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Update inbox/cap.xml

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Florian Schmaus 2017-10-06 11:10:58 +02:00
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inbox/cap.xml
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@ -22,6 +22,18 @@
<supersededby/>
<shortname>cap</shortname>
&flow;
<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>
@ -50,28 +62,91 @@
<section1 topic='Compare-And-Publish PubSub Items' anchor='cap'>
<p>In order to atomically compare-and-publish an item, a client
sends an &IQ; with a 'pubsub' element qualified by the
'urn:xmpp:pubsub:cap:0' namespace. The element MUST contain the same
attributes and elements as the &lt;publish/&gt; element defined in
&xep0060; <em>and</em> it MUST contain a previd attribute containing
an item ID.</p>
<section2 topic='PubSub Item Compare-And-Publish Value (CAP-V)' anchor='cap-v'>
<p>The PubSub service MUST only publish the item if the node's
latest item ID is equal to the ID found in the 'previd'
attribute.</p>
<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>
<example caption='Publisher publishes an item using Compare-And-Publish'><![CDATA[
<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='compare-and-publish1'>
<pubsub xmlns='urn:xmpp:pubsub:cap:0'>
<compare-and-publish
node='princely_musings'
previd='1'>
<item>
<entry xmlns='http://www.w3.org/2005/Atom'>
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:
@ -80,63 +155,73 @@ The slings and arrows of outrageous fortune,
Or to take arms against a sea of troubles,
And by opposing end them?
</summary>
<link rel='alternate' type='text/html'
href='http://denmark.lit/2003/12/13/atom03'/>
<id>tag:denmark.lit,2003:entry-32397</id>
<published>2003-12-13T18:30:02Z</published>
<updated>2003-12-13T18:30:02Z</updated>
</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 topic='Successfully published an item using Compare-And-Publish'>
<p>If the 'previd' matched the latest item's ID and if the service
was able to successfully process the request then the protocol
continues as defined in <cite>XEP-0060 7.1.2</cite>.</p>
</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 &IQ; result with 'pubsub'
element qualified by the 'urn:xmpp:pubsub:cap:0' namespace which
contains a &lt;compare-and-publish-failed/&gt; element. The
element MUST have a 'id' attribute with the ID of the lastest
item.</p>
<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='result'
<iq type='error'
from='pubsub.shakespeare.lit'
to='hamlet@denmark.lit/blogbot'
id='compare-and-ublish1'>
<pubsub xmlns='urn:xmpp:pubsub:cap:0'>
<compare-and-publish-failed id='2'/>
</pubsub>
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>
<section2 topic='Error cases'>
</section1>
<p>All other error cases are handled as specified in
<cite>XEP-0060 § 7.1.3</cite></p>
<section1 topic='Rationale' anchor='rationale'>
</section2>
<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>
<p>This extension protocol does not add any further security considerations to the ones mentioned
in <cite>XEP-0060 § 14.</cite>.</p>
</section1>
@ -160,10 +245,32 @@ And by opposing end them?
<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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