json-activity-2.xml

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<?rfc private="Activity Streams Working Group"?>
<rfc>
  <front>
    <title abbrev="ActivityStreams">Draft: JSON Activity Streams 2.0</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="2013" />

    <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.
      </t>
    </note>

  </front>

  <middle>
    <section anchor="introduction" title="Introduction">

      <t>
        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.
      </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 about the activity,
        visual representations about he 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 JSON Activity Streams 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.
        </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>
                
      </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 such that 
          implementations can utilize both with minimal conflict or confusion.
        </t>
        
        <t>
          Most significantly, the JSON Activity Streams 2.0 syntax  
          directly leverages the "@id", "@type", and "@language" properties 
          defined by JSON-LD.
        </t>
        
        <t>
          There are a few differences between JSON-LD and the 
          serialization syntax described here, specifically:
          <list style="symbols">
            <t>
              While JSON-LD allows using relative IRI references
              in the values of @id properties, this specification 
              limits @id properties 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.
            </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 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.
        </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 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
          <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 is a simple example of an activity:</t>

<figure><artwork><![CDATA[
  {
    "@type": "post",
    "published": "2011-02-10T15:04:55Z",
    "actor": {
      "@type": "person",
      "@id": "urn:example:person:martin",
      "name": "Martin Smith",
      "url": "http://example.org/martin",
      "image": {
        "@id": "http://example.org/martin/image",
        "width": 250,
        "height": 250
      }
    },
    "object" : {
      "@id": "urn:example:blog:abc123/xyz"
      "url": "http://example.org/blog/2011/02/entry"
    },
    "target" : {
      "@type": "blog",
      "@id": "urn:example:blog:abc123",
      "name": "Martin's Blog",
      "url": "http://example.org/blog/"
    }
  }
]]></artwork></figure>

        <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[
  {
    "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 físeán nua a albam."
        },
        "actor": {
          "@type": "person",
          "@id": "urn:example:person:martin",
          "name": "Martin Smith",
          "url": "http://example.org/martin",
          "foo2": "some other extension property",
          "image": {
            "@id": "http://example.org/martin/image",
            "width": 250,
            "height": 250
          }
        },
        "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 Máirtín"
          },
          "image": {
            "@id": "http://example.org/album/thumbnail.jpg",
            "width": 250,
            "height": 250
          }
        }
      }
    ]
  }
]]></artwork></figure>

    </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>@type</c>
          <c>IRI</c>
          <c>
            Identifies the type of object. An object MAY contain a "@type"
            property whose value matches 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. If no "@type" 
            property is specified, the object has no specific type.
          </c>
          
          <c>@language</c>
          <c><xref target="RFC5646"/> Language Tag</c>
          <c>
            Identifies the language for all human-readable, natural-language 
            metadata values included in the object. An object MAY contain a
            "@language" property.
          </c>
          
          <c>name</c>
          <c>String</c>
          <c>
            A simple human-readable, plain-text name for the 
            object. HTML markup MUST NOT be included. An object MAY contain a 
            "name" property. If the object does not specify a "@type"
            property, the object SHOULD specify a "name".
          </c>
          <c>url</c>
          <c>Link</c>
          <c>
            A <xref target="link">Link</xref> value describing a resource 
            that provides a visual 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 "title" property in all objects is a
          Natural Language value. This means that the "name" can 
          be expressed in one or more languages:
        </t>
        
        <figure>
          <preamble>A single String value using the default language:</preamble>
          <artwork>
  {
    "@language": "en",
    "title": "This is the title"
  }
          </artwork>
        </figure>
        
        <figure>
          <preamble>Multiple, language-specific values:</preamble>
          <artwork>
  {
    "title": {
      "en": "This is the name",
      "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="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"/>, but have a serialization that is 
          compatible with the JSON-serialization for Linked Data.
        </t>
        
        <t>
          For instance, as defined in 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. Based on the 
          JSON-LD approach, there are essentially three ways of 
          describing these 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>
  {
    "@type": "application",
    "@id": "http://example.org/application/123",
    "name": "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 that uses the @id or url 
          property:</preamble>
          <artwork>
  {
    "@type": "application",
    "@id": "http://example.org/application/123",
    "name": "My Application",
    "image": {
      "@id": "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>
  {
    "@type": "application",
    "@id": "http://example.org/application/123",
    "name": "My Application",
    "image": [
      "http://example.org/application/abc.gif",
      {
        "@id": "http://example.org/application/123.png",
        "mediaType": "image/png",
        "height": 320,
        "width": 320
      }
    ]
  }
        </artwork></figure>
        
        <t>
          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.
        </t>

        <t>
          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. 
        </t>
        
        <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 "@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.
        </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>"actor"</c>
          <c><xref target="link">Link</xref> value</c>
          <c>
            Describes the entity that either peformed or is 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>"context"</c>
          <c><xref target="link">Link</xref> value</c>
          <c>
            The "context" property allows the Activity to further include 
            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 
            <xref target="link">Link</xref> value.
          </c>
          
          <c>"status"</c>
          <c>String</c>
          <c>
            A string value indicating the current status of the activity.
            The value MUST be one of: "tentative", "pending", "completed",
            or "canceled".
          </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 object considers the object 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 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.
          </c>
        </texttable>
        
        <section title="Considerations on the use of &quot;priority&quot;">
          
          <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 typically represent priority values as
            numeric ranges.  Such systems typically 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>
      
      <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>"attachment"</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>"duplicate"</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 "duplicate" 
            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>"source"</c>
          <c><xref target="link">Link</xref> value</c>
          <c>
            A <xref target="link">Link</xref> value referencing the original
            source of 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.
          </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 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>"tag"</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 "tag" and "attachment" properties differ from one another
            in that the "tag" property asserts "association by reference"
            while "attachment" asserts "association by enclosure". 
          </c>
          
          <c>"title"</c>
          <c><xref target="lang">Natural Language</xref> value</c>
          <c>
            A Natural-language title of the object encoded as a single
            JSON String containing HTML markup.
          </c>
          
          <c>"duration"</c>
          <c><xref target="RFC4627">JSON</xref> int</c>
          <c>
            When the object describes a time-based resource, such as audio 
            or video, the "duration" property indicates the approximate 
            length of time in seconds.
          </c>

          <c>"height"</c>
          <c><xref target="RFC4627">JSON</xref> int</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><xref target="RFC4627">JSON</xref> int</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>
          
        </texttable>
        
      </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 "@type" 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>

    </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 a stream of activities.
      </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="id-comparison" title="Comparison of Identifier Values">
      <t>
        The values of multiple <xref target="object">Object</xref>
        and <xref target="link">Link</xref> 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.
      </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 make sure 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">
      <t>None.</t>
    </section>

    <section anchor="license" title="License">
      <t>As of [date], the following persons or entities have made this
      Specification available under the Open Web Foundation Agreement Version
      1.0, which is available at
      <eref target="http://www.openwebfoundation.org/legal/" />.

        <list>
          <t>[List of persons or entities]</t>
        </list>
      </t>

      <t>You can review the signed copies of the Open Web Foundation Agreement
      Version 1.0 for this Specification at
      <eref target="http://activitystrea.ms/licensing/" />, which may also
      include additional parties to those listed above.</t>

      <t>Your use of this Specification may be subject to other third party
      rights. THIS SPECIFICATION IS PROVIDED "AS IS." The contributors expressly
      disclaim any warranties (express, implied, or otherwise), including
      implied warranties of merchantability, non-infringement, fitness for a
      particular purpose, or title, related to the Specification. The entire
      risk as to implementing or otherwise using the Specification is assumed by
      the Specification implementer and user. IN NO EVENT WILL ANY PARTY BE
      LIABLE TO ANY OTHER PARTY FOR LOST PROFITS OR ANY FORM OF INDIRECT,
      SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES OF ANY CHARACTER FROM ANY
      CAUSES OF ACTION OF ANY KIND WITH RESPECT TO THIS SPECIFICATION OR ITS
      GOVERNING AGREEMENT, WHETHER BASED ON BREACH OF CONTRACT, TORT (INCLUDING
      NEGLIGENCE), OR OTHERWISE, AND WHETHER OR NOT THE OTHER PARTY HAS BEEN
      ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.</t>

    </section>

  </middle>

  <back>
    <references title="Normative References">      
      &rfc2119;
      &rfc3986;
      &rfc3987;
      &rfc4627;
      &rfc3339;
      &rfc5988;   
      &rfc5646;  
      &jsonld; 
    </references>
    <references title="Informational References">
      &rfc6963;
    </references>

    <section anchor="acknowledgements" title="Acknowledgements">
      <t>The authors wish 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 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 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,
      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, 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, Zach Copley, and Zach Shepherd.</t>
    </section>
  </back>
</rfc>
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