<?xml version='1.0' encoding='UTF-8'?> <!DOCTYPE xep SYSTEM 'xep.dtd' [ <!ENTITY % ents SYSTEM 'xep.ent'> %ents; ]> <?xml-stylesheet type='text/xsl' href='xep.xsl'?> <xep> <header> <title>PubSub Queueing</title> <abstract>This specification defines an extension to XMPP publish-subscribe for queueing information at a node.</abstract> &LEGALNOTICE; <number>0254</number> <status>Deferred</status> <type>Standards Track</type> <sig>Standards</sig> <approver>Council</approver> <dependencies> <spec>XMPP Core</spec> <spec>XEP-0060</spec> </dependencies> <supersedes/> <supersededby/> <shortname>NOT_YET_ASSIGNED</shortname> &hildjj; &stpeter; <revision> <version>0.1</version> <date>2008-11-13</date> <initials>psa</initials> <remark><p>Initial published version.</p></remark> </revision> <revision> <version>0.0.3</version> <date>2008-10-07</date> <initials>jjh/psa</initials> <remark><p>Added more detailed error flows; added additional implementation notes.</p></remark> </revision> <revision> <version>0.0.2</version> <date>2008-10-07</date> <initials>jjh/psa</initials> <remark> <ul> <li>Specified that notifications are sent to only one subscriber at a time.</li> <li>Specified that notifications include payloads.</li> <li>Added protocol extension for unlock feature.</li> <li>Modified namespace to incorporate namespace versioning.</li> </ul> </remark> </revision> <revision> <version>0.0.1</version> <date>2008-08-14</date> <initials>psa/jjh</initials> <remark><p>First draft.</p></remark> </revision> </header> <section1 topic='Introduction' anchor='intro'> <p>The &xep0060; 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. <note>See for instance <<link url='http://en.wikipedia.org/wiki/Java_Message_Service'>http://en.wikipedia.org/wiki/Java_Message_Service</link>>.</note> 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".</p> </section1> <section1 topic='How It Works' anchor='howitworks'> <section2 topic='Subscribing' anchor='subscribe'> <p>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).</p> <example caption='Service indicates that subscription configuration is required'><![CDATA[ <iq from='workflows.shakespeare.lit' id='sub1' to='workerbee237@shakespeare.lit/foo' type='error'> <pubsub xmlns='http://jabber.org/protocol/pubsub'> <subscribe jid='workerbee237@shakespeare.lit' node='a290fjsl29j19kjb'/> <options jid='workerbee237@shakespeare.lit' node='a290fjsl29j19kjb'> <x xmlns='jabber:x:data' type='form'> <field var='FORM_TYPE' type='hidden'> <value>http://jabber.org/protocol/pubsub#subscribe_options</value> </field> <field type='text-single' var='pubsub#queue_requests'> <required/> </field> </x> </options> </pubsub> <error type='modify'> <not-acceptable xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/> <configuration-required xmlns='http://jabber.org/protocol/pubsub#errors'/> </error> </iq> ]]></example> <p>The subscriber would then send a new subscription request, this time with options.</p> <example caption='Subscribing with options'><![CDATA[ <iq from='workerbee237@shakespeare.lit/foo' id='sub2' to='workflows.shakespeare.lit' type='set'> <pubsub xmlns='http://jabber.org/protocol/pubsub'> <subscribe jid='workerbee237@shakespeare.lit' node='a290fjsl29j19kjb'/> <options jid='workerbee237@shakespeare.lit' node='a290fjsl29j19kjb'> <x xmlns='jabber:x:data' type='submit'> <field var='FORM_TYPE' type='hidden'> <value>http://jabber.org/protocol/pubsub#subscribe_options</value> </field> <field var='pubsub#queue_requests'> <value>5</value> </field> </x> </options> </pubsub> </iq> ]]></example> <p>If the subscription request can be processed successfully, the service returns an IQ-result and includes the configuration options established during the negotiation.</p> <example caption='Service replies with success (including configuration options)'><![CDATA[ <iq from='workflows.shakespeare.lit' id='sub2' to='workerbee237@shakespeare.lit/foo' type='result'> <pubsub xmlns='http://jabber.org/protocol/pubsub'> <subscription jid='workerbee237@shakespeare.lit' node='a290fjsl29j19kjb' subid='ba49252aaa4f5d320c24d3766f0bdcade78c78d3' subscription='subscribed'/> <options> <x xmlns='jabber:x:data' type='result'> <field var='FORM_TYPE' type='hidden'> <value>http://jabber.org/protocol/pubsub#subscribe_options</value> </field> <field var='pubsub#queue_requests'> <value>5</value> </field> </x> </options> </pubsub> </iq> ]]></example> </section2> <section2 topic='Assigning an Item' anchor='assign'> <p>At any time, a publisher can push an item to the queue node.</p> <example caption='Publisher publishes an item'><![CDATA[ <iq from='engine.shakespeare.lit' id='pub1' to='workflow.shakespeare.lit' type='set'> <pubsub xmlns='http://jabber.org/protocol/pubsub'> <publish node='a290fjsl29j19kjb'> <item id='ae890ac52d0df67ed7cfdf51b644e901'> <example xmlns='urn:xmpp:example'>payload</example> </item> </publish> </pubsub> </iq> ]]></example> <p>The item is published to <em>one</em> of the subscribers.</p> <example caption='One subscriber receives a notification'><![CDATA[ <message from='workflow.shakespeare.lit' id='foo' to='workerbee237@shakespeare.lit'> <event xmlns='http://jabber.org/protocol/pubsub#event'> <items node='a290fjsl29j19kjb'> <item id='ae890ac52d0df67ed7cfdf51b644e901'> <example xmlns='urn:xmpp:example'>payload</example> </item> </items> </event> </message> ]]></example> </section2> <section2 topic='Deleting an Item from the Queue' anchor='delete'> <p>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.</p> <example caption='Entity deletes an item from a node'><![CDATA[ <iq from='workerbee237@shakespeare.lit/foo' id='delete1' to='workflows.shakespeare.lit' type='get'> <pubsub xmlns='http://jabber.org/protocol/pubsub'> <retract node='a290fjsl29j19kjb'> <item id='ae890ac52d0df67ed7cfdf51b644e901'/> </retract> </pubsub> </iq> ]]></example> <p>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.</p> <example caption='Subscriber receives delete notification'><![CDATA[ <message from='workflow.shakespeare.lit' id='bar' to='workerbee237@shakespeare.lit'> <event xmlns='http://jabber.org/protocol/pubsub#event'> <items node='a290fjsl29j19kjb'> <retract id='ae890ac52d0df67ed7cfdf51b644e901'/> </items> </event> </message> ]]></example> <p>Note: The subscriber SHOULD NOT commit any pending transactions until it receives the delete notification.</p> <p>If the item does not exist, the service MUST return an ¬found; error as described in <cite>XEP-0060</cite>.</p> <p>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 <cite>XEP-0060</cite>.</p> <p>If the item is locked by another subscriber, the service MUST return a &conflict; error (this flow is not defined in <cite>XEP-0060</cite>.</p> <example caption='Item is locked by another subscriber'><![CDATA[ <iq from='workflows.shakespeare.lit' id='delete1' to='workerbee237@shakespeare.lit/foo' type='get'> <pubsub xmlns='http://jabber.org/protocol/pubsub'> <retract node='a290fjsl29j19kjb'> <item id='ae890ac52d0df67ed7cfdf51b644e901'/> </retract> </pubsub> <error type='auth'> <forbidden xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/> </error> </iq> ]]></example> <p>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; error (this flow is not defined in <cite>XEP-0060</cite>.</p> <example caption='Item is no longer locked by subscriber'><![CDATA[ <iq from='workflows.shakespeare.lit' id='delete1' to='workerbee237@shakespeare.lit/foo' type='get'> <pubsub xmlns='http://jabber.org/protocol/pubsub'> <retract node='a290fjsl29j19kjb'> <item id='ae890ac52d0df67ed7cfdf51b644e901'/> </retract> </pubsub> <error type='wait'> <unexpected-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/> </error> </iq> ]]></example> </section2> <section2 topic='Unlocking an Item' anchor='unlock'> <p>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.</p> <example caption='Entity unlocks an item'><![CDATA[ <iq from='workerbee237@shakespeare.lit/foo' id='unlock1' to='workflows.shakespeare.lit' type='get'> <pubsub xmlns='http://jabber.org/protocol/pubsub'> <unlock xmlns='urn:xmpp:pubsub:queueing:0' node='a290fjsl29j19kjb'> <item id='ae890ac52d0df67ed7cfdf51b644e901'/> </unlock> </pubsub> </iq> ]]></example> <p>The service then MUST unlock the item and notify only this subscriber that the item has been unlocked.</p> <example caption='Subscriber receives unlock notification'><![CDATA[ <message from='workflow.shakespeare.lit' id='baz' to='workerbee237@shakespeare.lit'> <event xmlns='http://jabber.org/protocol/pubsub#event'> <items node='a290fjsl29j19kjb'> <unlock xmlns='urn:xmpp:queueing:0' id='ae890ac52d0df67ed7cfdf51b644e901'/> </items> </event> </message> ]]></example> <p>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.</p> <p>If the item does not exist, the service MUST return an ¬found; error.</p> <example caption='Item does not exist'><![CDATA[ <iq from='workflows.shakespeare.lit' id='delete1' to='workerbee237@shakespeare.lit/foo' type='error'> <pubsub xmlns='http://jabber.org/protocol/pubsub'> <unlock xmlns='urn:xmpp:pubsub:queueing:0' node='a290fjsl29j19kjb'> <item id='ae890ac52d0df67ed7cfdf51b644e901'/> </unlock> </pubsub> <error type='cancel'> <item-not-found xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/> </error> </iq> ]]></example> <p>If the entity that attempts to unlock the item is not the subscriber that received the item, the service MUST return a &forbidden; error.</p> <example caption='Requesting entity did not receive item'><![CDATA[ <iq from='workflows.shakespeare.lit' id='delete1' to='workerbee237@shakespeare.lit/foo' type='error'> <pubsub xmlns='http://jabber.org/protocol/pubsub'> <unlock xmlns='urn:xmpp:pubsub:queueing:0' node='a290fjsl29j19kjb'> <item id='ae890ac52d0df67ed7cfdf51b644e901'/> </unlock> </pubsub> <error type='auth'> <forbidden xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/> </error> </iq> ]]></example> <p>If the item is locked by another subscriber, the service MUST return a &conflict; error.</p> <example caption='Item is locked by another subscriber'><![CDATA[ <iq from='workflows.shakespeare.lit' id='delete1' to='workerbee237@shakespeare.lit/foo' type='error'> <pubsub xmlns='http://jabber.org/protocol/pubsub'> <unlock xmlns='urn:xmpp:pubsub:queueing:0' node='a290fjsl29j19kjb'> <item id='ae890ac52d0df67ed7cfdf51b644e901'/> </unlock> </pubsub> <error type='cancel'> <conflict xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/> </error> </iq> ]]></example> <p>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; error.</p> <example caption='Item is no longer locked by subscriber'><![CDATA[ <iq from='workflows.shakespeare.lit' id='delete1' to='workerbee237@shakespeare.lit/foo' type='error'> <pubsub xmlns='http://jabber.org/protocol/pubsub'> <unlock xmlns='urn:xmpp:pubsub:queueing:0' node='a290fjsl29j19kjb'> <item id='ae890ac52d0df67ed7cfdf51b644e901'/> </unlock> </pubsub> <error type='wait'> <unexpected-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/> </error> </iq> ]]></example> </section2> </section1> <section1 topic='Determining Support' anchor='support'> <p>If a pubsub service supports the queueing mode, it MUST advertise support for the "urn:xmpp:pubsub:queueing:0" namespace in response to &xep0030; information requests.</p> </section1> <section1 topic='Implementation Notes' anchor='impl'> <p>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.</p> <p>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.</p> <p>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.</p> </section1> <section1 topic='Security Considerations' anchor='security'> <p>To follow.</p> </section1> <section1 topic='IANA Considerations' anchor='iana'> <p>This document requires no interaction with &IANA;.</p> </section1> <section1 topic='XMPP Registrar Considerations' anchor='registrar'> <section2 topic='Protocol Namespaces' anchor='registrar-ns'> <p>This specification defines the following XML namespace:</p> <ul> <li>urn:xmpp:pubsub:queueing:0</li> </ul> <p>Upon advancement of this specification from a status of Experimental to a status of Draft, the ®ISTRAR; shall add the foregoing namespaces to the registry located at &NAMESPACES;, as described in Section 4 of &xep0053;.</p> </section2> <section2 topic='Protocol Versioning' anchor='registrar-versioning'> &NSVER; </section2> <section2 topic='Service Discovery Features' anchor='registrar-features'> <p>The ®ISTRAR; maintains a registry of service discovery features (see &DISCOFEATURES;), which includes a number of features that can be returned by pubsub services. The following registry submission supplements the existing list.</p> <code><![CDATA[ <var> <name>urn:xmpp:pubsub:queueing:0</name> <desc>The node or service supports the queueing mode.</desc> <doc>XEP-xxxx</doc> </var> ]]></code> </section2> </section1> </xep>