The Publish-Subscribe (XEP-0060)  extension to XMPP provides a comprehensive technology for alerts, notifications, data syndication, rich presence, and other real-time messaging use cases. In terms of traditional publish-subscribe systems like Java Message Service (JMS), the core XMPP PubSub specification covers the Observer design pattern only; however, traditional publish-subscribe systems often include support for a second design pattern, usually called the "point-to-point" or "queueing" pattern.  This specification defines a few small extensions to XMPP PubSub that enable support for a queueing mode in XMPP. The queueing mode is an add-on feature that a service can support, and that a node owner can enable if supported by the service. The feature name is "urn:xmpp:pubsub:queueing:0".
If a node has enabled support for the queueing mode, in response to a subscription request without configuration options it MUST return an IQ-error containing a subscription options form; this form MUST include the "queue_requests" field (which specifies the number of parallel requests a subscriber is willing to process).
The subscriber would then send a new subscription request, this time with options.
If the subscription request can be processed successfully, the service returns an IQ-result and includes the configuration options established during the negotiation.
At any time, a publisher can push an item to the queue node.
The item is published to one of the subscribers.
When the subscriber that received the item has successfully processed it (whatever that means in the context of the queue), the subscriber deletes the item from the queue.
In the context of a queue node, the service MUST treat a delete request from a subscriber that received the item as if the sender were a publisher; i.e., it MUST delete the item from the queue and notify only this subscriber that the item has been deleted.
Note: The subscriber SHOULD NOT commit any pending transactions until it receives the delete notification.
If the item does not exist, the service MUST return an <item-not-found/> error as described in XEP-0060.
If the entity that attempts to delete the item is not the subscriber that received the item, the service MUST return a <forbidden/> error as described in XEP-0060.
If the item is locked by another subscriber, the service MUST return a <conflict/> error (this flow is not defined in XEP-0060.
If the subscriber that received the item attempts to delete the item but the item is no longer locked by the subscriber (e.g., because of a race condition or a lost notification), the service MUST return an <unexpected-request/> error (this flow is not defined in XEP-0060.
The subscriber might determine that it cannot process the item (whatever that means in the context of the queue); if so, the subscriber unlocks the item.
The service then MUST unlock the item and notify only this subscriber that the item has been unlocked.
When an item is unlocked, the service would then send a publish notification to another subscriber according to application-specific logic for determining the "next" subscriber.
If the item does not exist, the service MUST return an <item-not-found/> error.
If the entity that attempts to unlock the item is not the subscriber that received the item, the service MUST return a <forbidden/> error.
If the item is locked by another subscriber, the service MUST return a <conflict/> error.
If the subscriber that received the item attempts to unlock the item but the item is no longer locked by the subscriber (e.g., because of a race condition or a lost notification), the service MUST return an <unexpected-request/> error.
If a pubsub service supports the queueing mode, it MUST advertise support for the "urn:xmpp:pubsub:queueing:0" namespace in response to Service Discovery (XEP-0030)  information requests.
If the service receives unavailable presence from a subscriber, it SHOULD unlock all outstanding queue items associated with the subscriber and unsubscribe the subscriber to prevent delivery of further publish notifications.
If a subscriber cannot process queue items because of an unrecoverable error (e.g., disk full), the subscriber SHOULD unsubscribe and then unlock all of its outstanding queue items.
If the service does not receive a delete or unlock request from a subscriber that received a queue item in a configurable amount of time, it SHOULD timeout the request, send an unlock notification to the subscriber, and send a publish notification to the "next" subscriber.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) .
This specification defines the following XML namespace:
Upon advancement of this specification from a status of Experimental to a status of Draft, the XMPP Registrar  shall add the foregoing namespaces to the registry located at <https://xmpp.org/registrar/namespaces.html>, as described in Section 4 of XMPP Registrar Function (XEP-0053) .
If the protocol defined in this specification undergoes a revision that is not fully backwards-compatible with an older version, the XMPP Registrar shall increment the protocol version number found at the end of the XML namespaces defined herein, as described in Section 4 of XEP-0053.
The XMPP Registrar  maintains a registry of service discovery features (see <https://xmpp.org/registrar/disco-features.html>), which includes a number of features that can be returned by pubsub services. The following registry submission supplements the existing list.
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The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) specifications contributed by the XMPP Standards Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.
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Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/
Initial published version.
Added more detailed error flows; added additional implementation notes.