xeps/xep-0395.xml

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<xep>
<header>
  <title>Atomically Compare-And-Publish PubSub Items</title>
  <abstract>This specification provides a mechanism to atomically Compare-And-Publish items to a PubSub node.</abstract>
  &LEGALNOTICE;
  <number>0395</number>
  <status>Experimental</status>
  <type>Standards Track</type>
  <sig>Standards</sig>
  <approver>Council</approver>
  <dependencies>
    <spec>XEP-0030</spec>
    <spec>XEP-0060</spec>
  </dependencies>
  <supersedes/>
  <supersededby/>
  <shortname>cap</shortname>
  &flow;
  <revision>
    <version>0.1</version>
    <date>2017-11-29</date>
    <initials>XEP Editor (jwi)</initials>
    <remark><p>Accepted by Council as Expremental XEP</p></remark>
  </revision>
  <revision>
    <version>0.0.3</version>
    <date>2017-10-06</date>
    <initials>fs</initials>
    <remark><p>Use a custom item value (CAP-V).</p></remark>
  </revision>
  <revision>
    <version>0.0.2</version>
    <date>2017-08-25</date>
    <initials>fs</initials>
    <remark><p>Use PubSub publish-options preconditions.</p></remark>
  </revision>
  <revision>
    <version>0.0.1</version>
    <date>2017-04-20</date>
    <initials>fs</initials>
    <remark><p>First draft.</p></remark>
  </revision>
</header>

<section1 topic='Introduction' anchor='intro'>

  <p>This specification provides a mechanism to atomically publish
  items to a PubSub node depending on the item ID of the node's latest
  item. This allows to prevent race conditions and avoids data
  loss in certain situations.</p>

</section1>

<section1 topic='Discoverying Support' anchor='disco'>

  <p>If an entity supports the Compared-And-Publish feature it MUST
  advertise the fact by returning a &lt;feature/&gt; with the 'var'
  attribute set to 'urn:xmpp:pubsub:cap:0' in response to a &xep0030;
  query for information.</p>

</section1>

<section1 topic='Compare-And-Publish PubSub Items' anchor='cap'>

  <section2 topic='PubSub Item Compare-And-Publish Value (CAP-V)' anchor='cap-v'>

    <p>PubSub services supporting the Compare-And-Publish PubSub extension MUST include a Comapre-and-Publish value
    (CAP-V) for every item in every response. The CAP-V value MUST change if the content of the item changed and
    different item content under the same node MUST NOT yield the same CAP-V. A simple computation of the CAP-ID would
    be to hash the String representation of the item's content.</p>

	<p>CAP-Vs are assoicated with PubSub node's items via the item ID. The maping information is placed by the PubSub
	service in a &lt;cap-v-map/&gt; extension element, qualified by the 'urn:xmpp:pubsub:cap:0' namespace, as child
	element of the &lt;items/&gt; element. The &lt;cap-v-map/&gt; element contains one or more &lt;cap-v-map-entry/&gt;
	elements, of which each MUST have a 'item-id' and a 'cap-value' attribute. The former contains the PubSub item ID
	value and the later contains the according CAP-V of the item.</p>

    <example caption='Service returns some items and their according CAP-Vs'><![CDATA[
<iq type='result'
    from='pubsub.shakespeare.lit'
    to='francisco@denmark.lit/barracks'
    id='items1'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <items node='princely_musings'>
      <item id='368866411b877c30064a5f62b917cffe'>
        <entry xmlns='http://www.w3.org/2005/Atom'>
          <title>The Uses of This World</title>
          <summary>
O, that this too too solid flesh would melt
Thaw and resolve itself into a dew!
          </summary>
        </entry>
      </item>
      <item id='3300659945416e274474e469a1f0154c'>
        <entry xmlns='http://www.w3.org/2005/Atom'>
          <title>Ghostly Encounters</title>
          <summary>
O all you host of heaven! O earth! what else?
And shall I couple hell? O, fie! Hold, hold, my heart;
And you, my sinews, grow not instant old,
But bear me stiffly up. Remember thee!
          </summary>
        </entry>
      </item>
      <item id='4e30f35051b7b8b42abe083742187228'>
        <entry xmlns='http://www.w3.org/2005/Atom'>
          <title>Alone</title>
          <summary>
Now I am alone.
O, what a rogue and peasant slave am I!
          </summary>
        </entry>
      </item>
      <cap-v-map xmlns='urn:xmpp:pubsub:cap:0'>
        <cap-value-map-entry
          item-id='368866411b877c30064a5f62b917cffe'
          cap-value='35a204c2-5d6c-43a2-8e0d-a235a627b04a'/>
        <cap-value-map-entry
          item-id='3300659945416e274474e469a1f0154c'
          cap-value='166b7c04-ed4d-4872-aa56-a58268da84e2'/>
        <cap-value-map-entry
          item-id='4e30f35051b7b8b42abe083742187228'
          cap-value='67f7f792-f2ee-4918-8894-36a3c4a6dd5f'/>
      </cap-v-map>
    </items>
  </pubsub>
</iq>
]]></example>

  </section2>

  <section2 topic='PubSub publishing using Compare-And-Publish'>

    <p>In order to atomically compare-and-publish an item, a client sends a <cite>XEP-0060</cite> &lt;publish/&gt; IQ
    with a 'pubsub#prev_item_cap_value' precondition publishing option, set to the value of the currently assumed CAP-V
    of the latest item of the node.</p>

    <p>The PubSub service MUST only publish the item if the node's latest item CAP-V is equal to the
    CAP-V found in the 'pubsub#prev_item_cap_value' field.</p>

    <example caption='Atomically publishing with Compare-And-Publish'><![CDATA[
<iq type='set'
    from='hamlet@denmark.lit/blogbot'
    to='pubsub.shakespeare.lit'
    id='pub1'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <publish node='princely_musings'>
      <item id='2'>
        <entry xmlns='https://example.org'>
          <title>Soliloquy</title>
          <summary>
To be, or not to be: that is the question:
Whether 'tis nobler in the mind to suffer
The slings and arrows of outrageous fortune,
Or to take arms against a sea of troubles,
And by opposing end them?
          </summary>
        </entry>
      </item>
    </publish>
    <publish-options>
      <x xmlns='jabber:x:data' type='submit'>
        <field var='FORM_TYPE' type='hidden'>
          <value>http://jabber.org/protocol/pubsub#publish-options</value>
        </field>
        <field var='pubsub#prev_item_cap_value'>
          <value>1</value>
        </field>
      </x>
    </publish-options>
  </pubsub>
</iq>
]]></example>

  </section2>

  <section2 topic='Could not publish because newest item ID did not match'>

    <p>In case the Compare-And-Publish operation failed because the latest node id is not the same
    as given in the 'previd' attribute in the request, the server returns an &lt;conflict/&gt; error
    of type 'modify' which a pubsub-specific condition of &lt;precondition-not-met/&gt; and a
    &lt;compare-and-publish-failed/&gt; element qualifed by the 'urn:xmpp:pubsub:cap:0'
    namespace. The element MUST have a 'cap-id' attribute with the CAP-V of the lastest item.</p>

    <example caption='Service returns IQ response notifying of failed Compare-And-Publish operation'><![CDATA[
<iq type='error'
    from='pubsub.shakespeare.lit'
    to='hamlet@denmark.lit/elsinore'
    id='retract1'>
  <error type='modify'>
    <conflict xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
    <precondition-not-met xmlns='http://jabber.org/protocol/pubsub#errors'/>
    <compare-and-publish-failed xmlns='urn:xmpp:pubsub:cap:0' cap-id='2'/>
  </error>
</iq>
]]></example>

  </section2>

</section1>

<section1 topic='Rationale' anchor='rationale'>

  <p>Unfortunately it was not possible to re-use the PubSub item ID for the "Atomically
  Compare-And-Publish" purpose. This is mostly due <cite>XEP-0060 § 12.8</cite> stating that:</p>
  <p class='box'>
  "If a publisher publishes an item and the ItemID
  matches that of an existing item, the pubsub service MUST overwrite the existing item and generate a new event
  notification."
  </p>
  <p> Which means that the content of an item could change without its ID, rendering the item ID
  unusable for CAP. </p>

  <p>Injecting a "cap"-namespaced attribute carrying the item's CAP-V into PubSub's &lt;item/&gt;
  would be a very elegant approach to assign CAP-Vs to PubSub items (and the favored one of the
  XEP's author). But the usage of namespaces attributes within XMPP is controversial. Therefore this
  XEP resorts to using the &lt;cap-v-map/&gt; approach for now.</p>

</section1>

<section1 topic='Security Considerations' anchor='security'>

  <p>This extension protocol does not add any further security considerations to the ones mentioned
  in <cite>XEP-0060 § 14.</cite>.</p>

</section1>

<section1 topic='IANA Considerations' anchor='iana'>

  <p>This document requires no interaction with the Internet Assigned
  Numbers Authority (IANA).</p>

</section1>

<section1 topic='XMPP Registrar Considerations' anchor='registrar'>

  <p>This specification defines the following XML namespaces:</p>
  <ul>
    <li>urn:xmpp:pubsub:cap:0</li>
  </ul>
    <code caption='Registry Submission'><![CDATA[
<var>
    <name>urn:xmpp:pubsub:cap:0</name>
    <desc>Indicates support for Compare-And-Publish</desc>
    <doc>XEP-XXXX</doc>
</var>]]></code>

    <p>This specification defines the following &lt;publish-options/&gt; fields:</p>
    <ul>
      <li>pubsub#prev_item_cap_value</li>
    </ul>
    <code caption='Registry Submission'><![CDATA[
<field var='pubsub#prev_item_cap_value'
         type='text-single'
         label='Precondition: The assumed value of the latest item&apos; CAP-V of the node'/>]]></code>

</section1>

<section1 topic='XML Schema' anchor='schema'>

  <p>TODO: Add after the XEP leaves the 'experimental' state.</p>

</section1>

<section1 topic='Acknowledgements' anchor='acknowledgements'>

  <p>Thanks to Kim Alvefur and Dave Cridland for their feedback.</p>

</section1>

</xep>

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