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<xep>
<header>
  <title>PubSub Queueing</title>
  <abstract>This specification defines an extension to XMPP publish-subscribe for queueing information at a node.</abstract>
  &LEGALNOTICE;
  <number>0254</number>
  <status>Deferred</status>
  <type>Standards Track</type>
  <sig>Standards</sig>
  <approver>Council</approver>
  <dependencies>
    <spec>XMPP Core</spec>
    <spec>XEP-0060</spec>
  </dependencies>
  <supersedes/>
  <supersededby/>
  <shortname>NOT_YET_ASSIGNED</shortname>
  &hildjj;
  &stpeter;
  <revision>
    <version>0.1</version>
    <date>2008-11-13</date>
    <initials>psa</initials>
    <remark><p>Initial published version.</p></remark>
  </revision>
  <revision>
    <version>0.0.3</version>
    <date>2008-10-07</date>
    <initials>jjh/psa</initials>
    <remark><p>Added more detailed error flows; added additional implementation notes.</p></remark>
  </revision>
  <revision>
    <version>0.0.2</version>
    <date>2008-10-07</date>
    <initials>jjh/psa</initials>
    <remark>
      <ul>
        <li>Specified that notifications are sent to only one subscriber at a time.</li>
        <li>Specified that notifications include payloads.</li>
        <li>Added protocol extension for unlock feature.</li>
        <li>Modified namespace to incorporate namespace versioning.</li>
      </ul>
    </remark>
  </revision>
  <revision>
    <version>0.0.1</version>
    <date>2008-08-14</date>
    <initials>psa/jjh</initials>
    <remark><p>First draft.</p></remark>
  </revision>
</header>

<section1 topic='Introduction' anchor='intro'>
  <p>The &xep0060; extension to XMPP provides a comprehensive technology for alerts, notifications, data syndication, rich presence, and other real-time messaging use cases. In terms of traditional publish-subscribe systems like Java Message Service (JMS), the core XMPP PubSub specification covers the Observer design pattern only; however, traditional publish-subscribe systems often include support for a second design pattern, usually called the "point-to-point" or "queueing" pattern. <note>See for instance &lt;<link url='http://en.wikipedia.org/wiki/Java_Message_Service'>http://en.wikipedia.org/wiki/Java_Message_Service</link>&gt;.</note> This specification defines a few small extensions to XMPP PubSub that enable support for a queueing mode in XMPP. The queueing mode is an add-on feature that a service can support, and that a node owner can enable if supported by the service. The feature name is "urn:xmpp:pubsub:queueing:0".</p>
</section1>

<section1 topic='How It Works' anchor='howitworks'>
  <section2 topic='Subscribing' anchor='subscribe'>
    <p>If a node has enabled support for the queueing mode, in response to a subscription request without configuration options it MUST return an IQ-error containing a subscription options form; this form MUST include the "queue_requests" field (which specifies the number of parallel requests a subscriber is willing to process).</p>
    <example caption='Service indicates that subscription configuration is required'><![CDATA[
<iq from='workflows.shakespeare.lit'
    id='sub1'
    to='workerbee237@shakespeare.lit/foo'
    type='error'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <subscribe
        jid='workerbee237@shakespeare.lit'
        node='a290fjsl29j19kjb'/>
    <options 
        jid='workerbee237@shakespeare.lit'
        node='a290fjsl29j19kjb'>
      <x xmlns='jabber:x:data' type='form'>
        <field var='FORM_TYPE' type='hidden'>
          <value>http://jabber.org/protocol/pubsub#subscribe_options</value>
        </field>
        <field type='text-single' var='pubsub#queue_requests'>
          <required/>
        </field>
      </x>
    </options>
  </pubsub>
  <error type='modify'>
    <not-acceptable xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
    <configuration-required xmlns='http://jabber.org/protocol/pubsub#errors'/>
  </error>
</iq>
    ]]></example>
    <p>The subscriber would then send a new subscription request, this time with options.</p>
    <example caption='Subscribing with options'><![CDATA[
<iq from='workerbee237@shakespeare.lit/foo'
    id='sub2'
    to='workflows.shakespeare.lit'
    type='set'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <subscribe
        jid='workerbee237@shakespeare.lit'
        node='a290fjsl29j19kjb'/>
    <options 
        jid='workerbee237@shakespeare.lit'
        node='a290fjsl29j19kjb'>
      <x xmlns='jabber:x:data' type='submit'>
        <field var='FORM_TYPE' type='hidden'>
          <value>http://jabber.org/protocol/pubsub#subscribe_options</value>
        </field>
        <field var='pubsub#queue_requests'>
          <value>5</value>
        </field>
      </x>
    </options>
  </pubsub>
</iq>
    ]]></example>
    <p>If the subscription request can be processed successfully, the service returns an IQ-result and includes the configuration options established during the negotiation.</p>
    <example caption='Service replies with success (including configuration options)'><![CDATA[
<iq from='workflows.shakespeare.lit'
    id='sub2'
    to='workerbee237@shakespeare.lit/foo'
    type='result'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <subscription
        jid='workerbee237@shakespeare.lit'
        node='a290fjsl29j19kjb'
        subid='ba49252aaa4f5d320c24d3766f0bdcade78c78d3'
        subscription='subscribed'/>
    <options>
      <x xmlns='jabber:x:data' type='result'>
        <field var='FORM_TYPE' type='hidden'>
          <value>http://jabber.org/protocol/pubsub#subscribe_options</value>
        </field>
        <field var='pubsub#queue_requests'>
          <value>5</value>
        </field>
      </x>
    </options>
  </pubsub>
</iq>
    ]]></example>
  </section2>
  <section2 topic='Assigning an Item' anchor='assign'>
    <p>At any time, a publisher can push an item to the queue node.</p>
    <example caption='Publisher publishes an item'><![CDATA[
<iq from='engine.shakespeare.lit'
    id='pub1'
    to='workflow.shakespeare.lit'
    type='set'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <publish node='a290fjsl29j19kjb'>
      <item id='ae890ac52d0df67ed7cfdf51b644e901'>
        <example xmlns='urn:xmpp:example'>payload</example>
      </item>
    </publish>
  </pubsub>
</iq>
    ]]></example>
    <p>The item is published to <em>one</em> of the subscribers.</p>
    <example caption='One subscriber receives a notification'><![CDATA[
<message from='workflow.shakespeare.lit' 
         id='foo'
         to='workerbee237@shakespeare.lit'>
  <event xmlns='http://jabber.org/protocol/pubsub#event'>
    <items node='a290fjsl29j19kjb'>
      <item id='ae890ac52d0df67ed7cfdf51b644e901'>
        <example xmlns='urn:xmpp:example'>payload</example>
      </item>
    </items>
  </event>
</message>
    ]]></example>
  </section2>
  <section2 topic='Deleting an Item from the Queue' anchor='delete'>
    <p>When the subscriber that received the item has successfully processed it (whatever that means in the context of the queue), the subscriber deletes the item from the queue.</p>
    <example caption='Entity deletes an item from a node'><![CDATA[
<iq from='workerbee237@shakespeare.lit/foo'
    id='delete1'
    to='workflows.shakespeare.lit'
    type='get'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <retract node='a290fjsl29j19kjb'>
      <item id='ae890ac52d0df67ed7cfdf51b644e901'/>
    </retract>
  </pubsub>
</iq>
    ]]></example>
    <p>In the context of a queue node, the service MUST treat a delete request from a subscriber that received the item as if the sender were a publisher; i.e., it MUST delete the item from the queue and notify only this subscriber that the item has been deleted.</p>
    <example caption='Subscriber receives delete notification'><![CDATA[
<message from='workflow.shakespeare.lit' 
         id='bar'
         to='workerbee237@shakespeare.lit'>
  <event xmlns='http://jabber.org/protocol/pubsub#event'>
    <items node='a290fjsl29j19kjb'>
      <retract id='ae890ac52d0df67ed7cfdf51b644e901'/>
    </items>
  </event>
</message>
    ]]></example>
    <p>Note: The subscriber SHOULD NOT commit any pending transactions until it receives the delete notification.</p>
    <p>If the item does not exist, the service MUST return an &notfound; error as described in <cite>XEP-0060</cite>.</p>
    <p>If the entity that attempts to delete the item is not the subscriber that received the item, the service MUST return a &forbidden; error as described in <cite>XEP-0060</cite>.</p>
    <p>If the item is locked by another subscriber, the service MUST return a &conflict; error (this flow is not defined in <cite>XEP-0060</cite>.</p>
    <example caption='Item is locked by another subscriber'><![CDATA[
<iq from='workflows.shakespeare.lit'
    id='delete1'
    to='workerbee237@shakespeare.lit/foo'
    type='get'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <retract node='a290fjsl29j19kjb'>
      <item id='ae890ac52d0df67ed7cfdf51b644e901'/>
    </retract>
  </pubsub>
  <error type='auth'>
    <forbidden xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
  </error>
</iq>
    ]]></example>
    <p>If the subscriber that received the item attempts to delete the item but the item is no longer locked by the subscriber (e.g., because of a race condition or a lost notification), the service MUST return an &unexpected; error (this flow is not defined in <cite>XEP-0060</cite>.</p>
    <example caption='Item is no longer locked by subscriber'><![CDATA[
<iq from='workflows.shakespeare.lit'
    id='delete1'
    to='workerbee237@shakespeare.lit/foo'
    type='get'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <retract node='a290fjsl29j19kjb'>
      <item id='ae890ac52d0df67ed7cfdf51b644e901'/>
    </retract>
  </pubsub>
  <error type='wait'>
    <unexpected-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
  </error>
</iq>
    ]]></example>
  </section2>
  <section2 topic='Unlocking an Item' anchor='unlock'>
    <p>The subscriber might determine that it cannot process the item (whatever that means in the context of the queue); if so, the subscriber unlocks the item.</p>
    <example caption='Entity unlocks an item'><![CDATA[
<iq from='workerbee237@shakespeare.lit/foo'
    id='unlock1'
    to='workflows.shakespeare.lit'
    type='get'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <unlock xmlns='urn:xmpp:pubsub:queueing:0'
            node='a290fjsl29j19kjb'>
      <item id='ae890ac52d0df67ed7cfdf51b644e901'/>
    </unlock>
  </pubsub>
</iq>
    ]]></example>
    <p>The service then MUST unlock the item and notify only this subscriber that the item has been unlocked.</p>
    <example caption='Subscriber receives unlock notification'><![CDATA[
<message from='workflow.shakespeare.lit' 
         id='baz'
         to='workerbee237@shakespeare.lit'>
  <event xmlns='http://jabber.org/protocol/pubsub#event'>
    <items node='a290fjsl29j19kjb'>
      <unlock xmlns='urn:xmpp:queueing:0'
              id='ae890ac52d0df67ed7cfdf51b644e901'/>
    </items>
  </event>
</message>
    ]]></example>
    <p>When an item is unlocked, the service would then send a publish notification to another subscriber according to application-specific logic for determining the "next" subscriber.</p>
    <p>If the item does not exist, the service MUST return an &notfound; error.</p>
    <example caption='Item does not exist'><![CDATA[
<iq from='workflows.shakespeare.lit'
    id='delete1'
    to='workerbee237@shakespeare.lit/foo'
    type='error'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <unlock xmlns='urn:xmpp:pubsub:queueing:0'
            node='a290fjsl29j19kjb'>
      <item id='ae890ac52d0df67ed7cfdf51b644e901'/>
    </unlock>
  </pubsub>
  <error type='cancel'>
    <item-not-found xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
  </error>
</iq>
    ]]></example>
    <p>If the entity that attempts to unlock the item is not the subscriber that received the item, the service MUST return a &forbidden; error.</p>
    <example caption='Requesting entity did not receive item'><![CDATA[
<iq from='workflows.shakespeare.lit'
    id='delete1'
    to='workerbee237@shakespeare.lit/foo'
    type='error'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <unlock xmlns='urn:xmpp:pubsub:queueing:0'
            node='a290fjsl29j19kjb'>
      <item id='ae890ac52d0df67ed7cfdf51b644e901'/>
    </unlock>
  </pubsub>
  <error type='auth'>
    <forbidden xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
  </error>
</iq>
    ]]></example>
    <p>If the item is locked by another subscriber, the service MUST return a &conflict; error.</p>
    <example caption='Item is locked by another subscriber'><![CDATA[
<iq from='workflows.shakespeare.lit'
    id='delete1'
    to='workerbee237@shakespeare.lit/foo'
    type='error'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <unlock xmlns='urn:xmpp:pubsub:queueing:0'
            node='a290fjsl29j19kjb'>
      <item id='ae890ac52d0df67ed7cfdf51b644e901'/>
    </unlock>
  </pubsub>
  <error type='cancel'>
    <conflict xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
  </error>
</iq>
    ]]></example>
    <p>If the subscriber that received the item attempts to unlock the item but the item is no longer locked by the subscriber (e.g., because of a race condition or a lost notification), the service MUST return an &unexpected; error.</p>
    <example caption='Item is no longer locked by subscriber'><![CDATA[
<iq from='workflows.shakespeare.lit'
    id='delete1'
    to='workerbee237@shakespeare.lit/foo'
    type='error'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <unlock xmlns='urn:xmpp:pubsub:queueing:0'
            node='a290fjsl29j19kjb'>
      <item id='ae890ac52d0df67ed7cfdf51b644e901'/>
    </unlock>
  </pubsub>
  <error type='wait'>
    <unexpected-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
  </error>
</iq>
    ]]></example>
  </section2>
</section1>

<section1 topic='Determining Support' anchor='support'>
  <p>If a pubsub service supports the queueing mode, it MUST advertise support for  the "urn:xmpp:pubsub:queueing:0" namespace in response to &xep0030; information requests.</p>
</section1>

<section1 topic='Implementation Notes' anchor='impl'>
  <p>If the service receives unavailable presence from a subscriber, it SHOULD unlock all outstanding queue items associated with the subscriber and unsubscribe the subscriber to prevent delivery of further publish notifications.</p>
  <p>If a subscriber cannot process queue items because of an unrecoverable error (e.g., disk full), the subscriber SHOULD unsubscribe and then unlock all of its outstanding queue items.</p>
  <p>If the service does not receive a delete or unlock request from a subscriber that received a queue item in a configurable amount of time, it SHOULD timeout the request, send an unlock notification to the subscriber, and send a publish notification to the "next" subscriber.</p>
</section1>

<section1 topic='Security Considerations' anchor='security'>
  <p>To follow.</p>
</section1>

<section1 topic='IANA Considerations' anchor='iana'>
  <p>This document requires no interaction with &IANA;.</p> 
</section1>

<section1 topic='XMPP Registrar Considerations' anchor='registrar'>
  <section2 topic='Protocol Namespaces' anchor='registrar-ns'>
    <p>This specification defines the following XML namespace:</p>
    <ul>
      <li>urn:xmpp:pubsub:queueing:0</li>
    </ul>
    <p>Upon advancement of this specification from a status of Experimental to a status of Draft, the &REGISTRAR; shall add the foregoing namespaces to the registry located at &NAMESPACES;, as described in Section 4 of &xep0053;.</p>
  </section2>
  <section2 topic='Protocol Versioning' anchor='registrar-versioning'>
    &NSVER;
  </section2>
  <section2 topic='Service Discovery Features' anchor='registrar-features'>
    <p>The &REGISTRAR; maintains a registry of service discovery features (see &DISCOFEATURES;), which includes a number of features that can be returned by pubsub services. The following registry submission supplements the existing list.</p>
    <code><![CDATA[
<var>
  <name>urn:xmpp:pubsub:queueing:0</name>
  <desc>The node or service supports the queueing mode.</desc>
  <doc>XEP-xxxx</doc>
</var>
    ]]></code>
  </section2>
</section1>
</xep>
