XEP-xxxx: Atomically Compare-And-Publish PubSub Items

3.1 PubSub Item Compare-And-Publish Value (CAP-V)¶

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.

CAP-Vs are assoicated with PubSub node's items via the item ID. The maping information is placed by the PubSub service in a <cap-v-map/> extension element, qualified by the 'urn:xmpp:pubsub:cap:0' namespace, as child element of the <items/> element. The <cap-v-map/> element contains one or more <cap-v-map-entry/> 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.

<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>

3.2 PubSub publishing using Compare-And-Publish¶

In order to atomically compare-and-publish an item, a client sends a XEP-0060 <publish/> 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.

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.

<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>

3.3 Could not publish because newest item ID did not match¶

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 <conflict/> error of type 'modify' which a pubsub-specific condition of <precondition-not-met/> and a <compare-and-publish-failed/> 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.

<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>

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1. XEP-0030: Service Discovery <https://xmpp.org/extensions/xep-0030.html>.