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Activity Streams (http://activitystrea.ms)                 J. Snell, Ed.
Internet-Draft                                                       IBM
Intended status: Standards Track                           July 11, 2013
Expires: January 12, 2014


                       JSON Activity Streams 2.0
                    draft-snell-activitystreams-00

Abstract

   This specification details a model for representing potential and
   completed activities using the JSON format.

Author's Note

   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.

   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).

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 12, 2014.





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Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Relationship to JSON Activity Streams 1.0 . . . . . . . .   3
     1.2.  Relationship to JSON-LD 1.0 . . . . . . . . . . . . . . .   3
     1.3.  Syntax Conventions  . . . . . . . . . . . . . . . . . . .   4
   2.  Example Activities  . . . . . . . . . . . . . . . . . . . . .   4
   3.  Object Model  . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Object  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Natural Language Values . . . . . . . . . . . . . . . . .   7
     3.3.  Link Values . . . . . . . . . . . . . . . . . . . . . . .   8
     3.4.  Activity  . . . . . . . . . . . . . . . . . . . . . . . .  10
       3.4.1.  Considerations on the use of "priority" . . . . . . .  11
       3.4.2.  Audience Targeting Properties . . . . . . . . . . . .  12
     3.5.  Additional Object Properties  . . . . . . . . . . . . . .  14
     3.6.  Collection  . . . . . . . . . . . . . . . . . . . . . . .  17
       3.6.1.  Using Collections as Summary Values . . . . . . . . .  19
   4.  The Activity Stream JSON Document . . . . . . . . . . . . . .  20
   5.  Deprecated Activity Streams 1.0 Syntax  . . . . . . . . . . .  20
   6.  Comparison of Identifier Values . . . . . . . . . . . . . . .  22
   7.  Extensibility . . . . . . . . . . . . . . . . . . . . . . . .  22
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  22
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  23
     9.1.  application/activity+xml Media Type . . . . . . . . . . .  23
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  24
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  24
     10.2.  Informational References . . . . . . . . . . . . . . . .  25
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  25
   Appendix B.  Processing as JSON-LD  . . . . . . . . . . . . . . .  26
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  26

1.  Introduction




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   In the most basic sense, an "activity" is a semantic description of
   either a potential or completed action.  In the former case,
   activities express what can be done with any particular object, while
   in the latter case, they express what has already been done.

   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.

   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 [RFC2119].

1.1.  Relationship to JSON Activity Streams 1.0

   The JSON Activity Streams 1.0 [activitystreams-1.0] 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.

   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.

1.2.  Relationship to JSON-LD 1.0

   The JSON-based Serialization for Linked Data (JSON-LD)
   [W3C.WD-json-ld-20130411] 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.

   There are a few differences between JSON-LD and the serialization
   syntax described here, specifically:

   o  JSON-LD uses field names with a leading "@" character, such as
      "@id", "@type" and "@language".  In this specification, the
      leading "@" is omitted.




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   o  While JSON-LD allows using relative IRI references in the values
      of ID properties, this specification limits identifiers to
      absolute IRIs.

   o  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.

1.3.  Syntax Conventions

   This specification defines a JSON-based [RFC4627] serialization
   syntax.

   When serialized, absent properties are represented either by setting
   the property value to null or 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.

   This specification uses IRIs [RFC3987].  Every URI [RFC3986] is also
   an IRI, so a URI may be used wherever below 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 [RFC3987] and (2) when an IRI is serving as
   an "id" value, it MUST NOT be so mapped.

   Unless otherwise specified, all properties with date and time values
   MUST conform to the "date-time" production in [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).

2.  Example Activities

   Following is a simple example of an activity:

   {
     "type": "post",
     "published": "2011-02-10T15:04:55Z",
     "actor": {
       "type": "person",
       "id": "urn:example:person:martin",
       "displayName": "Martin Smith",
       "url": "http://example.org/martin",
       "image": {
         "id": "http://example.org/martin/image",



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         "width": 250,
         "height": 250
       }
     },
     "object" : {
       "type": "article",
       "id": "urn:example:blog:abc123/xyz"
       "url": "http://example.org/blog/2011/02/entry"
     },
     "target" : {
       "type": "blog",
       "id": "urn:example:blog:abc123",
       "displayName": "Martin's Blog",
       "url": "http://example.org/blog/"
     }
   }


   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.

   {
     "items" : [
       {
         "type": "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",
         "title": {
           "en": "Martin posted a new video to his album.",
           "ga": "Martin phost le fisean nua a albam."
         },
         "actor": {
           "type": "person",
           "id": "urn:example:person:martin",
           "displayName": "Martin Smith",
           "url": "http://example.org/martin",
           "foo2": "some other extension property",
           "image": {
             "id": "http://example.org/martin/image",
             "width": 250,
             "height": 250
           }
         },



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         "object" : {
           "type": "photo",
           "id": "urn:example:album:abc123/my_fluffy_cat",
           "url": "http://example.org/album/my_fluffy_cat.jpg",
           "image": {
             "id": "http://example.org/album/my_fluffy_cat_thumb.jpg",
             "width": 250,
             "height": 250
           }
         },
         "target": {
           "url": "http://example.org/album/",
           "type": "photo-album",
           "id": "urn:example.org:album:abc123",
           "title": {
             "en": "Martin's Photo Album",
             "ga": "Grianghraif Mairtin"
           },
           "image": {
             "id": "http://example.org/album/thumbnail.jpg",
             "width": 250,
             "height": 250
           }
         }
       }
     ]
   }


3.  Object Model

3.1.  Object

   The following "core properties" apply to all JSON objects serialized
   within an Activity Stream document.

   +----------+-------------+------------------------------------------+
   | Property | Value       | Description                              |
   +----------+-------------+------------------------------------------+
   | id       | IRI         | Provides a permanent, universally unique |
   |          |             | identifier for the object in the form of |
   |          |             | an absolute IRI [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.                   |
   | type     | IRI         | Identifies the type of object. An object |



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   |          |             | MAY contain a "type" property whose      |
   |          |             | value matches either the "isegment-nz-   |
   |          |             | nc" or the "IRI" production in           |
   |          |             | [RFC3987]. The use of a relative         |
   |          |             | reference other than a simple name is    |
   |          |             | not allowed. If no "type" property is    |
   |          |             | specified, the object has no specific    |
   |          |             | type.                                    |
   | language | [RFC5646]   | Identifies the language for all human-   |
   |          | Language    | readable, natural-language metadata      |
   |          | Tag         | values included in the object. An object |
   |          |             | MAY contain a "language" property whose  |
   |          |             | value MUST be a [RFC5646] Language-Tag.  |
   | name     | String      | 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 "type" property, the  |
   |          |             | object SHOULD specify a "displayName".   |
   | url      | Link        | A Link (Section 3.3) value describing a  |
   |          |             | resource that provides a visual          |
   |          |             | representation of the object. An object  |
   |          |             | MAY contain a "url" property.            |
   +----------+-------------+------------------------------------------+


3.2.  Natural Language Values

   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 [RFC5646] Language-Tags
   to localized, equivalent translations of the same string value.

   For instance, the "title" property in all objects is a Natural
   Language value.

   A single String value using the default language:

   {
     "language": "en",
     "title": "This is the title"
   }


   Multiple, language-specific values:






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   {
     "title": {
       "en": "This is the name",
       "fr": "C'est le titre",
       "sp": "Este es el titulo"
     }
   }


   Each key in the JSON dictionary MUST be an [RFC5646] Language Tag.
   The associated values MUST be Strings.

3.3.  Link Values

   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 Object (Section 3.1), or (3) a JSON Array
   containing a mixture of IRIs or Objects (Section 3.1).  Link values
   are closely related to the conceptual model of Links as established
   in [RFC5988], but have a serialization that is compatible with the
   JSON-serialization for Linked Data.

   For instance, as defined in previously, all objects (Section 3.1) 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.  Based on
   the JSON-LD approach, there are essentially three ways of describing
   these references.

   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:

   {
     "type": "application",
     "id": "http://example.org/application/123",
     "displayName": "My Application",
     "image": "http://example.org/application/123.png"
   }


   Alternatively, if additional metadata is required, the link can be
   expressed as an object that uses the id or url property:





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   {
     "type": "application",
     "id": "http://example.org/application/123",
     "displayName": "My Application",
     "image": {
       "id": "http://example.org/application/123.png",
       "mediaType": "image/png",
       "height": 320,
       "width": 320
     }
   }


   If more than one link value is to be expressed, A JSON Array with a
   mix of string and object elements can be used:

   {
     "type": "application",
     "id": "http://example.org/application/123",
     "displayName": "My Application",
     "image": [
       "http://example.org/application/abc.gif",
       {
         "id": "http://example.org/application/123.png",
         "mediaType": "image/png",
         "height": 320,
         "width": 320
       }
     ]
   }


   Individual items contained in such an array value are independent of
   the others and no significance is given to the order of items in the
   array.

   RFC 5988 defines that all Links have a "link relation" that describes
   the contextual purpose of the link.  Within an Activity Streams
   document, in the absence of a specific "rel" property within the link
   itself, the name of the property whose value is a link serves as the
   "link relation".  Any legal 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.

   When an object (Section 3.1) is used to represent a Link value, the
   following additional properties MAY be used:




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   +---------------+--------------+------------------------------------+
   | Property      | Value        | Description                        |
   +---------------+--------------+------------------------------------+
   | "rel"         | RFC 5988     | The RFC 5988 Link Relation         |
   |               | Link         | associated with this link value.   |
   |               | Relation     | If absent, the name of the         |
   |               |              | property is assumed to specify the |
   |               |              | link relation.                     |
   | "mediaType"   | MIME Media   | The MIME media type of the         |
   |               | Type         | resource being referenced.         |
   +---------------+--------------+------------------------------------+


3.4.  Activity

   Activity objects are specializations of the base Object (Section 3.1)
   type that provide metadata about potential or completed actions.

   Within an Activity object, the "type" property is used to identify
   the type of activity.  Formerly, in JSON Activity Streams version
   1.0, this was represented using the "verb" property.  All existing
   verb definitions used in JSON Activity Streams 1.0 implementations
   can continue to be used as values of the "type" property and retain
   their existing semantics.

   Activity objects extend the core object (Section 3.1) definition with
   the following additional, optional properties:

   +-------------+------------+----------------------------------------+
   | Property    | Value      | Description                            |
   +-------------+------------+----------------------------------------+
   | "actor"     | Link       | Describes the entity that either       |
   |             | (Section   | peformed or is expected to perform the |
   |             | 3.3) value | Activity.                              |
   | "object"    | Link       | Describes the primary object of the    |
   |             | (Section   | activity. For instance, in the         |
   |             | 3.3) value | 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.                            |
   | "target"    | Link       | Describes the target of the activity.  |
   |             | (Section   | The precise meaning of the activity's  |
   |             | 3.3) value | target is dependent on the activities  |
   |             |            | "verb", but will often be the object   |
   |             |            | the English preposition "to".  For     |



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   |             |            | 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.          |
   | "result"    | Link       | Describes the result of the activity.  |
   |             | (Section   | For instance, if a particular action   |
   |             | 3.3) value | results in the creation of a new       |
   |             |            | resource, the "result" property can be |
   |             |            | used to describe that new resource.    |
   | "context"   | Link       | The "context" property allows the      |
   |             | (Section   | Activity to further include            |
   |             | 3.3) value | information about why a particular     |
   |             |            | action occurred by providing details   |
   |             |            | about the context within which a       |
   |             |            | particular action was performed. The   |
   |             |            | value of the context property is Link  |
   |             |            | (Section 3.3) value.                   |
   | "status"    | String     | A string value indicating the current  |
   |             |            | status of the activity.  The value     |
   |             |            | MUST be one of: "tentative",           |
   |             |            | "pending", "completed", or "canceled". |
   | "priority"  | Decimal    | An indicator of the relative priority  |
   |             | Number     | or importance that the creator of an   |
   |             | between    | Activity object considers the object   |
   |             | 0.00 and   | to have. Represented as a numeric      |
   |             | 1.00       | decimal between 0.00 and 1.00, with    |
   |             |            | two decimal places of precision. If    |
   |             |            | the property is omitted, or expicitly  |
   |             |            | set to null, the assumption is that no |
   |             |            | explicit priority or importance can be |
   |             |            | assumed. All other values falling      |
   |             |            | between 0.00 and 1.00 indicate         |
   |             |            | increasing priority.                   |
   +-------------+------------+----------------------------------------+


3.4.1.  Considerations on the use of "priority"

   The presence of the "priority" property does not impose any specific
   processing or display requirements on the part of any entity
   consuming the activity.

   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



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   utilize and expose the additional information in a number of
   different ways depending on the unique requirements of each
   application use case.

   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.

3.4.2.  Audience Targeting Properties

   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").

   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.

   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.

   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.

   +--------------+--------------------+-------------------------------+
   | Property     | Value              | Description                   |
   +--------------+--------------------+-------------------------------+
   | "to"         | Link (Section 3.3) | Specifies the public primary  |



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   |              | value              | audience.                     |
   | "cc"         | Link (Section 3.3) | Specifies the public          |
   |              | value              | secondary audience.           |
   | "bto"        | Link (Section 3.3) | Specifies the private primary |
   |              | value              | audience.                     |
   | "bcc"        | Link (Section 3.3) | Specifies the private         |
   |              | value              | secondary audience.           |
   +--------------+--------------------+-------------------------------+


   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.

   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.

   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.

   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.









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3.5.  Additional Object Properties

   The following "additional properties" MAY be used with any JSON
   Object serialized within an Activity Stream document.

   +---------------+--------------+------------------------------------+
   | Property      | Value        | Description                        |
   +---------------+--------------+------------------------------------+
   | "alias"       | IRI          | 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         |
   |               |              | [RFC3987]. The use of a relative   |
   |               |              | reference other than a simple name |
   |               |              | is not allowed.                    |
   | "attachments" | Link         | A Link (Section 3.3) value         |
   |               | (Section     | referencing one or more objects    |
   |               | 3.3) value   | associated with the containing     |
   |               |              | object. These are similiar in      |
   |               |              | concept to files attached to an    |
   |               |              | email message.                     |
   | "author"      | Link         | A Link (Section 3.3) value         |
   |               | (Section     | referencing one or more entity     |
   |               | 3.3) value   | that created or authored the       |
   |               |              | object.                            |
   | "content"     | Natural      | A Natural-language description of  |
   |               | Language     | the object encoded as a single     |
   |               | value        | JSON String containing HTML        |
   |               | (Section     | markup. Visual elements such as    |
   |               | 3.2)         | thumbnail images MAY be included.  |
   | "duplicates"  | Link         | A Link (Section 3.3)value          |
   |               | (Section     | referencing one or more objects    |
   |               | 3.3) value   | 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.          |
   | "icon"        | Link         | A Link (Section 3.3) value         |



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   |               | (Section     | referencing one or more visual,    |
   |               | 3.3) value   | 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.      |
   | "image"       | Link         | A Link (Section 3.3) value         |
   |               | (Section     | referencing one or more visual,    |
   |               | 3.3) value   | graphic represenations of the      |
   |               |              | object. Unlike the "icon"          |
   |               |              | property, there are no aspect      |
   |               |              | ratio or display restrictions.     |
   | "location"    | Link         | A Link (Section 3.3) value         |
   |               | (Section     | describing one or more physical or |
   |               | 3.3) value   | virtual locations associated with  |
   |               |              | which the object.                  |
   | "published"   | [RFC3339]    | The date and time at which the     |
   |               | date-time    | object was published.              |
   | "generator"   | Link         | A Link (Section 3.3) value         |
   |               | (Section     | referencing the application that   |
   |               | 3.3) value   | generated the object.              |
   | "provider"    | Link         | A Link (Section 3.3) value         |
   |               | (Section     | referencing the application that   |
   |               | 3.3) value   | published the object. Note that    |
   |               |              | this is not necessarily the same   |
   |               |              | entity that generated the object.  |
   | "source"      | Link         | A Link (Section 3.3) value         |
   |               | (Section     | referencing the original source of |
   |               | 3.3) value   | this object. The source property   |
   |               |              | is closely related to the          |
   |               |              | generator and provider properties  |
   |               |              | but serves the distinct purpose of |
   |               |              | identifying where the object was   |
   |               |              | originally published as opposed to |
   |               |              | identifying the applications that  |
   |               |              | generated or published it.         |
   | "summary"     | Natural      | A Natural-language summarization   |
   |               | Language     | of the object encoded as a single  |
   |               | value        | JSON String containing HTML        |
   |               | (Section     | markup. Visual elements such as    |
   |               | 3.2)         | thumbnail images can be included.  |
   | "updated"     | [RFC3339]    | The date and time at which a       |
   |               | date-time    | previously published object has    |
   |               |              | been modified.                     |
   | "startTime"   | [RFC3339]    | A date-time describing the actual  |
   |               | date-time    | or expected starting time of the   |



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   |               |              | object. When used within an        |
   |               |              | Activity object, for instance, the |
   |               |              | "startTime" specifies the moment   |
   |               |              | the activity began or is scheduled |
   |               |              | to begin.                          |
   | "endTime"     | [RFC3339]    | A date-time describing the actual  |
   |               | date-time    | 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.                       |
   | "rating"      | Decimal      | A quality rating expressed as a    |
   |               | Number       | number between 1.0 and 5.0         |
   |               | between 1.0  | (inclusive) with one decimal place |
   |               | and 5.0      | of precision.                      |
   | "tags"        | Link         | A Link (Section 3.3) value         |
   |               | (Section     | referencing one or more resources  |
   |               | 3.3) value   | 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".                     |
   | "title"       | Natural      | A Natural-language title of the    |
   |               | Language     | object encoded as a single JSON    |
   |               | (Section     | String containing HTML markup.     |
   |               | 3.2) value   |                                    |
   | "duration"    | Integer      | When the object describes a time-  |
   |               |              | based resource, such as audio or   |
   |               |              | video, the "duration" property     |
   |               |              | indicates the approximate length   |
   |               |              | of time in seconds.                |
   | "height"      | Integer      | 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.                            |
   | "width"       | Integer      | 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.                            |
   | "inReplyTo"   | Link         | A Link (Section 3.3) value         |



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   |               | (Section     | identifying one or more other      |
   |               | 3.3) value   | objects to which the containing    |
   |               |              | object can be considered a         |
   |               |              | response.                          |
   +---------------+--------------+------------------------------------+


3.6.  Collection

   Collection objects are a specialization of the base Object
   (Section 3.1) that contain a listing of other objects (Section 3.1)
   The Collection object is used primarily as the root of an Activity
   Streams document as described in Section 4, but can be used as the
   value of object properties.

   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 Link (Section 3.3) 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".

   The value of the Collection object's "type" property MUST be
   "collection" unless the fact that the object is a collection can be
   determined by context.

   Collection objects extend the core object (Section 3.1) definition
   with the following additional properties:

   +----------------+--------------+-----------------------------------+
   | Property       | Value        | Description                       |
   +----------------+--------------+-----------------------------------+
   | "totalItems"   | Integer      | 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.       |
   | "items"        | Array of     | An array containing a listing of  |
   |                | Objects      | Objects (Section 3.1) of any      |
   |                | (Section     | type.                             |
   |                | 3.1)         |                                   |
   | "itemsAfter"   | [RFC3339]    | A RFC 3339 date-time that         |
   |                | date-time    | indicates that the collection     |
   |                |              | contains only items published or  |



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   |                |              | updated strictly after the date   |
   |                |              | and time specified.               |
   | "itemsBefore"  | [RFC3339]    | A RFC 3339 date-time that         |
   |                | date-time    | indicates that the collection     |
   |                |              | contains only items published or  |
   |                |              | updated strictly before the date  |
   |                |              | and time specified.               |
   | "itemsPerPage" | Integer      | A non-negative integer specifying |
   |                |              | the maximum number of items that  |
   |                |              | will be included in the value of  |
   |                |              | the items array.                  |
   | "startIndex"   | Integer      | 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.                               |
   | "first"        | Link         | A Link (Section 3.3) value        |
   |                | (Section     | referencing the furthest          |
   |                | 3.3) value   | preceeding page of a multi-page   |
   |                |              | collection.                       |
   | "last"         | Link         | A Link (Section 3.3) value        |
   |                | (Section     | referencing the furthest          |
   |                | 3.3) value   | following page of a multi-page    |
   |                |              | collection.                       |
   | "prev"         | Link         | A Link (Section 3.3) value        |
   |                | (Section     | referencing the immediately       |
   |                | 3.3) value   | 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.                       |
   | "next"         | Link         | A Link (Section 3.3) value        |
   |                | (Section     | referencing the immediately       |
   |                | 3.3) value   | following page of the multi-page  |
   |                |              | collection.                       |
   | "current"      | Link         | A Link (Section 3.3) value        |
   |                | (Section     | referencing the page containing   |



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   |                | 3.3) value   | the items that have been updated  |
   |                |              | or published most recently.       |
   | "self"         | Link         | A Link (Section 3.3) value        |
   |                | (Section     | referencing this page.            |
   |                | 3.3) value   |                                   |
   +----------------+--------------+-----------------------------------+


3.6.1.  Using Collections as Summary Values

   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.

   This specification defines the following properties that MAY be used
   within any object (Section 3.1) as "summary values":

   +--------------+----------------+-----------------------------------+
   | Property     | Value          | Description                       |
   +--------------+----------------+-----------------------------------+
   | "replies"    | Collection     | Provides information about the    |
   |              | (Section 3.6)  | set of objects that can be        |
   |              |                | considered to be replies to the   |
   |              |                | containing object.                |
   +--------------+----------------+-----------------------------------+


   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:














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   {
     "type": "note",
     "id": "urn:example:note:1",
     "displayName": "Note",
     "title": "A Note about things",
     "content": "blah blah blah",
     "replies": {
       "id": "http://example.org/note/1/comments.json",
       "mediaType": "application/activity+json",
       "totalItems": 10,
       "items": [
         {
           "type": "note",
           "id": "urn:example:note:1:A",
           "content": "That's profound, man."
         }
       ]
     }
   }


4.  The Activity Stream JSON Document

   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 a stream of activities.

   Publishers using this format MUST produce a valid JSON document whose
   root value is a Collection (Section 3.6).

   The MIME media type of this document MUST be "application/
   activity+json".

5.  Deprecated Activity Streams 1.0 Syntax

   The JSON syntax defined by this specification differs somewhat from
   that defined in the original JSON Activity Streams 1.0
   [activitystreams-1.0] 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.

   Specifically:




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   1.  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.

   2.  Implementations that process serializations of an Activity Object
       identified using either the "application/stream+json" or more
       generic "application/json" MIME media type MUST follow the syntax
       and processing rules set by [activitystreams-1.0].  The 2.0
       syntax and processing rules apply only when handling
       serializations using the "application/activity+json" media type.

   3.  This document combines the 1.0 "verb" and "objectType" concepts
       into a single type system.  For backwards compatibility,
       implementations MUST treat the "verb" and "objectType" properties
       as aliases of the new "type" property.  In situations where an
       object contains both "verb" and "objectType" properties, the
       "verb" will take precedence when mapping to the new "type"
       property.

   4.  This document redefines the "title", "content" and "summary"
       properties as Natural Language Values (Section 3.2), 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.

   5.  This document redefines a large number of common properties
       defined originally as Objects in 1.0 as Link values
       (Section 3.3).  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.

   6.  This specification replaces the "upstreamDuplicates" and
       "downstreamDuplicates" properties defined in the 1.0 syntax with
       a singular "duplicates" property with a Link value (Section 3.3).
       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.

   By following these requirements, all JSON Activity Streams 1.0
   serializations can be processed successfully by 2.0 implementations.





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6.  Comparison of Identifier Values

   The values of multiple Object (Section 3.1) and Link (Section 3.3)
   value's "id" properties can be compared to determine if the objects
   represent duplicate content.  Processors MUST compare these values on
   a character-by-character, case-sensitive basis.  Comparison
   operations MUST be based solely on the IRI character strings and MUST
   NOT rely on dereferencing the IRIs or URIs mapped from them.

   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.

7.  Extensibility

   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.

8.  Security Considerations

   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.

   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.

   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.

   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.

   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.



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   Activity Streams are JSON Documents and are subject to the same
   security considerations described in [RFC4627].

   Activity Streams implementations handle URIs.  See Section 7 of
   [RFC3986].

   Activity Streams implementations handle IRIs.  See Section 8 of
   [RFC3987].

9.  IANA Considerations

9.1.  application/activity+xml Media Type

   This specification registers the application/activity+json MIME Media
   Type:

      Type name: application

      Subtype name: activity+json

      Required parameters: None

      Optional parameters: "charset" : Specifies the character set
      encoding.  If not specified, a default of "UTF-8" is assumed.

      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 Section 6 [RFC4627].

      Security considerations: As defined in this specification

      Published specification: This specification.

      Applications that use this media type: JSON Activity Streams are
      implemented by a wide range of existing applications.

      Additional information:

         Magic number(s): N/A

         File extension(s): N/A

         Macintosh file type code(s): TEXT

      Person & email address to contact for further information: James M
      Snell <jasnell@gmail.com>




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      Intended usage: COMMON

      Restrictions on usage: None.

      Author: James M Snell <jasnell@gmail.com>

      Change controller: IESG

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3339]  Klyne, G., Ed. and C. Newman, "Date and Time on the
              Internet: Timestamps", RFC 3339, July 2002.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66, RFC
              3986, January 2005.

   [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
              Identifiers (IRIs)", RFC 3987, January 2005.

   [RFC4627]  Crockford, D., "The application/json Media Type for
              JavaScript Object Notation (JSON)", RFC 4627, July 2006.

   [RFC5646]  Phillips, A. and M. Davis, "Tags for Identifying
              Languages", BCP 47, RFC 5646, September 2009.

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988, October 2010.

   [W3C.WD-json-ld-20130411]
              Sporny, M., Kellogg, G., and M. Lanthaler, "JSON-LD 1.0",
              World Wide Web Consortium LastCall WD-json-ld-20130411,
              April 2013,
              <http://www.w3.org/TR/2013/WD-json-ld-20130411>.

   [activitystreams-1.0]
              Snell, J., Atkins, M., Norris, W., Messina, C., Wilkinson,
              M., and R. Dolin, "JSON Activity Streams 1.0", May 2011.









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10.2.  Informational References

   [RFC6963]  Saint-Andre, P., "A Uniform Resource Name (URN) Namespace
              for Examples", BCP 183, RFC 6963, May 2013.

Appendix A.  Acknowledgements

   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 hOra, 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 Engestroem, 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 Mueller-Prove, Max
   Engel, Max Wegmueller, Melvin Carvalho, Michael Buckbee, Michael
   Chan, Michael Richardson, Michael Sullivan, Mike Macgirvin, Mislav
   Marohni&#263;, Mo Jangda, Monica Wilkinson, Nate Benes, NeilFred
   Picciotto, Nick Howard, Nick Lothian, Nissan Dookeran, Nitya
   Narasimhan, Pablo Martin, Padraic 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 Celik, Tatu Saloranta, Tim Moore, Timothy Young,
   Todd Barnard, Tosh Meston, Tyler Gillies, Will Norris, Zach Copley,
   and Zach Shepherd.





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Internet-Draft               ActivityStreams                   July 2013


Appendix B.  Processing as JSON-LD

   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.

   {
     "@context": {
       "@vocab": "http://activitystrea.ms/spec/2.0/",
       "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"
     },
     "type" : "purchase",
     "id" : "urn:example:purchase:123/abc",
     "title": "John purchased 'A Tale of Two Cities'",
     "startTime" : "2013-04-02T12:31Z",
     "endTime" : "2013-04-02T12:31Z",
     "actor": {
       "type": "person",
       "displayName": "John Doe"
     },
     "status":"completed",
     "object": {
       "type": "book",
       "title": "A Tale of Two Cities"
     }
   }


Author's Address

   James M Snell (editor)
   IBM












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