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.
If an entity supports the Compared-And-Publish feature it MUST advertise the fact by returning a <feature/> with the 'var' attribute set to 'urn:xmpp:pubsub:cap:0' in response to a Service Discovery (XEP-0030)  query for information.
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.
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.
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.
Unfortunately it was not possible to re-use the PubSub item ID for the "Atomically Compare-And-Publish" purpose. This is mostly due XEP-0060 § 12.8 stating that:
"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."
Which means that the content of an item could change without its ID, rendering the item ID unusable for CAP.
Injecting a "cap"-namespaced attribute carrying the item's CAP-V into PubSub's <item/> 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 <cap-v-map/> approach for now.
This extension protocol does not add any further security considerations to the ones mentioned in XEP-0060 § 14..
This document requires no interaction with the Internet Assigned Numbers Authority (IANA).
This specification defines the following XML namespaces:
This specification defines the following <publish-options/> fields:
TODO: Add after the XEP leaves the 'experimental' state.
Thanks to Kim Alvefur and Dave Cridland for their feedback.
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Use a custom item value (CAP-V).
Use PubSub publish-options preconditions.