mirror of
https://github.com/moparisthebest/xeps
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466 lines
18 KiB
XML
466 lines
18 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>Entity Versioning</title>
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<abstract>
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A method by which rosters and disco items may be versioned so that servers
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will not need to send the entire list if it has not been modified, saving
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bandwidth and time during session initialization with minimal state being
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stored by the server and client.
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</abstract>
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&LEGALNOTICE;
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<number>xxxx</number>
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<status>ProtoXEP</status>
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<type>Standards Track</type>
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<sig>Standards</sig>
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<approver>Council</approver>
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<dependencies>
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<spec>RFC 6120</spec>
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<spec>RFC 6121</spec>
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</dependencies>
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<supersedes />
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<supersededby />
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<shortname>EV</shortname>
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<author>
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<firstname>Sam</firstname>
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<surname>Whited</surname>
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<email>swhited@atlassian.com</email>
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<jid>sam@samwhited.com</jid>
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</author>
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<author>
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<firstname>Doug</firstname>
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<surname>Keen</surname>
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<email>dkeen@atlassian.com</email>
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</author>
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<revision>
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<version>0.0.1</version>
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<date>2015-08-25</date>
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<initials>ssw</initials>
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<remark><p>First draft</p></remark>
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</revision>
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</header>
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<section1 topic='Introduction' anchor='intro'>
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<p>
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This problem of "downloading the world" (downloading the entire roster
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every time a session is initialized) was partially addressed by &xep0237;
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which was later merged into &rfc6121; §2.6. While this solved the problem
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for the roster, it didn't account for other entities (eg. disco items).
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Furthermore, roster versioning requires that the server maintain a great
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deal of state (roster items which should be pushed for each entity on
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reconnect, or monotonically increasing counters, etc.) which can be
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difficult to store or synchronize in a large, distributed system. This XEP
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defines a method by which the roster and entities other than the roster can
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be versioned and cached, and which is optimized for distributed systems
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with large enity lists (but works equally well on small, single server
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deployments).
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</p>
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</section1>
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<section1 topic='Requirements' anchor='reqs'>
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<ul>
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<li>
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An extra round trip MUST NOT be required to initiate entity versioning.
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</li>
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<li>
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Clients that do not implement the protocol (but which use servers that
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do) MUST still be able to request and receive entities normally.
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</li>
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<li>
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Servers which implement this protocol MUST NOT be required to store
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multiple versions of an entity list or maintain other redundant state.
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</li>
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<li>
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Inconsistant state between servers in a cluster should not cause cache
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invalidation for the entire entity list.
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</li>
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<li>
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Large changes SHOULD NOT be required for existing servers / clients.
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</li>
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</ul>
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</section1>
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<section1 topic='Glossary' anchor='glossary'>
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<dl>
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<dt>Aggregate Token</dt>
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<dd>
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A hash which represents the state of a list of entities, and changes if
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any of those entities changes.
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</dd>
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<dt>Versioned Entity</dt>
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<dd>Any abstract object which may be versioned (eg. rooms, users).</dd>
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<dt>Version Token</dt>
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<dd>
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A generally short, case sensitive string which represents an entity and
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changes if that entity changes.
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</dd>
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</dl>
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</section1>
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<section1 topic='Use Cases' anchor='use_cases'>
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<section2 topic='Clients' anchor='client_use_cases'>
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<ul>
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<li>
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A client on a mobile device where bandwidth and throughput are limited
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has a very large roster which cause connections to take an unacceptable
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amount of time. With entity versioning, connections after the first
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connection do not take as long, and use less bandwidth.
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</li>
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<li>
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A client often wants to view the list of multi-user chat rooms
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available on a servers MUC service. However, the list is very long and
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takes a long time to download. After enabling entity versioning the
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client can fetch the list, and then poll for changes at a later date
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without re-requesting the entire list.
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</li>
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<li>
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A client wishes to cache the features supported by servers of the
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contacts in their roster since their disco items is not likely to
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change often.
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</li>
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</ul>
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</section2>
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<section2 topic='Servers' anchor='server_use_cases'>
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<ul>
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<li>
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A server is running in an environment where storing multiple versions
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of each users roster may put too much pressure on the storage backend.
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After enabling entity versioning, they only have to store a small token
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per user and can calculate the diffs to send to the client afterwards.
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</li>
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<li>
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A server maintains an out-of-band HTTP API for fetching information
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about MUC rooms to display on their web page. They wish to use a
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reverse proxy to cache API requests based on etags. Instead of
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attempting to check if the backend page has changed and generate etags,
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the room's entity version token is used as a weakly-validated ETag.
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</li>
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</ul>
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</section2>
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</section1>
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<section1 topic='Discovering Support' anchor='disco'>
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<p>
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If a server supports entity versioning, it MUST inform the connecting
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client when returning stream features during the stream negotiation
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process. This is done by including a <ver/> element, qualified by
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the 'urn:xmpp:entityver:0' namespace. At the latest, this SHOULD
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be done when informing a client that resource binding is required. For
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example:
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</p>
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<example caption="Stream Features"><![CDATA[
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<stream:features>
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<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
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<required/>
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</bind>
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<ver xmlns='urn:xmpp:entityver:0'/>
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</stream:features>
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]]></example>
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<p>
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The entity versioning stream feature is merely informative and therefore
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is never mandatory-to-negotiate.
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</p>
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</section1>
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<section1 topic='Version Tokens' anchor='version_tokens'>
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<p>
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Version tokens are short case-sensitive strings which are generated by the
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server. Their format is not defined in this spec, but a recommendation may
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be found in the <link url="#impl">Implementation Notes</link>. Version
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tokens are akin to a weakly-validated etag for the entity in question.
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</p>
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<p>
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Servers that implement this protocol must assign such a version token to
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each entity that is controlled by the server. The server MUST then update
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this version every time any mutable property of the entity changes (eg.
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when the subscription status of a user changes). The server MAY choose to
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update this token at any time (to force the clients to invalidate their
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cached representation fo the object). This version token MUST then be
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included with every object representation of that entity sent down in the
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stream. This is done by including a sub-node called "version" qualified
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by the entity versioning XML namespace defined in this document.
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Similarly, clients MAY also add version nodes for each version token they
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possess to the request for a list (not specifying a version token will
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force the server to send information on that entity to the client). If a
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server sends up a list of version tokens, the server MUST then check to
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see if those tokens correspond to any entity which it knows about, and
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not send down any entities with matching version tokens in the response.
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</p>
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<p>For example, a roster request might look like:</p>
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<example caption="Roster Request"><![CDATA[
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<!-- Client -->
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<iq from='romeo@montague.lit/home' id='56' to='romeo@montague.lit' type='get'>
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<query xmlns='jabber:iq:roster'>
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<item jid='bill@shakespeare.lit'>
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<version xmlns='urn:xmpp:entityver:0'>25P2A7H8</version>
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</item>
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<item jid='anne@shakespeare.lit'>
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<version xmlns='urn:xmpp:entityver:0'>VIZSVF0D</version>
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</item>
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</query>
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</iq>
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<!-- Server -->
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<iq from='romeo@montague.lit' id='56' to='romeo@montague.lit/home' type='result>
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<query xmlns='jabber:iq:roster'>
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<item subscription='both' jid='bill@shakespeare.lit'>
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<version xmlns='urn:xmpp:entityver:0'>9ZFZXVP9</version>
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</item>
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</query>
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</iq>
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]]></example>
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<p>
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Note that in this case there may be three roster items total (and the
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client only knows about two of them), or there may be two total roster
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items and the server is informing the client about a change to
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"bill@shakespeare.lit". Version tokens MUST also be present in roster
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pushes:
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</p>
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<example caption="Roster Push"><![CDATA[
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<!-- Server -->
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<iq from='romeo@montague.lit' id='ah382g678jka7' to='romeo@montague.lit/home' type='set'>
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<query xmlns='jabber:iq:roster' ver='ver34'>
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<item jid='tybalt@shakespeare.lit' subscription='remove'>
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<version xmlns='urn:xmpp:entityver:0'>XWE4MUUP</version>
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</item>
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</query>
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</iq>
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]]></example>
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<p>A disco request for rooms (as defined in &xep0045;) might look like:</p>
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<example caption="MUC Disco"><![CDATA[
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<!-- Client -->
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<iq from='hag66@shakespeare.lit/phone'
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id='zb8q41fas6yn4'
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to='chat.shakespeare.lit'
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type='get'>
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<query xmlns='http://jabber.org/protocol/disco#items>
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<item jid='coven@chat.shakespeare.lit'>
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<version xmlns='urn:xmpp:entityver:0'>25P2A7H8</version>
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</item>
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<item jid='inverness@chat.shakespeare.lit'>
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<version xmlns='urn:xmpp:entityver:0'>4OLGSVNY</version>
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</item>
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</query>
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</iq>
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<!-- Server -->
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<iq from='chat.shakespeare.lit'
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id='zb8q41fas6yn4'
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to='hag66@shakespeare.lit/phone'
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type='result'>
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<query xmlns='http://jabber.org/protocol/disco#items'>
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<item jid='coven@chat.shakespeare.lit'
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name='A Dark Cave'>
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<version xmlns='urn:xmpp:entityver:0'>VIZSVF0D</version>
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</item>
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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 example coven@chat.shakespeare.lit has been modified (eg. the
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room name might have been changed), but inverness@chat.shakespeare.lit
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has not changed, therefore no update is sent down.
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</p>
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<p>
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Clients that implement this protocol SHOULD then cache the entity in
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question when a version token is received.
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</p>
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<section2 topic='Cache Invalidation' anchor='deletes'>
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<p>
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When a client syncs with the server and indicates that it has a version
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token in its cache that does not match any entity on the server (or when
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the server wants to remove an entity from the clients cache for any other
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reason), the server MUST reply with an empty <version/> node. When
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the client receives such an empty version node it SHOULD purge the entity
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from its cache. For example, the following exchange would trigger the
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removal of 'inverness@chat.shakespeare.lit' from the cached MUC list:
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<example caption="MUC deletion"><![CDATA[
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<!-- Client -->
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<iq from='hag66@shakespeare.lit/phone'
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id='zb8q41fas6yn4'
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to='chat.shakespeare.lit'
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type='get'>
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<query xmlns='http://jabber.org/protocol/disco#items>
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<item jid='coven@chat.shakespeare.lit'>
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<version xmlns='urn:xmpp:entityver:0'>25P2A7H8</version>
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</item>
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<item jid='inverness@chat.shakespeare.lit'>
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<version xmlns='urn:xmpp:entityver:0'>4OLGSVNY</version>
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</item>
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</query>
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</iq>
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<!-- Server -->
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<iq from='chat.shakespeare.lit'
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id='zb8q41fas6yn4'
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to='hag66@shakespeare.lit/phone'
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type='result'>
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<query xmlns='http://jabber.org/protocol/disco#items'>
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<item jid='inverness@chat.shakespeare.lit'>
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<version xmlns='urn:xmpp:entityver:0'/>
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</item>
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</query>
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</iq>
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]]></example>
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</p>
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<p>
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If the client receives an indication that it should delete an item from a
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list by any other means (eg. via a roster push), it SHOULD remove the
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version token associated with that entity from its cache.
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</p>
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</section2>
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</section1>
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<section1 topic='Aggregate Tokens' anchor='agg_tokens'>
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<p>
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While the version token approach to caching does not require a great deal
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of state to be stored on the client or the server, it does require a lot
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more information to be sent by the client when requesting a list of
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entities. For a very large list which is not likely to have changed, it may
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be useful know in advance if the roster has changed or not (so that we can
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avoid sending the large request entirely). To do this, we can request an
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aggregate version token from the server. This aggregate token is calculated
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by constructing a string of comma separated "JID:version" pairs sorted in
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byte-wise order (because the JID:version pair is constructed before
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sorting, if two items in the list have the same JID they can still be
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sorted by the version token), and taking the MD5 hash of the constructed
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string. For example, if the server is calculating the aggregate version
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token for a roster, it might end up with the following string:
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</p>
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<example caption="Aggregate token list"><![CDATA[
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anne@shakespeare.lit:VIZSVF0D,bill@shakespeare.lit:25P2A7H8
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]]></example>
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<p>
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Which results in the aggregate token:
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</p>
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<example caption="Aggregate token"><![CDATA[
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0514fc90e6c7981b06bbb2173bb8ef03
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]]></example>
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<p>
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The actual request is an IQ sent to the server, or entity handling the
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versioned list which contains a query that specifies the namespace of the
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list we want to fetch. Eg. to fetch the aggregate token for the roster one
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would query the server with the type set to the `jabber:iq:roster`
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namespace:
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</p>
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<example caption="Roster aggregate token request"><![CDATA[
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<!-- Client -->
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<iq to='bill@shakespeare.lit' type='get' id='bill1'>
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<query xmlns='urn:xmpp:entityver:0' type='jabber:iq:roster' />
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</iq>
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<!-- Server -->
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<iq to='bill@shakespeare.lit/home' type='result' id='bill1'>
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<query xmlns='urn:xmpp:entityver:0' type='jabber:iq:roster'>
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0514fc90e6c7981b06bbb2173bb8ef03
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</query>
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</iq>
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]]></example>
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<p>
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Similarly, to fetch the aggregate token for a list of MUC rooms, one would
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query the MUC component directly with the type set to the 'disco#items'
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namespace:
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</p>
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<example caption="MUC rooms aggregate token request"><![CDATA[
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<!-- Client -->
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<iq to='chat.shakespeare.lit' type='get' id='bill2'>
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<query xmlns='urn:xmpp:entityver:0' type='http://jabber.org/protocol/disco#items' />
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</iq>
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<!-- Server -->
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<iq to='bill@shakespeare.lit/home' type='result' id='bill2'>
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<query xmlns='urn:xmpp:entityver:0' type='http://jabber.org/protocol/disco#items'>
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32151d1d01440d5536a7f106afd3f4d8
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</query>
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</iq>
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]]></example>
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<p>
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Because aggregate tokens are OPTIONAL to implement, clients MUST fall back
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to a normal request if any error is returned in response to an aggregate
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token IQ.
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</p>
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<p>
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If an aggregate token is requested for a list that may contain more than
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one type of entity (eg. MUC rooms and pubsub nodes that live on the same
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component), then the server MUST return the aggregate token constructed
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with the entire list (rooms and pubsub nodes).
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</p>
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<p>
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Clients are also NOT REQUIRED to check aggregate tokens. However, clients
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MAY wish to check aggregate tokens before making a roster or MUC request
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when the cached roster or MUC list is very large. When to check aggregate
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tokens (if at all) is left up to the implementation.
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</p>
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</section1>
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<section1 topic='Implementation Notes' anchor='impl'>
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<p>
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Version tokens may not provide enough collision resistance across versioned
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entities (hereafter simply called "entities"), and may vary from server to
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server, and therefore they MUST NOT be used as an entity identifier.
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</p>
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<p>
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Version tokens SHOULD always be considered opaque to the client (eg. even
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if the version token is a derivable and consistent hash on the server side,
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clients should not need to know how the server is calculating the token).
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</p>
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<p>
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The author RECOMMENDS using 8 character (32-bit) random alphanumeric ASCII
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strings (eg. AABd7z9T) for version tokens.
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</p>
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<p>
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If a server which supports this XEP provides an HTTP API which can be used
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to fetch information about entities (eg. for listing information about MUC
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rooms that a server provides on the providers web page), the entities
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version token MAY be used as a weakly validated ETag for any API requests
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for that entity.
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</p>
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</section1>
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<section1 topic='Security Considerations' anchor='security'>
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<p>
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Client-side caching of entity information across sessions (rather than
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holding them in memory only for the life of a session) could pose a privacy
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risk, especially on shared systems. Implementations SHOULD protect cached
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entity data with strong encryption or other appropriate means.
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</p>
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</section1>
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<section1 topic='IANA Considerations' anchor='iana'>
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<p>This document requires no interaction with &IANA;.</p>
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</section1>
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<section1 topic='XMPP Registrar Considerations' anchor='registrar'>
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<section2 topic='Protocol Namespaces' anchor='registrar-ns'>
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<p>This specification defines the following XML namespace:</p>
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<ul>
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<li>urn:xmpp:entityver:0</li>
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</ul>
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<p>
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Upon advancement of this specification from a status of Experimental to a
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status of Draft, the ®ISTRAR; shall add the foregoing namespace to the
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registry located at &STREAMFEATURES;, as described in Section 4 of
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&xep0053;.
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</p>
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</section2>
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</section1>
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<section1 topic='XML Schema' anchor='schema'>
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TODO
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</section1>
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<section1 topic='Acknowledgements' anchor='ack'>
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<p>
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The original entity versioning proposal was engineered and written by
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HipChat's Doug Keen.
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</p>
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</section1>
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</xep>
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