-language description of the object encoded as a single
JSON String containing HTML markup. Visual elements such as
thumbnail images MAY be included.
</c>
<c>duplicates</c>
<c><xref target="link">Link</xref> value</c>
<c>
A <xref target="link">Link</xref>value referencing one or more
objects that are semantically equivalent to this object or duplicate
this objects content. An object SHOULD contain a "duplicates"
property when there are known objects, possibly in a different
system, that are semantically equivalent or duplicate the content.
</c>
<c>icon</c>
<c><xref target="link">Link</xref> value</c>
<c>
A <xref target="link">Link</xref> value referencing one or more
visual, graphic representations of the object, intended for human
consumption. The visual element SHOULD have an aspect ratio of
one (horizontal) to one (vertical) and SHOULD be suitable for
presentation at a small size.
</c>
<c>image</c>
<c><xref target="link">Link</xref> value</c>
<c>
A <xref target="link">Link</xref> value referencing one or more
visual, graphic represenations of the object. Unlike the "icon"
property, there are no aspect ratio or display restrictions.
</c>
<c>location</c>
<c><xref target="link">Link</xref> value</c>
<c>
A <xref target="link">Link</xref> value describing one or more
physical or virtual locations associated with which the object.
</c>
<c>published</c>
<c><xref target="RFC3339"/> date-time</c>
<c>
The date and time at which the object was published.
</c>
<c>generator</c>
<c><xref target="link">Link</xref> value</c>
<c>
A <xref target="link">Link</xref> value referencing the
application that generated the object.
</c>
<c>provider</c>
<c><xref target="link">Link</xref> value</c>
<c>
A <xref target="link">Link</xref> value referencing the
application that published the object. Note that this
is not necessarily the same entity that generated the
object.
</c>
<c>summary</c>
<c><xref target="lang">Natural Language value</xref></c>
<c>
A Natural-language summarization of the object encoded as a single
JSON String containing a fragment of HTML markup. Visual elements
such as thumbnail images can be included.
</c>
<c>updated</c>
<c><xref target="RFC3339"/> date-time</c>
<c>
The date and time at which a previously published object has
been modified.
</c>
<c>startTime</c>
<c><xref target="RFC3339"/> date-time</c>
<c>
A date-time describing the actual or expected starting time of
the object. When used within an Activity object, for instance,
the "startTime" specifies the moment the activity began or is
scheduled to begin.
</c>
<c>endTime</c>
<c><xref target="RFC3339" /> date-time</c>
<c>
A date-time describing the actual or expected ending time of
the object. When used within an Activity object, for instance,
the "endTime" specifies the moment the activity concluded or is
scheduled to conclude.
</c>
<c>rating</c>
<c>Decimal Number between 1.0 and 5.0</c>
<c>
A quality rating expressed as a number between 1.0 and 5.0
(inclusive) with one decimal place of precision.
</c>
<c>tags</c>
<c><xref target="link">Link</xref> value</c>
<c>
A <xref target="link">Link</xref> value referencing one or more
resources that are loosely associated with the containing object.
The "tags" and "attachments" properties differ from one another
in that the "tags" property asserts "association by reference"
while "attachments" asserts "association by enclosure".
</c>
<c>title</c>
<c><xref target="lang">Natural Language</xref> value</c>
<c>
A Natural-language title for the object expressed as a fragment
of HTML markup. The "title" and "displayName" properties are closely
related and overlap in function with the key difference being
that "title" is permitted to contain HTML markup, while "displayName"
is not.
</c>
<c>duration</c>
<c>Integer or <xref target="RFC3339" /> duration</c>
<c>
When the object describes a time-based resource, such as audio
or video, the "duration" property indicates the approximate
duration of time expressed as an either an RFC 3339 "duration" (e.g.
a duration of 5 seconds is represented as "PT5S") or as a
non-negative integer specifying the duration in seconds.
</c>
<c>height</c>
<c>Integer</c>
<c>
When the object describes a visual resource, such as an
image, video or embeddable HTML page, the "height" property
indicates the recommended display height in pixels.
</c>
<c>width</c>
<c>Integer</c>
<c>
When the object describes a visual resource, such as an
image, video or embeddable HTML page, the "width" property
indicates the recommended display width in pixels.
</c>
<c>inReplyTo</c>
<c><xref target="link">Link</xref> value</c>
<c>
A <xref target="link">Link</xref> value identifying one or
more other objects to which the containing object can be
considered a response.
</c>
<c>actions</c>
<c><xref target="action">Action</xref> value</c>
<c>
An optional <xref target="action">Action</xref> value that
describes potential activities that can be performed with the
object.
</c>
</texttable>
<section anchor="action" title="Action Values">
<t>
The "actions" property on an Activity Streams object is used
to describe the kinds of activities that can be taken with regards
to the object. The value is expressed as a JSON dictionary
mapping verbs to <xref target="link">Link</xref> values referencing
resources or objects that can be used to carry out those verbs.
</t>
<t>
For instance, a hypothetical object with "video" as the objectType
might have "watch", "share" and "embed" as potential actions:
</t>
<figure><artwork><![CDATA[
{
"objectType": "video",
"id": "http://example.org/cats.mpg",
"actions": {
"watch": "movie://example.org/cats.mpg",
"share": {
"objectType": "service",
"displayName": "My Sharing Service",
"url": "http://example.net/share"
},
"embed": [
"http://example.org/gadgets/video.xml?v=cats.mpg",
{
"objectType": "inline-html",
"content": "<video ... />"
}
]
}
}
]]></artwork></figure>
<t>
Each key in the Action value MUST be a valid verb identifier
conforming to either the "isegment-nz-nc" or "IRI" productions in
<xref target="RFC3987"/> and be suitable for use as a value for
<xref target="activity">Activity</xref> object's "verb" property.
The value of each key MUST be a valid <xref target="link">Link</xref>
value.
</t>
</section>
</section>
<section anchor="collection" title="Collection">
<t>
Collection objects are a specialization of the base
<xref target="object">Object</xref> that contain a
listing of other <xref target="object">objects</xref>
The Collection object is used primarily as the root of an
Activity Streams document as described in <xref target="document"/>,
but can be used as the value of object properties.
</t>
<t>
Collections have both a logical model and a physical
serialization. While the logical view of a collection
might contain a large number of objects, any single
serialized representation might include only a subset
of those objects, with specific <xref target="link">Link</xref>
values used to reference additional serialized representations
that include additional subsets. Such representations are known
as "multi-page collections", with each serialized subset
representing a single "page".
</t>
<t>
The value of the Collection object's "objectType" property MUST
be "collection" unless the fact that the object is a collection
can be determined by context.
</t>
<t>
Collection objects extend the core <xref target="object">object</xref>
definition with the following additional properties:
</t>
<texttable>
<ttcol align="left" width="15%">Property</ttcol>
<ttcol align="left" width="21%">Value</ttcol>
<ttcol align="left">Description</ttcol>
<c>totalItems</c>
<c>Integer</c>
<c>
Non-negative integer specifying the total number of objects
contained by the logical view of the collection. This number
might not reflect the actual number of items serialized within
the Collection object instance.
</c>
<c>items</c>
<c>
Array of <xref target="object">Objects</xref>
</c>
<c>
An array containing a listing of <xref target="object">Objects</xref>
of any type.
</c>
<c>itemsAfter</c>
<c><xref target="RFC3339" /> date-time</c>
<c>
A RFC 3339 date-time that indicates that the collection contains
only items published or updated strictly after the date and time
specified.
</c>
<c>itemsBefore</c>
<c><xref target="RFC3339" /> date-time</c>
<c>
A RFC 3339 date-time that indicates that the collection contains
only items published or updated strictly before the date and time
specified.
</c>
<c>itemsPerPage</c>
<c>Integer</c>
<c>
A non-negative integer specifying the maximum number of items
that will be included in the value of the items array.
</c>
<c>startIndex</c>
<c>Integer</c>
<c>
A non-negative integer value identifying the relative position
within the logical view of collection of the first object
contained in the items property. For instance, if there are 20
items that are considered to be members of a collection, but
only the last 10 of those items are included in the items
property, the value of startIndex would be 10.
</c>
<c>first</c>
<c><xref target="link">Link</xref> value</c>
<c>
A <xref target="link">Link</xref> value referencing the
furthest preceeding page of a multi-page collection.
</c>
<c>last</c>
<c><xref target="link">Link</xref> value</c>
<c>
A <xref target="link">Link</xref> value referencing the
furthest following page of a multi-page collection.
</c>
<c>prev</c>
<c><xref target="link">Link</xref> value</c>
<c>
A <xref target="link">Link</xref> value referencing the
immediately preceding page of the multi-page collection. Note
that the property name previous can be used as an equivalent
alternative; however implementations SHOULD use prev and MUST
NOT use both prev AND previous within the same collection.
</c>
<c>next</c>
<c><xref target="link">Link</xref> value</c>
<c>
A <xref target="link">Link</xref> value referencing the immediately
following page of the multi-page collection.
</c>
<c>current</c>
<c><xref target="link">Link</xref> value</c>
<c>
A <xref target="link">Link</xref> value referencing the page
containing the items that have been updated or published most
recently.
</c>
<c>self</c>
<c><xref target="link">Link</xref> value</c>
<c>
A <xref target="link">Link</xref> value referencing this page.
</c>
</texttable>
<section anchor="summary-properties" title="Using Collections as Summary Values">
<t>
It is a common practice to use Collection objects to provide
summary information on the number of specific types of events
that have occurred with respect to any given object. For instance,
a "note" object may have been "shared" or "liked" a number of
times by different individuals. In such cases, the Collection
object is used as a property value with the "totalItems" field
used to indicate the total number of occurrences, the "items"
property used to provide details for a subset of the most
recent occurrences, and the "id" property used to reference a
separate Activity Streams document providing additional information.
</t>
<t>
This specification defines the following properties that
MAY be used within any <xref target="object">object</xref>
as "summary values":
</t>
<texttable>
<ttcol align="left" width="15%">Property</ttcol>
<ttcol align="left" width="21%">Value</ttcol>
<ttcol align="left">Description</ttcol>
<c>replies</c>
<c><xref target="collection">Collection</xref></c>
<c>
Provides information about the set of objects that can be
considered to be replies to the containing object.
</c>
</texttable>
<figure>
<preamble>In the following example, the "replies" property
is used to indicate that a note has 10 responses, and
provides information on the most recently received
response:</preamble>
<artwork>{
"objectType": "note",
"id": "urn:example:note:1",
"displayName": "A note about things",
"content": "blah blah blah",
"replies": {
"url": "http://example.org/note/1/comments.json",
"mediaType": "application/activity+json",
"totalItems": 10,
"items": [
{
"objectType": "note",
"id": "urn:example:note:1:A",
"content": "That's profound, man."
}
]
}
}</artwork></figure>
</section>
</section>
</section>
<section anchor="document" title="The Activity Stream JSON Document">
<t>
The above defined JSON serialization can be used to represent
activities, objects and media links in any context. This section
defines one particular use of the above formats to publish a JSON
document representing an ordered listing of Activity objects.
</t>
<t>
Publishers using this format MUST produce a valid JSON document
whose root value is a <xref target="collection">Collection</xref>.
</t>
<t>
The MIME media type of this document MUST be
"application/activity+json".
</t>
</section>
<section anchor="deprecated-syntax" title="Deprecated Activity Streams 1.0 Syntax">
<t>
The JSON syntax defined by this specification differs somewhat
from that defined in the original
<xref target="activitystreams-1.0">JSON Activity Streams 1.0</xref>
specification in ways that are not backwards compatible. Implementations
can choose to continue supporting the JSON Activity Streams 1.0 syntax
but SHOULD consider it to be deprecated. This means that while
implementations MAY continue to consume the 1.0 syntax, they SHOULD NOT
output the 1.0 syntax unless specifically interacting with older
non-2.0 compliant implementations.
</t>
<t>
Specifically:
<list style="numbers">
<t>
Implementations MUST use the "application/stream+json" MIME media
type when producing a JSON serialization of an Activity Object
conforming to the 1.0 syntax, and "application/activity+json"
when producing a serialization conforming to the 2.0 syntax.
</t>
<t>
Implementations that process serializations of an Activity
Object identified using either the "application/stream+json"
or the more generic "application/json" MIME media type MUST follow
the syntax and processing rules set by
<xref target="activitystreams-1.0"/>. The 2.0 syntax and
processing rules apply only when handling serializations
using the "application/activity+json" media type.
</t>
<t>
This document redefines the "displayName", "title", "content"
and "summary" properties as <xref target="lang">Natural Language
values</xref>, which means their values can be expressed as either a
String or a JSON-LD Language Map. In the 1.0 syntax, these are
expressed solely as String values. Because the 1.0 values
are a valid subset allowed by this specification, implementations
are not required to take any specific action to continue supporting
those values.
</t>
<t>
This document redefines a large number of common properties
defined originally as Objects in 1.0 as
<xref target="link">Link values</xref>. This means the
property values can be expressed as either an IRI String,
an Object, or an Array of IRI Strings and Objects. Because the 1.0
values are a valid subset allowed by this specification,
implementations are not required to take any specific action to
continue supporting those values.
</t>
<t>
This specification replaces the "upstreamDuplicates" and
"downstreamDuplicates" properties defined in the 1.0 syntax
with a singular "duplicates" property with a
<xref target="link">Link value</xref>. The "upstreamDuplicates"
and "downstreamDuplicates" property values in 1.0 are defined
as Arrays of strings. Implementations MUST consider the union
of these two values as an alias for the "duplicates" property.
</t>
</list>
</t>
<t>
By following these requirements, all JSON Activity Streams 1.0
serializations can be processed successfully by 2.0 implementations.
</t>
</section>
<section anchor="id-comparison" title="Comparison of Identifier Values">
<t>
The values of "id" properties can be compared to determine if the
identifiers represent duplicate content. The values MUST be compared
on a character-by-character, case-sensitive basis. Comparisons
MUST be based solely on the character strings themselves and MUST NOT
rely on dereferencing the IRIs or URIs mapped from them.
</t>
<t>
As a consequence, two IRIs that resolve to the same resource but are
not character-for-character identical will be considered different for
the purposes of identifier comparison. In such cases, the "duplicates"
property can be used to expressly relate such objects to one another.
</t>
</section>
<section anchor="extensions" title="Extensibility">
<t>
Processors that encounter unfamiliar properties within any
Activity Streams object MUST NOT stop processing or signal
an error and MUST continue processing the items as if those
properties were not present.
</t>
</section>
<section anchor="security" title="Security Considerations">
<t>
Publishers or Consumers implementing Activity Streams as a stream of
public data may also want to consider the potential for unsolicited
commercial or malicious content and should take preventative measures to
recognize such content and either identify it or not include it in their
implementations.
</t>
<t>
Publishers should take reasonable measures to ensure potentially
malicious user input such as cross-site scripting attacks are not
included in the Activity Streams data they publish.
</t>
<t>
Consumers that re-emit ingested content to end-users MUST take
reasonable measures if emitting ingested content to make sure
potentially malicious ingested input is not re-emitted.
</t>
<t>
Consumers that re-emit ingested content for crawling by search
engines should take reasonable measures to limit any use of their site
as a Search Engine Optimization loophole. This may include converting
un-trusted hyperlinks to text or including a rel="nofollow"
attribute.
</t>
<t>
Consumers should be aware of the potential for spoofing attacks
where the attacker publishes activities or objects with falsified
property values with the intent of injecting malicious content,
hiding or corrupting legitimate content, or misleading users.
</t>
<t>
Activity Streams are JSON Documents and are subject to the same
security considerations described in <xref target="RFC4627"/>.
</t>
<t>
Activity Streams implementations handle URIs. See Section 7 of
<xref target="RFC3986"/>.
</t>
<t>
Activity Streams implementations handle IRIs. See Section 8 of
<xref target="RFC3987"/>.
</t>
</section>
<section anchor="iana" title="IANA Considerations">
<section anchor="media-type" title="application/activity+xml Media Type">
<t>
This specification registers the application/activity+json
MIME Media Type:
</t>
<t><list>
<t>Type name: application</t>
<t>Subtype name: activity+json</t>
<t>Required parameters: None</t>
<t>
Optional parameters: "charset" : Specifies the character set encoding.
If not specified, a default of "UTF-8" is assumed.
</t>
<t>
Encoding considerations: Resources that use the "application/activity+json"
media type are required to conform to the "application/json" Media Type
and are therefore subject to the same encoding considerations
specified in <xref target="RFC4627">Section 6</xref>.
</t>
<t>Security considerations: As defined in this specification</t>
<t>Published specification: This specification.</t>
<t>
Applications that use this media type: JSON Activity Streams are implemented
by a wide range of existing applications.
</t>
<t>Additional information:
<list>
<t>Magic number(s): N/A</t>
<t>File extension(s): N/A</t>
<t>Macintosh file type code(s): TEXT</t>
</list></t>
<t>Person & email address to contact for further information: James M Snell <jasnell@gmail.com></t>
<t>Intended usage: COMMON</t>
<t>Restrictions on usage: None.</t>
<t>Author: James M Snell <jasnell@gmail.com></t>
<t>Change controller: IESG</t>
</list></t>
</section>
</section>
</middle>
<back>
<references title="Normative References">
&rfc2119;
&rfc3986;
&rfc3987;
&rfc4627;
&rfc3339;
&rfc5988;
&rfc5646;
&jsonld;
<reference anchor="activitystreams-1.0" target="http://activitystrea.ms/specs/json/1.0/">
<front>
<title abbrev="ActivityStreams">JSON Activity Streams 1.0</title>
<author fullname="James M Snell" initials="J.M." surname="Snell">
<organization>IBM</organization>
</author>
<author fullname="Martin Atkins" initials="M." surname="Atkins">
<organization>SAY Media</organization>
</author>
<author fullname="Will Norris" initials="W." surname="Norris">
<organization>Google</organization>
</author>
<author fullname="Chris Messina" initials="C." surname="Messina">
<organization>Citizen Agency, Google</organization>
</author>
<author fullname="Monica Wilkinson" initials="M." surname="Wilkinson">
<organization>MySpace, Facebook, VMware</organization>
</author>
<author fullname="Rob Dolin" initials="R." surname="Dolin">
<organization>Microsoft</organization>
</author>
<date month="May" year="2011" />
<abstract>
<t>This specification details the serialization of a stream of social
activities using the JSON format. Activities are important in that
they allow individuals to process the latest news of people and things
they care about.</t>
</abstract>
</front>
</reference>
</references>
<references title="Informational References">
&rfc6963;
<reference anchor="base-schema" target="https://github.com/activitystreams/activity-schema/blob/master/activity-schema.md">
<front>
<title abbrev="ActivityStreams">Activity Streams - Base Schema</title>
<author>
<organization>Activity Streams Workgroup</organization>
</author>
<date month="September" year="2012" />
</front>
</reference>
<reference anchor="audience" target="http://activitystrea.ms/specs/json/targeting/1.0/">
<front>
<title abbrev="Audience Targeting">Audience Targeting for JSON Activity Streams</title>
<author fullname="James M Snell" initials="J.M." surname="Snell">
<organization>IBM</organization>
</author>
<date month="March" year="2012" />
</front>
</reference>
<reference anchor="responses" target="http://activitystrea.ms/specs/json/replies/1.0/">
<front>
<title abbrev="Responses">Responses for Activity Streams</title>
<author fullname="James M Snell" initials="J.M." surname="Snell">
<organization>IBM</organization>
</author>
<date month="March" year="2012" />
</front>
</reference>
<reference anchor="priority" target="https://raw.github.com/jasnell/specs/master/activitystrea.ms/priority_extension.txt">
<front>
<title abbrev="Priority">Priority Extension for JSON Activity Streams</title>
<author fullname="James M Snell" initials="J.M." surname="Snell">
<organization>IBM</organization>
</author>
<date month="June" year="2012" />
</front>
</reference>
</references>
<section anchor="acknowledgements" title="Acknowledgements">
<t>The author wishes to thank the Activity Streams community and
implementers for their support, encouragement, and enthusiasm including
but not limited to: Abdul Qabiz, Adina Levin, Adrian Chan, Adriana Javier,
Alan Hoffman, Alex Kessinger, Alexander Ovchinnikov, Alexander Zhuravlev,
Alexandre Loureiro Solleiro, Amy Walgenbach, Andres Vidal, Angel Robert
Marquez, Ari Steinberg, Arjan Scherpenisse, Arne Roomann-Kurrik, Beau
Lebens, Ben Hedrington, Ben Metcalfe, Ben Werdmuller, Benjamin Goering,
Bill de hÓra, Bo Xing, Bob Aman, Bob Wyman, Brett Slatkin, Brian Walsh,
Brynn Evans, Charlie Cauthen, Chris Chabot, Chris Messina, Chris Toomey, Christian
Crumlish, Dan Brickley, Dan Scott, Daniel Chapman, Danny Ayers, Dare
Obasanjo, Darren Bounds, David Cramer, David Nelson, David Recordon,
DeWitt Clinton, Douglas Pearce, Ed Summers, Elias Bizannes, Elisabeth
Norris, Eric Marcoullier, Eric Woods, Evan Prodromou, Gee-Hsien Chuang,
Greg Biggers, Gregory Foster, Henry Saputra, Hillary Madsen, Howard
Liptzin, Hung Tran, Ian Kennedy, Ian Mulvany, Ivan Pulleyn, Jacob Kim,
James Falkner, James Pike, James Walker, Jason Kahn, Jason Kantz, Jeff
Kunins, Jeff Martin, Jian Lin, Johannes Ernst, John Panzer, Jon Lebkowsky,
Jon Paul Davies, Jonathan Coffman, Jonathan Dugan, Joseph Boyle, Joseph
Holsten, Joseph Smarr, Josh Brewer, Jud Valeski, Julien Chaumond, Julien
Genestoux, Jyri Engeström, Kaliya Hamlin, Kevin Marks, Laurent Eschenauer,
Laurie Voss, Leah Culver, Libby Miller, Manu Mukerji, Mark Weitzel, Marko
Degenkolb, Marshall Kirkpatrick, Martin Atkins, Martin Svensson, Marty Alchin, Mary
Hoder, Matt Leventi, Matt Wilkinson, Matthias Müller-Prove, Max Engel, Max
Wegmueller, Melvin Carvalho, Michael Buckbee, Michael Chan, Michael
Richardson, Michael Sullivan, Mike Macgirvin, Mislav Marohnić, Mo Jangda,
Monica Wilkinson, Nate Benes, NeilFred Picciotto, Nick Howard, Nick Lothian, Nissan
Dookeran, Nitya Narasimhan, Pablo Martin, Pádraic Brady, Pat G.
Cappalaere, Patrick Aljord, Peter Ferne, Peter Reiser, Peter Saint-Andre,
Phil Wolff, Philip (flip) Kromer, Richard Cunningham, Richard Zhao, Rick
Severson, Robert Hall, Robert Langbert, Robert Dolin, Robin Cover, Ryan Boyd, Sam Sethi,
Scott Raymond, Scott Seely, Simon Grant, Simon Wistow, Stephen Garcia,
Stephen Sisk, Stephen Paul Weber, Steve Ivy, Steve Midgley, Steven
Livingstone-Perez, Sylvain Carle, Sylvain Hellegouarch, Tantek Çelik, Tatu
Saloranta, Tim Moore, Timothy Young, Todd Barnard, Tosh Meston, Tyler
Gillies, Will Norris, Zach Copley, and Zach Shepherd.</t>
</section>
<section anchor="Appendix-A" title="Processing as JSON-LD">
<t>
While the Activity Streams 2.0 syntax is designed to be compatible
with JSON-LD, in order to successfully process an Activity Streams
document as JSON-LD, a "@context" description needs to be provided.
The following example illustrates an Activity Streams document
that can be processed as JSON-LD containing Schema.org defined
metadata elements.
</t>
<figure><artwork>
{
"@context": {
"@vocab": "http://activitystrea.ms/spec/2.0/",
"verb": "@type",
"objectType": "@type",
"id": "@id",
"actor": "http://schema.org/Action/performedBy",
"object": "http://schema.org/BuyAction/bought",
"purchase": "http://schema.org/BuyAction",
"person": "http://schema.org/Person",
"book": "http://schema.org/Book"
},
"verb" : "purchase",
"id" : "urn:example:purchase:123/abc",
"displayName": "John purchased 'A Tale of Two Cities'",
"startTime" : "2013-04-02T12:31Z",
"endTime" : "2013-04-02T12:31Z",
"actor": {
"objectType": "person",
"displayName": "John Doe"
},
"object": {
"objectType": "book",
"displayName": "A Tale of Two Cities"
}
}
</artwork></figure>
</section>
<section anchor="Appendix-B" title="Motivational Use Cases">
<t>
This specification defines a number of syntax changes relative
to the JSON Activity Streams 1.0 specification. The sections that
follow describe some of the general motivations for these changes
with illustrative examples.
</t>
<section title="Internationalization (i18n)">
<t>
The JSON Activity Streams 1.0 syntax has no inherent notion
of a "language context". That is, the core syntax has no
internal mechanism a publisher can use to identify the language
used when constructing the Activity Streams document. Nor are
there any existing mechanisms at the JSON syntax level that an
Activity Streams implementation can inherit. This specification
introduces the "language" property and Natural Language Value
concepts to fill this gap.
</t>
<t>
Imagine a scenario with a service that receives Activity objects
from users and republishes those to a distributed audience of
interested parties. This service spans international boundaries
and the users speak a multitude of different languages. Within this
system, a native English speaker might subscribe to notifications
about activities posted by a native French speaker.
</t>
<t>
For instance, let's suppose that our native French speaker posts
the following activity to this system:
</t>
<figure><artwork><![CDATA[
POST /activity/feed HTTP/1.1
Content-Type: application/activity+xml
{
"verb": "post",
"language": "fr",
"object": {
"objectType": "article",
"displayName": "Un exemple basique",
...
}
}
]]></artwork></figure>
<t>
The system receives this activity post and prepares to notify
our native English speaking user. Knowing that this user prefers
English and does not speak a word of French, the system can
inspect the Activity and detect automatically that a translation
ought to be provided. Rather than replacing the original French
text, however, the service can simply add in the English translation
along side it.
</t>
<figure><artwork><![CDATA[
{
"verb": "post",
"language": "fr",
"actor": {
"id": "urn:example:person:abc",
"displayName": "Jean Valjean"
},
"object": {
"type": "article",
"displayName": {
"fr": "Un exemple basique",
"en": "A basic example"
}
...
}
}
]]></artwork></figure>
<t>
It is also possible for a Natural Language Value to express
alternative same-language representations of a string that
utilize different writing systems or regions. For instance,
it is common for Japanese translations to provide equivalent
ideographic (kanji) and phonetic (katakana or hiragana)
alternatives:
</t>
<figure><artwork><![CDATA[
{
"title": {
"ja-Hani": "...",
"ja-Kana": "..."
}
}
]]></artwork></figure>
</section>
<section title="Extensibility (e11y)">
<t>
Arguably, the two most important extensibility points in
the Activity Streams format are the object type and verb
properties. Implementations are free to come up with their
own types and verbs at any point. While such extensibility
is extremely powerful, it comes with a cost. Namely,
implementations that encounter previously unknown verbs and
object types may not have enough to knowledge about those
to do anything significant with them.
</t>
<t>
For instance, the most common use case for Activity Streams
today is the generation of a human-readable "activity feed"
that translates Activity objects into sentences just as
"John uploaded a new photo" or "Jane checked in at a hotel",
etc. Given an extension verb such as "http://example.org/whatever",
an implementation might not have sufficient information about
that verb to generate a readable sentence describing the
activity that occurred.
</t>
<t>
With Activity Streams 1.0, a number of different approaches have
been tried to address this problem, but all of the solutions
essentially deal with the need to provide additional metadata
about extension verbs and object types so that an implementation
can dynamically learn and adapt. The notion of "type values"
is added by this specification to specifically deal with this
issue.
</t>
<t>
For example, suppose I have an implementation that generates
Activity objects that use a new extension verb
"urn:example:verbs:upload". Knowing that consumers of these
objects might not have encountered this verb before, I want
to make it possible for those implementations to automatically
discover metadata about the new verb. To do so, I can use
a type value to provide some basic information.
</t>
<figure><artwork><![CDATA[
{
"verb": {
"id": "urn:example:verbs:upload",
"url": "http://example.org/verbs.json",
"mediaType": "application/ld+json",
"displayName": {
"en": "upload",
"fr": "televersement"
},
"alias": "post"
},
"actor": {
"type": "person",
"displayName": "John"
},
"object": {
"type": "photo",
"displayName": "cats.jpg"
}
}
]]></artwork></figure>
<t>
An implementation receiving this has several choices. It could
choose to ignore everything other than the verb's identifier,
treating it generically as one would have to today using the
1.0 syntax; or, it could inspect the metadata provided and notice
that the extension verb can be treated generally as an alias of
"post" or displayed in English as "upload" and in French as
"televersement"; or, it can choose to attempt discovering more
information about the verb by dereferencing the provided URL.
</t>
<t>
The point is, these options are built into the core syntax,
making extension verbs and object types significantly more
usable, particularly when combined with the new language
context features.
</t>
</section>
<section title="First Class Links">
<t>
Linking in the 1.0 syntax is largely undefined and inconsistent.
There is a general notion of Media Link objects that are used for
some things like images and videos, along with a "url" property
that in some cases is used to always point to HTML represenations
while in other cases might point to JSON documents or image files,
and there is no reusable concept of a generic link provided for
extensions to leverage which has led to inconsistent implementation.
The 2.0 syntax introduced here deals with these issues by introducing
a clear, consistent, reusable first class linking model.
</t>
<t>
For instance, using the 2.0 syntax, an "image" object type can
be represented simply as:
</t>
<figure><artwork><![CDATA[
{
"objectType": "image",
"url": "http://example.org/cats.jpg",
"mediaType": "image/jpeg",
"displayName": "A picture of my cats",
"alternate": {
"url": "http://example.org/gallery?i=cats.jpg",
"mediaType": "text/html"
}
"preview": "http://example.org/thumbnails/cats.jpg"
}
]]></artwork></figure>
<t>
Essentially, any 2.0 object that contains a "url" property can
be interpreted as a link. That "url" property points to a
representation of the object, while the "mediaType" property
identifies the content type of that linked resource. RFC 5988
Link Relations can be used directly within the 2.0 syntax to
provide additional data -- in this case, an alternative HTML
representation of the image as well as a thumbnail preview.
</t>
<t>
Another case that the more flexible linking approach allows us
to address is providing multiple links for a single property.
For instance, it is not uncommon for there to be several alternative
versions of an image resource offered at various resolutions to
support multiple types of devices. With the 2.0 syntax, multiple
choices for a single link can be easily provided.
</t>
<figure><artwork><![CDATA[
{
"objectType": "application",
"displayName": "My application",
"icon": [
{
"url": "http://example.org/sd/icon.png",
"width": 57,
"height": 57
},
{
"url": "http://example.org/hd/icon.png",
"width": 114,
"height": 114
}
],
"preview": [
"http://www.example.org/screenshots/1.jpg",
"http://www.example.org/screenshots/2.jpg",
"http://www.example.org/screenshots/3.jpg"
]
}
]]></artwork></figure>
</section>
<section title="Use of External Vocabularies">
<t>
Use of an "external vocabulary" within Activity Streams means
using object types, verbs and properties that are not defined
by the core Activity Streams specification. An example would be
using concepts defined within a microdata vocabulary such as
that defined by Schema.org (http://schema.org).
</t>
<t>
Implementations that wish to use a Activity Streams with such
external vocabularies face the challenge that, often times,
identical or overlapping concepts can be expressed in a multitude
of ways depending on which vocabulary is selected. This can
make it difficult to map abstract data models into a specific
JSON serialization.
</t>
<t>
For instance, suppose an application uses the Schema.org model
to represent an article, described here: http://schema.org/Article.
Within this model, there are several properties defined that directly
overlap properties defined by the core Activity Streams syntax.
Such properties include "name", "contentLocation", and "articleBody". In
order to encode the abstact model of a Schema.org/Article into the
JSON Activity Streams model, the application needs to determine
precisely how to map the abstract properties to the serialized
format. In Activity Streams 1.0, no guidance was given on how to
achieve such a mapping, within the 2.0 syntax, the JSON Serialization
for Linked Data (JSON-LD) provides a foundation.
</t>
<t>
Using JSON-LD I can maintain basic Activity Streams 2.0 syntax
while mapping the physical serialization to the abstract model
inline.
</t>
<figure><artwork><![CDATA[
{
"objectType": "article",
"displayName": "My article about things",
"content": "This is my article",
"@context": {
"objectType": "@type",
"article": "http://schema.org/Article",
"displayName": "http://schema.org/name",
"content": "http://schema.org/Article/articleBody"
}
}
]]></artwork></figure>
<t>
For any non-JSON-LD aware implementation, this can be processed just
as if it were an ordinary Activity Streams object, without any
additional consideration given. For a JSON-Ld aware implementation,
however, the addition of the "@context" property allows the
serialized JSON to be unambiguously mapped to the Schema.org
concept of an "Article". The fact that we can support such a mapping
allows the Activity Streams format to extend to a broader range of
scenarios without requiring alternative, incompatible vocabulary
specific models of "actions" or "activities" to be developed.
</t>
</section>
</section>
</back>
</rfc>
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