mirror of
https://github.com/moparisthebest/xeps
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1131 lines
38 KiB
XML
1131 lines
38 KiB
XML
<?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>Publish/Subscribe</title>
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<abstract>A publish-subscribe protocol for Jabber.</abstract>
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&PUBLICDOMAINNOTICE;
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<number>0024</number>
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<status>Retracted</status>
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<type>Standards Track</type>
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<sig>Standards</sig>
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<dependencies/>
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<supersedes/>
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<supersededby><spec>XEP-0060</spec></supersededby>
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<shortname>None</shortname>
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<author>
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<firstname>DJ</firstname>
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<surname>Adams</surname>
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<email>dj.adams@pobox.com</email>
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<jid>dj@gnu.mine.nu</jid>
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</author>
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<author>
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<firstname>Piers</firstname>
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<surname>Harding</surname>
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<email>piers@ompa.net</email>
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<jid>piers@gnu.mine.nu</jid>
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</author>
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<revision>
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<version>0.2</version>
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<date>2003-04-22</date>
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<initials>psa</initials>
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<remark>At the request of the authors, the status of this document has been changed to Retracted since it has been superseded by XEP-0060.</remark>
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</revision>
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<revision>
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<version>0.1</version>
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<date>2002-03-05</date>
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<initials>dja</initials>
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<remark>Initial draft.</remark>
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</revision>
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</header>
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<section1 topic='Abstract'>
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<p>
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Pubsub ("publish/subscribe") is a technique for coordinating the efficient
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delivery of information from publisher to consumer. This specification
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describes the use of pubsub within a Jabber context and is a result of
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two separate but related goals:
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</p>
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<ul>
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<li>to be able to exchange information _within_ a Jabber environment
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(for example continuously changing personal information between users)</li>
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<li>to be able to exchange information _using_ Jabber as a mechanism for
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<ul>
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<li>organising that exchange</li>
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<li>providing transport for the information</li>
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</ul>
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</li>
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</ul>
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<p>
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The specification details the use of the Jabber protocol elements and
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introduces a new namespace, jabber:iq:pubsub.
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It also includes notes on actual implementation of such a
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mechanism in Jabber.
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</p>
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</section1>
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<section1 topic='Introduction'>
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<p>
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It's clear that as Jabber is deployed over a wider spectrum of platforms
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and circumstances, more and more information will be exchanged. Whether
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that information is specific to Jabber (JSM) users, or components, we need
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an mechanism to be able to manage the exchange of this information in an
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efficient way.
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</p>
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<p>
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For example, it is currently the trend to embed information about a
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particular client's circumstance inside presence packets, either in the
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<status/> tag or in an <x/> extension. One example that comes
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to mind is "song currently playing on my MP3 player" (to which I have to
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admit some responsibility for the meme in the first place). While embedding
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information inside presence packets and having that information diffused to
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the users who are subscribed to that user's presence has the desired effect,
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it has a couple of non-trivial drawbacks:
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</p>
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|
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<ul>
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<li>the diffusion is inefficient, sending potentially huge amounts of data
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to recipients who aren't interested</li>
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<li>the distribution is tied to closely to presence subscription; any entity
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|
that wants to receive information must be subscribed to the source's presence,
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and there is no mechanism for specifying _what_ information they wish to
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receive. It is also arguably too closely tied to the JSM to be useful for
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_component_-based information exchange.</li>
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</ul>
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<p>
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This is above and beyond the simple fact that this overloading of presence
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packets and the presence subscription and diffusion mechanism can only end
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in tears.
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</p>
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<p>
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It would be far better to have a separate (sub-)protocol that enabled
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entities to take part in publish/subscribe relationships, and have a service
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that facilitated the efficient exchange of information. Not only would it
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relax the undue pressure on the presence mechanism, but it would also allow
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people to use Jabber, which is, after all, about exchanging structured content
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between endpoints, as a publish/subscribe _mechanism_ in its own right.
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</p>
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<p>
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This specification describes a publish/subscribe protocol in terms
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of IQ packets with payload data in a new namespace, jabber:iq:pubsub. The
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choice for this namespace is slightly arbitrary - it was the same namespace
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used in temas's original document, seems to fit well, and we need a namespace
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to focus on.<note>It may well be that we will move to a URI-based namespace
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in the form of a URL pointing to this specification.</note>
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</p>
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<p>
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The aim of the specification is to provide for a facility where Jabber
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entities can subscribe to (consume) and publish (emit) information in an
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efficient and organised way. These entities could be JSM users or components.
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</p>
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<p>
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Push technology is back with a vengeance. Jabber can play a fundamental
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part.
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</p>
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</section1>
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<section1 topic='The Specification'>
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<p>
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The pubsub services will be typically provided by a component. In what
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follows, there are generally three parties involved:
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</p>
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<ul>
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<li>the subscriber</li>
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<li>the pubsub service</li>
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<li>the publisher</li>
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</ul>
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<p>
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Bear in mind that it is perfectly possible for a subscriber to be a
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publisher, and a publisher to be a subscriber, too.
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</p>
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<p>
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The pubsub traffic will be carried in info/query (IQ) packets. All of the
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data in these packets will be qualified by the jabber:iq:pubsub namespace.
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</p>
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<p>
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Pubsub scenarios can be seen in a subscribe (or unsubscribe) context or a
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publish context. In light of this, we will examine the IQ packets
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used in these contexts.
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</p>
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<section2 topic='Subscribe/Unsubscribe Context'>
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<p>
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A potential consumer, or recipient, of published information, needs to
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request that he be sent that published information. Requesting to receive,
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or be pushed, information is known as subscribing.
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</p>
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<p>
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A subscription request generally takes this form:
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</p>
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<example caption='General form of a subscription'>
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SEND: <iq type='set' from='subscriber' to='pubsub' id='s1'>
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<query xmlns='jabber:iq:pubsub'>
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<subscribe to='publisher'>
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<ns>namespace:1</ns>
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<ns>namespace:2</ns>
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...
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<ns>namespace:N</ns>
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</subscribe>
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</query>
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</iq>
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RECV: <iq type='result' to='subscriber' from='pubsub' id='s1'>
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<query xmlns='jabber:iq:pubsub'>
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<subscribe to='publisher'>
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<ns>namespace:1</ns>
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<ns>namespace:2</ns>
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...
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<ns>namespace:N</ns>
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</subscribe>
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</query>
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</iq>
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</example>
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<section3 topic='Publisher-Specific Subscriptions and Unsubscriptions'>
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<p>
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Subscriptions can be specific to a publisher, in which case a to attribute
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is specified in the <subscribe/> tag:
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</p>
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<example caption='Publisher-specific subscription'>
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SEND: <iq type='set' to='pubsub.localhost'
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from='subscriber@localhost/resource' id='s1'>
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<query xmlns='jabber:iq:pubsub'>
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<subscribe to='publisher'>
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<ns>namespace:1</ns>
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<ns>namespace:2</ns>
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</subscribe>
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</query>
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</iq>
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RECV: <iq type='result' from='pubsub.localhost'
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to='subscriber@localhost/resource' id='s1'>
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<query xmlns='jabber:iq:pubsub'>
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<subscribe to='publisher'>
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<ns>namespace:1</ns>
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<ns>namespace:2</ns>
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</subscribe>
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</query>
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</iq>
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</example>
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<p>
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In this case, the namespaces specified will be added to any existing list
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of namespaces already recorded for that subscriber:publisher relationship.
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In other words, it's a relative, not an absolute, subscription request.
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</p>
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<p>
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It is also possible in a publisher-specific subscription to omit specific
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namespaces, if you want to be sent everything that particular publisher
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might publish:
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</p>
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<example caption='Publisher-specific subscription without namespace specification'>
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SEND: <iq type='set' to='pubsub.localhost'
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from='subscriber.localhost' id='s1'>
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<query xmlns='jabber:iq:pubsub'>
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<subscribe to='publisher'/>
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</query>
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</iq>
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RECV: <iq type='result' from='pubsub.localhost'
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|
to='subscriber.localhost' id='s1'>
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<query xmlns='jabber:iq:pubsub'>
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<subscribe to='publisher'/>
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</query>
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</iq>
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</example>
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<p>
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|
This type of subscription should have the effect of absolutely replacing any
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previous namespace-specific subscription to the publisher specified.
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|
</p>
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<p>
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If a subscriber wishes to cancel a subscription from a particular publisher,
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he can send an unsubscribe like this:
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</p>
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<example caption='Publisher-specific unsubscription'>
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SEND: <iq type='set' to='pubsub.localhost'
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from='subscriber@localhost/resource' id='s1'>
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<query xmlns='jabber:iq:pubsub'>
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<unsubscribe to='publisher'>
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<ns>namespace:1</ns>
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</unsubscribe>
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</query>
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</iq>
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RECV: <iq type='result' from='pubsub.localhost'
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to='subscriber@localhost/resource' id='s1'>
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<query xmlns='jabber:iq:pubsub'>
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<unsubscribe to='publisher'>
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<ns>namespace:1</ns>
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</unsubscribe>
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</query>
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</iq>
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</example>
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<p>
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This should have the effect of removing the subscription from that publisher
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for the namespaces specified.
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</p>
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<p>
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You can also send an unsubscribe without specifying any namespaces:
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</p>
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|
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<example caption='Publisher-specific general unsubscription'>
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|
SEND: <iq type='set' to='pubsub.localhost'
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from='subscriber.localhost' id='s1'>
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<query xmlns='jabber:iq:pubsub'>
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<unsubscribe to='publisher'/>
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</query>
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</iq>
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RECV: <iq type='result' from='pubsub.localhost'
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to='subscriber.localhost' id='s1'>
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<query xmlns='jabber:iq:pubsub'>
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<unsubscribe to='publisher'/>
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</query>
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</iq>
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</example>
|
|
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<p>
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This should have the effect of removing any subscription relationship with
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the publisher specified. Note, however, that this won't stop the subscriber
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being pushed information from that publisher if he's specified a
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"publisher-generic" subscription (see next section).
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</p>
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</section3>
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<section3 topic='Non-Publisher-Specific Subscriptions and Unsubscriptions'>
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<p>
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As well as being able to subscribe to specific publishers, it is also
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|
possible to subscribe to receive data, according to namespace, regardless
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of publisher:
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|
</p>
|
|
|
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<example caption='General namespace specific subscription'>
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SEND: <iq type='set' to='pubsub.localhost'
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from='subscriber@localhost/resource' id='s1'>
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<query xmlns='jabber:iq:pubsub'>
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<subscribe>
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<ns>namespace:1</ns>
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<ns>namespace:2</ns>
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</subscribe>
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</query>
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</iq>
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RECV: <iq type='result' from='pubsub.localhost'
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to='subscriber@localhost/resource' id='s1'>
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<query xmlns='jabber:iq:pubsub'>
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<subscribe>
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|
<ns>namespace:1</ns>
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<ns>namespace:2</ns>
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</subscribe>
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</query>
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</iq>
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</example>
|
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<p>
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This means that the subscriber wishes to be pushed information in the
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|
namespaces specified, regardless of who publishes it. Like the
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publisher-specific subscribe that specifies namespaces, this request is
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|
relative, in the namespaces are added to any existing namespaces already
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recorded for this generic subscription.
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</p>
|
|
|
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<p>
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Subscribing to everything from everyone is probably not a good idea and
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we should not allow this. (The format of the request is actually used in
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an IQ-get context - see later).
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</p>
|
|
|
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<example caption='This is not allowed'>
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SEND: <iq type='set' to='pubsub.localhost'
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from='subscriber@localhost/resource' id='s1'>
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<query xmlns='jabber:iq:pubsub'>
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<subscribe/>
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</query>
|
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</iq>
|
|
|
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RECV: <iq type='error' from='pubsub.localhost'
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|
to='subscriber@localhost/resource' id='s1'>
|
|
<query xmlns='jabber:iq:pubsub'>
|
|
<subscribe/>
|
|
</query>
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|
<error code='405'>Not Allowed</error>
|
|
</iq>
|
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</example>
|
|
|
|
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<p>
|
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Likewise, you can unsubscribe from certain namespaces in this non-publisher-specific context like this:
|
|
</p>
|
|
|
|
<example caption='General unsubscription to specific namespaces'>
|
|
SEND: <iq type='set' to='pubsub.localhost'
|
|
from='subscriber.localhost' id='s1'>
|
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<query xmlns='jabber:iq:pubsub'>
|
|
<unsubscribe>
|
|
<ns>namespace:1</ns>
|
|
<ns>namespace:2</ns>
|
|
</unsubscribe>
|
|
</query>
|
|
</iq>
|
|
|
|
RECV: <iq type='result' from='pubsub.localhost'
|
|
to='subscriber.localhost' id='s1'>
|
|
<query xmlns='jabber:iq:pubsub'>
|
|
<unsubscribe>
|
|
<ns>namespace:1</ns>
|
|
<ns>namespace:2</ns>
|
|
</unsubscribe>
|
|
</query>
|
|
</iq>
|
|
</example>
|
|
|
|
<p>
|
|
If there are any subscriptions to specific publishers for the namespaces
|
|
specified here, they should be removed (for those namespaces) in addition
|
|
to the removal from the 'all publishers' list.
|
|
</p>
|
|
|
|
<p>
|
|
Finally, a subscriber can wipe the slate clean like this:
|
|
</p>
|
|
|
|
<example caption='Wiping the slate'>
|
|
SEND: <iq type='set' to='pubsub.localhost'
|
|
from='subscriber.localhost' id='s1'>
|
|
<query xmlns='jabber:iq:pubsub'>
|
|
<unsubscribe/>
|
|
</query>
|
|
</iq>
|
|
|
|
RECV: <iq type='result' from='pubsub.localhost'
|
|
to='subscriber.localhost' id='s1'>
|
|
<query xmlns='jabber:iq:pubsub'>
|
|
<unsubscribe/>
|
|
</query>
|
|
</iq>
|
|
</example>
|
|
|
|
<p>
|
|
which should have the effect of removing all namespace subscriptions
|
|
from everywhere.
|
|
</p>
|
|
|
|
</section3>
|
|
|
|
<section3 topic='Further Notes'>
|
|
|
|
<p>
|
|
All the examples so far have shown actions on the subscriber's part, and
|
|
have consisted of IQ-sets. In an IQ-set, within the jabber:iq:pubsub
|
|
namespace, multiple children can exist in the query payload, but those
|
|
children must be of the same type. In other words, you can send multiple
|
|
<subscribe/>s, or multiple <unsubscribe/>s, but not a combination
|
|
of the two.
|
|
</p>
|
|
|
|
<p>
|
|
This is allowed:
|
|
</p>
|
|
|
|
<example caption='Subscribing to more than one publisher at once'>
|
|
SEND: <iq type='set' to='pubsub.localhost'
|
|
from='subscriber@localhost/resource' id='s1'>
|
|
<query xmlns='jabber:iq:pubsub'>
|
|
<subscribe to='publisherA'>
|
|
<ns>namespace:1</ns>
|
|
<ns>namespace:2</ns>
|
|
</subscribe>
|
|
<subscribe to='publisherB'>
|
|
<ns>namespace:3</ns>
|
|
</subscribe>
|
|
</query>
|
|
</iq>
|
|
|
|
RECV: <iq type='result' from='pubsub.localhost'
|
|
to='subscriber@localhost/resource' id='s1'>
|
|
<query xmlns='jabber:iq:pubsub'>
|
|
<subscribe to='publisherA'>
|
|
<ns>namespace:1</ns>
|
|
<ns>namespace:2</ns>
|
|
</subscribe>
|
|
<subscribe to='publisherB'>
|
|
<ns>namespace:3</ns>
|
|
</subscribe>
|
|
</query>
|
|
</iq>
|
|
</example>
|
|
|
|
<p>
|
|
But this is not allowed:
|
|
</p>
|
|
|
|
<example caption='Subscribes and unsubscribes in same IQ-set is not allowed'>
|
|
SEND: <iq type='set' to='pubsub.localhost'
|
|
from='subscriber.localhost' id='s1'>
|
|
<query xmlns='jabber:iq:pubsub'>
|
|
<subscribe to='publisherA'>
|
|
<ns>namespace:1</ns>
|
|
<ns>namespace:2</ns>
|
|
</subscribe>
|
|
<unsubscribe to='publisherB'>
|
|
<ns>namespace:3</ns>
|
|
</unsubscribe>
|
|
</query>
|
|
</iq>
|
|
|
|
RECV: <iq type='result' from='pubsub.localhost'
|
|
to='subscriber.localhost' id='s1'>
|
|
<query xmlns='jabber:iq:pubsub'>
|
|
<subscribe to='publisherA'>
|
|
<ns>namespace:1</ns>
|
|
<ns>namespace:2</ns>
|
|
</subscribe>
|
|
<unsubscribe to='publisherB'>
|
|
<ns>namespace:3</ns>
|
|
</unsubscribe>
|
|
</query>
|
|
<error code='400'>
|
|
Bad Request: only subscribes or unsubscribes
|
|
</error>
|
|
</iq>
|
|
</example>
|
|
|
|
<p>
|
|
In the case where multiple <subscribe/>s or <unsubscribe/>s
|
|
appear in an action, each element will be processed in turn, as they appear
|
|
in the payload.
|
|
</p>
|
|
|
|
<p>
|
|
As well as actions, the subscriber can query his subscription using an
|
|
IQ-get in the jabber:iq:pubsub namespace. This should return a list of
|
|
the subscribers current subscriptions, like this:
|
|
</p>
|
|
|
|
<example caption='Querying current subscription'>
|
|
SEND: <iq type='get' to='pubsub.localhost'
|
|
from='subscriber@localhost/resource' id='s1'>
|
|
<query xmlns='jabber:iq:pubsub'>
|
|
<subscribe/>
|
|
</query>
|
|
</iq>
|
|
|
|
RECV: <iq type='result' from='pubsub.localhost'
|
|
to='subscriber@localhost/resource' id='s1'>
|
|
<query xmlns='jabber:iq:pubsub'>
|
|
<subscribe>
|
|
<ns>namespace:1</ns>
|
|
<ns>namespace:2</ns>
|
|
</subscribe>
|
|
<subscribe to='publisherA'>
|
|
<ns>namespace:2</ns>
|
|
<ns>namespace:4</ns>
|
|
</subscribe>
|
|
<subscribe to='publisherB'>
|
|
<ns>namespace:5</ns>
|
|
</subscribe>
|
|
</query>
|
|
</iq>
|
|
</example>
|
|
|
|
<p>
|
|
Note the two references to namespace:2 - one inside the non-publisher-specific
|
|
subscription list and one inside the subscription list specific to publisherA.
|
|
This example implies that the non-publisher-specific and publisher-specific
|
|
subscription information should be kept separately. This is designed to make
|
|
it easier on the subscriber to manage his specific subscriptions over time.
|
|
</p>
|
|
|
|
</section3>
|
|
|
|
</section2>
|
|
|
|
<section2 topic='Publish Context'>
|
|
|
|
<p>
|
|
In contrast to the subscribe and unsubscribe context, the publishing
|
|
context is a lot simpler to explain.
|
|
</p>
|
|
|
|
<p>
|
|
A publisher can publish information within a certain namespace, like this:
|
|
</p>
|
|
|
|
<example caption='Publishing information'>
|
|
SEND: <iq type='set' to='pubsub.localhost'
|
|
from='publisher@localhost/resource' id='s1'>
|
|
<query xmlns='jabber:iq:pubsub'>
|
|
<publish ns='foo'>
|
|
<foo xmlns='foo'>bar</foo>
|
|
</publish>
|
|
</query>
|
|
</iq>
|
|
|
|
RECV: <iq type='result' from='pubsub.localhost'
|
|
to='publisher@localhost/resource' id='s1'>
|
|
<query xmlns='jabber:iq:pubsub'>
|
|
<publish ns='foo'>
|
|
<foo xmlns='foo'>bar</foo>
|
|
</publish>
|
|
</query>
|
|
</iq>
|
|
</example>
|
|
|
|
<p>
|
|
It's also possible for a publisher to publish more than one item at once,
|
|
like this:
|
|
</p>
|
|
|
|
<example caption='Publishing information in different namespaces'>
|
|
SEND: <iq type='set' to='pubsub.localhost'
|
|
from='publisher.localhost' id='s1'>
|
|
<query xmlns='jabber:iq:pubsub'>
|
|
<publish ns='foo'>
|
|
<foo xmlns='foo'>bar</foo>
|
|
</publish>
|
|
<publish ns='jabber:x:oob'>
|
|
<x xmlns='jabber:x:oob'>
|
|
<url>http://www.pipetree.com/jabber/</url>
|
|
<desc>Some stuff about Jabber</desc>
|
|
</x>
|
|
</publish>
|
|
</query>
|
|
</iq>
|
|
|
|
RECV: <iq type='result' from='pubsub.localhost'
|
|
to='publisher.localhost' id='s1'>
|
|
<query xmlns='jabber:iq:pubsub'>
|
|
<publish ns='foo'>
|
|
<foo xmlns='foo'>bar</foo>
|
|
</publish>
|
|
<publish ns='jabber:x:oob'>
|
|
<x xmlns='jabber:x:oob'>
|
|
<url>http://www.pipetree.com/jabber/</url>
|
|
<desc>Some stuff about Jabber</desc>
|
|
</x>
|
|
</publish>
|
|
</query>
|
|
</iq>
|
|
</example>
|
|
|
|
<!--
|
|
<p>
|
|
Optionally, a pubsub component may respond with an empty IQ-result, to
|
|
reduce traffic:
|
|
</p>
|
|
|
|
<example>
|
|
RECV: <iq type='result' from='pubsub.localhost'
|
|
to='publisher.localhost' id='s1'>
|
|
</iq>
|
|
</example>
|
|
-->
|
|
|
|
<p>
|
|
Each published item is wrapped in a <publish/> tag. This tag
|
|
must contain the namespace of the item being publishes, in an ns
|
|
attribute, as shown. This is distinct from the xmlns attribute of
|
|
the fragment of XML actually being published. It is theoretically
|
|
none of the pubsub component's business to go poking around in the
|
|
real published data, nor should it have to. It needs to know what
|
|
namespace is qualifying the published information that has been
|
|
received, so that the list of appropriate recipients can be
|
|
determined.
|
|
</p>
|
|
|
|
</section2>
|
|
|
|
|
|
<section2 topic='Distributing Published Information'>
|
|
<p>
|
|
While it's the responsibility of the publishing entities to publish
|
|
information, it's the responsibility of the pubsub
|
|
component to push out that published data to the subscribers. The
|
|
list of recipient subscribers must be determined by the information
|
|
stored by the pubsub component as a result of receiving subscription
|
|
requests (which are described earlier).
|
|
</p>
|
|
<p>
|
|
On receipt of an IQ-set containing published information, the pubsub
|
|
entity must determine the list of subscribers to which that information
|
|
should be pushed. If the IQ-set contains multiple <publish/>
|
|
fragments, this process must be carried out for each one in turn.
|
|
<note>Whether a pubsub component implementation should be allowed to
|
|
batch up individual published information fragments for one recipient
|
|
as a result of a large, multi-part incoming publishing IQ-set, is not
|
|
specified here, the choice is down to the implementer. Receiving entities
|
|
should be able to cope with being pushed an IQ-set with multiple
|
|
fragments of published data.</note>
|
|
</p>
|
|
<p>
|
|
Taking the earlier example of the publishing of data in the 'foo'
|
|
namespace, the following example shows what the pubsub component
|
|
must send to push this foo data out to a subscriber.
|
|
</p>
|
|
<example caption='Pushing out published information to a subscriber'>
|
|
SEND: <iq type='set' to='subscriber@localhost/foosink'
|
|
from='pubsub.localhost' id='push1'>
|
|
<query xmlns='jabber:iq:pubsub'>
|
|
<publish ns='foo' from='publisher@localhost'>
|
|
<foo xmlns='foo'>bar</foo>
|
|
</publish>
|
|
</query>
|
|
</iq>
|
|
</example>
|
|
<p>
|
|
The recipient is _not_ required to send an 'acknowledgement' in the
|
|
form of an IQ-result; the idea that this _push_ of information is
|
|
akin to how information is pushed in a live browsing context (see
|
|
jabber:iq:browse documentation for more details).
|
|
</p>
|
|
|
|
</section2>
|
|
|
|
<section2 topic='Delivery Sensitivity'>
|
|
|
|
<p>
|
|
When a pubsub service receives a publish packet like the ones above, it
|
|
needs to deliver (push) the information out according to the subscriptions
|
|
that have been made.
|
|
</p>
|
|
|
|
<p>
|
|
However, we can introduce a modicum of sensitivity by using a presence
|
|
subscription between the pubsub service and the subscriber(s). If the
|
|
subscriber wishes only to receive information when he's online (this is
|
|
a JSM-specific issue), then he needs to set up a presence subscription
|
|
relationship with the pubsub service. The pubsub service should respond
|
|
to presence subscriptions and unsubscriptions by
|
|
</p>
|
|
|
|
<ul>
|
|
<li>accepting the (un)subscription request</li>
|
|
<li>reciprocating the (un)subscription request</li>
|
|
</ul>
|
|
|
|
<p>
|
|
If the pubsub service deems that a published piece of information should
|
|
be pushed to a subscriber, and there is a presence subscription relationship
|
|
with that subscriber, the service should only push that information to the
|
|
subscriber if he is available. If he is not available, the information is not
|
|
to be sent.
|
|
</p>
|
|
|
|
<p>
|
|
Thus the subscriber can control the sensitivity by initiating (or not) a
|
|
presence relationship with the service. If the subscriber wishes to receive
|
|
information regardless of availability, he should not initiate a (or cancel
|
|
any previous) presence relationship with the service.
|
|
</p>
|
|
|
|
<p>
|
|
This loose coupling of presence relationships for sensitivity allows this
|
|
specification to be used in the wider context of component-to-component
|
|
publish/subscribe where presence is not a given.
|
|
</p>
|
|
|
|
</section2>
|
|
|
|
<section2 topic='Use of Resources'>
|
|
<p>
|
|
When in receipt of a pubsub subscription request from an entity
|
|
where a resource is specified in the JID, the pubsub component must
|
|
honour the resource specified in the from attribute of the request.
|
|
For example, here's a typical subscription request from a JSM user:
|
|
</p>
|
|
<example caption='Incoming subscription request from a JSM user'>
|
|
RECV: <iq type='set' to='pubsub.localhost'
|
|
from='subscriber@localhost/resource' id='s1'>
|
|
<query xmlns='jabber:iq:pubsub'>
|
|
<subscribe to='publisher'>
|
|
<ns>namespace:1</ns>
|
|
<ns>namespace:2</ns>
|
|
</subscribe>
|
|
</query>
|
|
</iq>
|
|
</example>
|
|
<p>
|
|
When storing the subscriber/publisher/namespace relationship matrix for
|
|
eventual querying when a publisher publishes some information, the
|
|
pubsub component must use the full JID, not just the username@host part.
|
|
</p>
|
|
<p>
|
|
Similarly, in this example:
|
|
</p>
|
|
<example caption='Incoming subscription request from a component'>
|
|
RECV: <iq type='set' to='pubsub.localhost'
|
|
from='news.server/politics-listener' id='s1'>
|
|
<query xmlns='jabber:iq:pubsub'>
|
|
<subscribe to='publisher'>
|
|
<ns>news:politics:home</ns>
|
|
<ns>news:politics:foreign:usa</ns>
|
|
</subscribe>
|
|
</query>
|
|
</iq>
|
|
</example>
|
|
<p>
|
|
the full JID of the component subscriber - news.server/politics-listener,
|
|
should be used to qualify the matrix.
|
|
</p>
|
|
<p>
|
|
This is because it allows the subscribing entities to arrange the
|
|
receipt of pushed items by resource. In the case of a JSM user, it
|
|
allows him to organise his clients, which may have different capabilities
|
|
(some being able to handle the jabber:iq:pubsub data, others not) to
|
|
receive the 'right' data. In the case of a component, it allows the
|
|
component to associate component-specific data with incoming published
|
|
namespace-qualified information.
|
|
</p>
|
|
|
|
</section2>
|
|
|
|
</section1>
|
|
|
|
|
|
<section1 topic='Implementation Notes'>
|
|
|
|
<p>
|
|
While the specification describes the fundamental building blocks of the
|
|
pubsub protocol, there are ideas that are not discussed above but nonetheless
|
|
may be incorporated into an implementation. There are other considerations
|
|
that have to be made in the wider context of publish and subscribe. Some of
|
|
the main ones are discussed briefly here too.
|
|
</p>
|
|
|
|
<section2 topic='Publisher Discovery'>
|
|
<p>
|
|
There is no part of this pubsub specification that determines how a
|
|
potential subscriber might discover publishers. After all, there are
|
|
no rules governing which pubsub component a publisher could or should
|
|
publish to. And since pubsub subscriptions are specific to a pubsub
|
|
component, there is an information gap - "how do I find out what
|
|
publishers there are, and through which pubsub components they're publishing
|
|
information?"
|
|
</p>
|
|
<p>
|
|
This problem domain should be solved using other methods, not with the
|
|
actual jabber:iq:pubsub specific namespace. A combination of jabber:iq:browse
|
|
usage (the magic ointment that heals all things) and perhaps a DNS style
|
|
(or at least root-node-based) knowledge hierarchy might be the right
|
|
direction.
|
|
</p>
|
|
<p>
|
|
In the case where a server administrator wishes to facilitate pubsub
|
|
flow between JSM users on a server, a pubsub component can be plugged
|
|
into the jabberd backbone, and there is potentially no real issue with
|
|
knowing which pubsub component to use, and where it is.
|
|
But what about if the JSM users on one server wish to build pubsub
|
|
relationships with JSM users on another server? (Note that this general
|
|
question is not specific to JSM users, although that example will be used
|
|
here). The next two sections look at how these things might pan out.
|
|
</p>
|
|
</section2>
|
|
|
|
<section2 topic='Cross-Server Relationships'>
|
|
<p>
|
|
When JSM users on server1 wish to subscribe to information published
|
|
by JSM users on server2 (let's say it's the mp3 player info, or avatars)
|
|
then there are some issues that come immediately to mind:
|
|
</p>
|
|
<ul>
|
|
<li>Does a JSM user on server1 (userA@server1) send his IQ-set subscription
|
|
to the pubsub component on server2 (pubsub.server2), or server1
|
|
(pubsub.server1)?</li>
|
|
<li>If he sends it to pubsub.server2, can we expect
|
|
pubsub.server2 to always accept that subscription request, i.e. to
|
|
be willing to serve userA@server1 (if pubsub.server2 knows that
|
|
pubsub.server1 exists)?</li>
|
|
<li>Will there be performance (or at least server-to-server traffic)
|
|
implications if many subscription relationships exist between subscribers on
|
|
server1 and publishers on server2?</li>
|
|
</ul>
|
|
|
|
<section3 topic='Proxy Subscriptions'>
|
|
<p>
|
|
To reduce the amount of server-to-server traffic, we can employ the
|
|
concept of "proxy subscriptions". This is simply getting a pubsub component
|
|
to act on behalf of a (server-local) subscriber. Benefit comes when a pubsub
|
|
component acts on behalf of multiple (server-local) subscribers.
|
|
</p>
|
|
<p>
|
|
Here's how such proxy subscriptions can work, to reduce the amount of
|
|
server-to-server traffic:
|
|
</p>
|
|
<p>
|
|
Step 1: Subscriber sends original subscription
|
|
</p>
|
|
<p>
|
|
JSM users on server1 wish to subscribe to information published by an
|
|
entity on server2. Each of them sends a subscription request to the
|
|
_local_ pubsub component:
|
|
</p>
|
|
<example>
|
|
SEND: <iq type='set' to='pubsub.server1'
|
|
from='subscriber@server1/resource' id='s1'>
|
|
<query xmlns='jabber:iq:pubsub'>
|
|
<subscribe to='publisher.server2'>
|
|
<ns>namespace:1</ns>
|
|
</subscribe>
|
|
</query>
|
|
</iq>
|
|
</example>
|
|
|
|
<p>
|
|
Step2: Pubsub component subscribes on subscriber's behalf
|
|
</p>
|
|
<p>
|
|
The pubsub component knows about the publisher, and where (to which
|
|
pubsub component) that publisher publishes information. It formulates
|
|
a subscription request and sends it to the remote pubsub component:
|
|
</p>
|
|
<example>
|
|
SEND: <iq type='set' to='pubsub.server2'
|
|
from='pubsub.server1' id='s1'>
|
|
<query xmlns='jabber:iq:pubsub'>
|
|
<subscribe to='publisher.server2'>
|
|
<ns>namespace:1</ns>
|
|
</subscribe>
|
|
</query>
|
|
</iq>
|
|
</example>
|
|
|
|
<p>
|
|
The remote pubsub component receives and acknowledges the subscription
|
|
request, and the local pubsub component relays the response back to
|
|
the original requester:
|
|
</p>
|
|
<example>
|
|
SEND: <iq type='result' from='pubsub.server1'
|
|
to='subscriber@server1/resource' id='s1'>
|
|
<query xmlns='jabber:iq:pubsub'>
|
|
<subscribe to='publisher.server2'>
|
|
<ns>namespace:1</ns>
|
|
</subscribe>
|
|
</query>
|
|
</iq>
|
|
</example>
|
|
|
|
<p>
|
|
If the remote pubsub server was unable or unwilling to accept the
|
|
subscription request, this should be reflected in the response:
|
|
</p>
|
|
<example>
|
|
SEND: <iq type='error' from='pubsub.server1'
|
|
to='subscriber@server1/resource' id='s1'>
|
|
<query xmlns='jabber:iq:pubsub'>
|
|
<subscribe to='publisher.server2'>
|
|
<ns>namespace:1</ns>
|
|
</subscribe>
|
|
</query>
|
|
<error code='406'>Not Acceptable</error>
|
|
</iq>
|
|
</example>
|
|
|
|
<p>
|
|
Step3: Publisher publishes information
|
|
</p>
|
|
<p>
|
|
The publisher, publisher.server2, publishes information in the
|
|
namespace:1 namespace, to the remote pubsub component pubsub.server2:
|
|
</p>
|
|
<example>
|
|
SEND: <iq type='set' from='publisher.server2'
|
|
to='pubsub.server2' id='p1'>
|
|
<query xmlns='jabber:iq:pubsub'>
|
|
<publish ns='namespace;1'>
|
|
<stuff xmlns='namespace:1'>nonsense</stuff>
|
|
</publish>
|
|
</query>
|
|
</iq>
|
|
</example>
|
|
|
|
|
|
<p>
|
|
Step4: Pubsub component receives published information
|
|
</p>
|
|
<p>
|
|
The pubsub component pushes the published information to pubsub.server1,
|
|
who has been determined to be a valid recipient:
|
|
</p>
|
|
<example>
|
|
RECV: <iq type='set' from='pubsub.server2'
|
|
to='pubsub.server1' id='p1'>
|
|
<query xmlns='jabber:iq:pubsub'>
|
|
<publish ns='namespace;1' from='publisher.server2'>
|
|
<stuff xmlns='namespace:1'>nonsense</stuff>
|
|
</publish>
|
|
</query>
|
|
</iq>
|
|
</example>
|
|
|
|
<p>
|
|
Step5: Pubsub component forwards published information to original subscriber
|
|
</p>
|
|
<p>
|
|
The local pubsub component then diffuses the information received to the
|
|
original subscriber:
|
|
</p>
|
|
<example>
|
|
SEND: <iq type='set' from='pubsub.server1'
|
|
to='subscriber@server1/resource' id='p1'>
|
|
<query xmlns='jabber:iq:pubsub'>
|
|
<publish ns='namespace;1' from='publisher.server2'>
|
|
<stuff xmlns='namespace:1'>nonsense</stuff>
|
|
</publish>
|
|
</query>
|
|
</iq>
|
|
</example>
|
|
|
|
<p>
|
|
This way, only a single published element must travel between servers
|
|
to satisfy a multiplex of subscribed entities at the delivery end.
|
|
</p>
|
|
|
|
<p>
|
|
Of course, this mechanism will rely upon knowledge about pubsub components
|
|
and where they're available; furthermore, it will require knowledge about
|
|
where publisher entities publish their information.
|
|
This knowledge, and the mechanisms to discover this sort of information,
|
|
is not to be covered in this spec, which purely deals with the subscription
|
|
and publishing of information. As SOAP is to UDDI (to use a slightly
|
|
controversial pair of technologies), so is jabber:iq:pubsub to this
|
|
discovery mechanism as yet undefined. To include the definition of such
|
|
a discovery mechanism in this specification is wrong on two counts:
|
|
</p>
|
|
<ul>
|
|
<li>Discovery mechanisms by nature should not be tied to specific areas</li>
|
|
<li>Trying to load too much onto jabber:iq:pubsub will only produce a
|
|
complex and hard-to-implement specification</li>
|
|
</ul>
|
|
<p>
|
|
After all, the jabber:iq:pubsub spec as defined here is usable out of the
|
|
box for the simple scenarios, and scenarios where discovery is not
|
|
necessary or the information can be exchanged in other ways.
|
|
</p>
|
|
|
|
</section3>
|
|
|
|
<section3 topic='Willingness to Serve'>
|
|
<p>
|
|
There are some situations where it might be appropriate for a pubsub
|
|
component to refuse particular subscription requests. Here are two
|
|
examples:
|
|
</p>
|
|
<ul>
|
|
<li>Where a pubsub component that's been designed, implemented, or
|
|
configured to handle local-only pubsub traffic, and a subscription request
|
|
is received, specifying a publisher that the local pubsub component knows
|
|
to be one that publishes to a remote pubsub component <note>under other
|
|
circumstances, this would trigger a 'Proxy Subscription', as described earlier, if supported</note>. In this case, the local pubsub component would be
|
|
unwilling to provoke a server-to-server connection and therefore unwilling to
|
|
honour the request.</li>
|
|
<li>Where a pubsub component receives a subscription request from a
|
|
remote subscriber, and that pubsub component knows that there's a
|
|
pubsub component local to the subscriber. In this case, the (administrator
|
|
of the) remote pubsub component might want to encourage proxy subscriptions.
|
|
</li>
|
|
</ul>
|
|
<p>
|
|
A refusal could take one of a number of guises:
|
|
</p>
|
|
<example caption='A flat refusal'>
|
|
SEND: <iq type='error' from='pubsub.server2'
|
|
to='subscriber@server1/resource' id='s1'>
|
|
<query xmlns='jabber:iq:pubsub'>
|
|
<subscribe to='publisher.server2'>
|
|
<ns>namespace:1</ns>
|
|
</subscribe>
|
|
</query>
|
|
<error code='406'>Local pubsub only</error>
|
|
</iq>
|
|
</example>
|
|
<example caption='A refusal with redirection'>
|
|
SEND: <iq type='error' from='pubsub.server2'
|
|
to='subscriber@server1/resource' id='s1'>
|
|
<query xmlns='jabber:iq:pubsub'>
|
|
<subscribe to='publisher.server2'>
|
|
<ns>namespace:1</ns>
|
|
</subscribe>
|
|
</query>
|
|
<error code='302' jid='pubsub.server1'/>
|
|
</iq>
|
|
</example>
|
|
<p>Note: This 302 redirect is not covered in the general protocol specification,
|
|
but it's an interesting concept :-)</p>
|
|
</section3>
|
|
|
|
</section2>
|
|
|
|
|
|
<section2 topic='Subscriber Anonymity and Acceptance?'>
|
|
<p>
|
|
The jabber:iq:pubsub specification makes no provision for
|
|
publishers to query a pubsub component to ask for a list of those entities
|
|
that are subscribed to (namespaces) it (publishes). This is deliberate.
|
|
Do we wish to add to the specification to allow the publisher to discover
|
|
this information? If so, it must be as an optional 'opt-in' (or 'opt-out')
|
|
tag for the subscriber, to determine whether his JID will show up on the
|
|
list.
|
|
<note>Even if there is no provision for querying the subscribers, perhaps
|
|
we should make a provision for the publisher to ask the pubsub component
|
|
for a list of namespaces that have been subscribed to (for that publisher).
|
|
</note>
|
|
</p>
|
|
<p>
|
|
Associated with this is the semi-reciprocal issue of acceptance? The
|
|
specification deliberately makes no provision for a subscription acceptance
|
|
mechanism (where the publisher must first accept a subscriber's request,
|
|
via the pubsub component). If we're to prevent the publishers knowing
|
|
who is subscribing, ought we to give them the power of veto, to 'balance
|
|
things out'?
|
|
</p>
|
|
<p>
|
|
Note that if we do, the acceptance issue is not necessarily one for the
|
|
pubsub specification to resolve; there are other ways of introducing
|
|
access control, at least in a component environment; use of a mechanism
|
|
that the Jabber::Component::Proxy Perl module represents is one example:
|
|
wedge a proxy component in front of a real (pubsub) component and have
|
|
the ability to use ACLs (access control lists) to control who gets to
|
|
connect to the real component.
|
|
</p>
|
|
|
|
</section2>
|
|
|
|
</section1>
|
|
|
|
</xep>
|