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288 lines
9.9 KiB
288 lines
9.9 KiB
<?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>Deferred</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>0.2.0</version> |
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<date>2018-12-06</date> |
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<initials>XEP Editor (jsc)</initials> |
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<remark>Defer due to lack of activity.</remark> |
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</revision> |
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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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|
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<section1 topic='Introduction' anchor='intro'> |
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|
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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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|
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<section1 topic='Discoverying Support' anchor='disco'> |
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|
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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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|
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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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|
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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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