mirror of
https://github.com/moparisthebest/xeps
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283 lines
9.7 KiB
XML
283 lines
9.7 KiB
XML
<?xml version='1.0' encoding='UTF-8'?>
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<!DOCTYPE xep SYSTEM 'xep.dtd' [
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<!ENTITY % ents SYSTEM 'xep.ent'>
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%ents;
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]>
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<?xml-stylesheet type='text/xsl' href='xep.xsl'?>
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<xep>
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<header>
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<title>Atomically Compare-And-Publish PubSub Items</title>
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<abstract>This specification provides a mechanism to atomically Compare-And-Publish items to a PubSub node.</abstract>
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&LEGALNOTICE;
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<number>0395</number>
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<status>Experimental</status>
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<type>Standards Track</type>
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<sig>Standards</sig>
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<approver>Council</approver>
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<dependencies>
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<spec>XEP-0030</spec>
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<spec>XEP-0060</spec>
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</dependencies>
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<supersedes/>
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<supersededby/>
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<shortname>cap</shortname>
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&flow;
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<revision>
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<version>0.1</version>
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<date>2017-11-29</date>
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<initials>XEP Editor (jwi)</initials>
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<remark><p>Accepted by Council as Expremental XEP</p></remark>
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</revision>
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<revision>
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<version>0.0.3</version>
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<date>2017-10-06</date>
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<initials>fs</initials>
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<remark><p>Use a custom item value (CAP-V).</p></remark>
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</revision>
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<revision>
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<version>0.0.2</version>
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<date>2017-08-25</date>
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<initials>fs</initials>
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<remark><p>Use PubSub publish-options preconditions.</p></remark>
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</revision>
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<revision>
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<version>0.0.1</version>
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<date>2017-04-20</date>
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<initials>fs</initials>
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<remark><p>First draft.</p></remark>
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</revision>
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</header>
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<section1 topic='Introduction' anchor='intro'>
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<p>This specification provides a mechanism to atomically publish
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items to a PubSub node depending on the item ID of the node's latest
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item. This allows to prevent race conditions and avoids data
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loss in certain situations.</p>
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</section1>
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<section1 topic='Discoverying Support' anchor='disco'>
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<p>If an entity supports the Compared-And-Publish feature it MUST
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advertise the fact by returning a <feature/> with the 'var'
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attribute set to 'urn:xmpp:pubsub:cap:0' in response to a &xep0030;
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query for information.</p>
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</section1>
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<section1 topic='Compare-And-Publish PubSub Items' anchor='cap'>
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<section2 topic='PubSub Item Compare-And-Publish Value (CAP-V)' anchor='cap-v'>
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<p>PubSub services supporting the Compare-And-Publish PubSub extension MUST include a Comapre-and-Publish value
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(CAP-V) for every item in every response. The CAP-V value MUST change if the content of the item changed and
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different item content under the same node MUST NOT yield the same CAP-V. A simple computation of the CAP-ID would
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be to hash the String representation of the item's content.</p>
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<p>CAP-Vs are assoicated with PubSub node's items via the item ID. The maping information is placed by the PubSub
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service in a <cap-v-map/> extension element, qualified by the 'urn:xmpp:pubsub:cap:0' namespace, as child
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element of the <items/> element. The <cap-v-map/> element contains one or more <cap-v-map-entry/>
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elements, of which each MUST have a 'item-id' and a 'cap-value' attribute. The former contains the PubSub item ID
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value and the later contains the according CAP-V of the item.</p>
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<example caption='Service returns some items and their according CAP-Vs'><![CDATA[
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<iq type='result'
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from='pubsub.shakespeare.lit'
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to='francisco@denmark.lit/barracks'
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id='items1'>
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<pubsub xmlns='http://jabber.org/protocol/pubsub'>
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<items node='princely_musings'>
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<item id='368866411b877c30064a5f62b917cffe'>
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<entry xmlns='http://www.w3.org/2005/Atom'>
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<title>The Uses of This World</title>
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<summary>
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O, that this too too solid flesh would melt
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Thaw and resolve itself into a dew!
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</summary>
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</entry>
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</item>
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<item id='3300659945416e274474e469a1f0154c'>
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<entry xmlns='http://www.w3.org/2005/Atom'>
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<title>Ghostly Encounters</title>
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<summary>
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O all you host of heaven! O earth! what else?
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And shall I couple hell? O, fie! Hold, hold, my heart;
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And you, my sinews, grow not instant old,
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But bear me stiffly up. Remember thee!
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</summary>
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</entry>
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</item>
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<item id='4e30f35051b7b8b42abe083742187228'>
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<entry xmlns='http://www.w3.org/2005/Atom'>
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<title>Alone</title>
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<summary>
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Now I am alone.
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O, what a rogue and peasant slave am I!
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</summary>
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</entry>
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</item>
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<cap-v-map xmlns='urn:xmpp:pubsub:cap:0'>
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<cap-value-map-entry
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item-id='368866411b877c30064a5f62b917cffe'
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cap-value='35a204c2-5d6c-43a2-8e0d-a235a627b04a'/>
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<cap-value-map-entry
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item-id='3300659945416e274474e469a1f0154c'
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cap-value='166b7c04-ed4d-4872-aa56-a58268da84e2'/>
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<cap-value-map-entry
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item-id='4e30f35051b7b8b42abe083742187228'
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cap-value='67f7f792-f2ee-4918-8894-36a3c4a6dd5f'/>
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</cap-v-map>
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</items>
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</pubsub>
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</iq>
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]]></example>
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</section2>
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<section2 topic='PubSub publishing using Compare-And-Publish'>
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<p>In order to atomically compare-and-publish an item, a client sends a <cite>XEP-0060</cite> <publish/> IQ
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with a 'pubsub#prev_item_cap_value' precondition publishing option, set to the value of the currently assumed CAP-V
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of the latest item of the node.</p>
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<p>The PubSub service MUST only publish the item if the node's latest item CAP-V is equal to the
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CAP-V found in the 'pubsub#prev_item_cap_value' field.</p>
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<example caption='Atomically publishing with Compare-And-Publish'><![CDATA[
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<iq type='set'
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from='hamlet@denmark.lit/blogbot'
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to='pubsub.shakespeare.lit'
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id='pub1'>
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<pubsub xmlns='http://jabber.org/protocol/pubsub'>
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<publish node='princely_musings'>
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<item id='2'>
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<entry xmlns='https://example.org'>
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<title>Soliloquy</title>
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<summary>
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To be, or not to be: that is the question:
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Whether 'tis nobler in the mind to suffer
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The slings and arrows of outrageous fortune,
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Or to take arms against a sea of troubles,
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And by opposing end them?
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</summary>
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</entry>
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</item>
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</publish>
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<publish-options>
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<x xmlns='jabber:x:data' type='submit'>
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<field var='FORM_TYPE' type='hidden'>
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<value>http://jabber.org/protocol/pubsub#publish-options</value>
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</field>
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<field var='pubsub#prev_item_cap_value'>
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<value>1</value>
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</field>
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</x>
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</publish-options>
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</pubsub>
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</iq>
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]]></example>
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</section2>
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<section2 topic='Could not publish because newest item ID did not match'>
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<p>In case the Compare-And-Publish operation failed because the latest node id is not the same
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as given in the 'previd' attribute in the request, the server returns an <conflict/> error
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of type 'modify' which a pubsub-specific condition of <precondition-not-met/> and a
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<compare-and-publish-failed/> element qualifed by the 'urn:xmpp:pubsub:cap:0'
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namespace. The element MUST have a 'cap-id' attribute with the CAP-V of the lastest item.</p>
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<example caption='Service returns IQ response notifying of failed Compare-And-Publish operation'><![CDATA[
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<iq type='error'
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from='pubsub.shakespeare.lit'
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to='hamlet@denmark.lit/elsinore'
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id='retract1'>
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<error type='modify'>
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<conflict xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
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<precondition-not-met xmlns='http://jabber.org/protocol/pubsub#errors'/>
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<compare-and-publish-failed xmlns='urn:xmpp:pubsub:cap:0' cap-id='2'/>
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</error>
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</iq>
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]]></example>
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</section2>
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</section1>
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<section1 topic='Rationale' anchor='rationale'>
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<p>Unfortunately it was not possible to re-use the PubSub item ID for the "Atomically
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Compare-And-Publish" purpose. This is mostly due <cite>XEP-0060 § 12.8</cite> stating that:</p>
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<p class='box'>
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"If a publisher publishes an item and the ItemID
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matches that of an existing item, the pubsub service MUST overwrite the existing item and generate a new event
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notification."
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</p>
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<p> Which means that the content of an item could change without its ID, rendering the item ID
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unusable for CAP. </p>
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<p>Injecting a "cap"-namespaced attribute carrying the item's CAP-V into PubSub's <item/>
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would be a very elegant approach to assign CAP-Vs to PubSub items (and the favored one of the
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XEP's author). But the usage of namespaces attributes within XMPP is controversial. Therefore this
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XEP resorts to using the <cap-v-map/> approach for now.</p>
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</section1>
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<section1 topic='Security Considerations' anchor='security'>
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<p>This extension protocol does not add any further security considerations to the ones mentioned
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in <cite>XEP-0060 § 14.</cite>.</p>
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</section1>
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<section1 topic='IANA Considerations' anchor='iana'>
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<p>This document requires no interaction with the Internet Assigned
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Numbers Authority (IANA).</p>
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</section1>
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<section1 topic='XMPP Registrar Considerations' anchor='registrar'>
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<p>This specification defines the following XML namespaces:</p>
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<ul>
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<li>urn:xmpp:pubsub:cap:0</li>
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</ul>
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<code caption='Registry Submission'><![CDATA[
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<var>
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<name>urn:xmpp:pubsub:cap:0</name>
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<desc>Indicates support for Compare-And-Publish</desc>
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<doc>XEP-XXXX</doc>
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</var>]]></code>
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<p>This specification defines the following <publish-options/> fields:</p>
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<ul>
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<li>pubsub#prev_item_cap_value</li>
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</ul>
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<code caption='Registry Submission'><![CDATA[
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<field var='pubsub#prev_item_cap_value'
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type='text-single'
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label='Precondition: The assumed value of the latest item' CAP-V of the node'/>]]></code>
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</section1>
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<section1 topic='XML Schema' anchor='schema'>
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<p>TODO: Add after the XEP leaves the 'experimental' state.</p>
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</section1>
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<section1 topic='Acknowledgements' anchor='acknowledgements'>
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<p>Thanks to Kim Alvefur and Dave Cridland for their feedback.</p>
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</section1>
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</xep>
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<!-- Local Variables: -->
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<!-- fill-column: 100 -->
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<!-- indent-tabs-mode: nil -->
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<!-- End: -->
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