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<xep>
<header>
  <title>Entity Versioning</title>
  <abstract>
    A method by which lists of items may be versioned so that servers will not
    need to send the entire list if it has not been modified, saving bandwidth
    and time with minimal state being stored by the server and client.
  </abstract>
  &LEGALNOTICE;
  <number>0366</number>
  <status>Experimental</status>
  <type>Standards Track</type>
  <sig>Standards</sig>
  <approver>Council</approver>
  <dependencies>
    <spec>RFC 6120</spec>
    <spec>RFC 6121</spec>
  </dependencies>
  <supersedes />
  <supersededby />
  <shortname>EV</shortname>
  <author>
    <firstname>Sam</firstname>
    <surname>Whited</surname>
    <email>swhited@atlassian.com</email>
    <jid>sam@samwhited.com</jid>
  </author>
  <author>
    <firstname>Doug</firstname>
    <surname>Keen</surname>
    <email>dkeen@atlassian.com</email>
  </author>
  <revision>
    <version>0.1</version>
    <date>2015-12-15</date>
    <initials>XEP Editor (mam)</initials>
    <remark><p>Initial published version approved by the XMPP Council.</p></remark>
  </revision>
  <revision>
    <version>0.0.2</version>
    <date>2015-09-17</date>
    <initials>ssw</initials>
    <remark><p>Add profiles / partial sync.</p></remark>
  </revision>
  <revision>
    <version>0.0.1</version>
    <date>2015-08-25</date>
    <initials>ssw</initials>
    <remark><p>First draft</p></remark>
  </revision>
</header>

<section1 topic='Introduction' anchor='intro'>
  <p>
    This problem of "downloading the world" (downloading the entire roster
    every time a session is initialized or receiving an entire disco items
    response every time a MUC list is queried, etc.) was partially addressed by
    &xep0237;  which was later merged into &rfc6121; §2.6. While this solved
    the problem for the roster, it didn't account for other entities.
    Furthermore, roster versioning requires that the server maintain a great
    deal of state (roster items which should be pushed for each entity on
    reconnect, or monotonically increasing counters, etc.) which can be
    difficult to store or synchronize in a large, distributed system. This XEP
    defines a method by which generic entity lists can be versioned and cached,
    and which is optimized for distributed systems with large entity lists (but
    works equally well on small, single server deployments).
  </p>
</section1>

<section1 topic='Requirements' anchor='reqs'>
  <ul>
    <li>
      An extra round trip MUST NOT be required to initiate entity versioning.
    </li>
    <li>
      Clients that do not implement the protocol (but which use servers that
      do) MUST still be able to request and receive entities normally.
    </li>
    <li>
      Servers which implement this protocol MUST NOT be required to store
      multiple versions of an entity list or maintain other redundant state.
    </li>
    <li>
      Inconsistant state between servers in a cluster should not cause cache
      invalidation for the entire entity list.
    </li>
    <li>
      Large changes SHOULD NOT be required for existing servers / clients.
    </li>
  </ul>
</section1>

<section1 topic='Glossary' anchor='glossary'>
  <dl>
    <dt>Aggregate Token</dt>
    <dd>
      A hash which represents the state of a list of entities, and changes if
      any of those entities changes.
    </dd>
    <dt>Versioned Entity</dt>
    <dd>Any abstract object which may be versioned (eg. rooms, users).</dd>
    <dt>Version Token</dt>
    <dd>
      A short, case sensitive string which represents an entity and changes if
      that entity changes.
    </dd>
  </dl>
</section1>

<section1 topic='Use Cases' anchor='use_cases'>
  <section2 topic='Clients' anchor='client_use_cases'>
    <ul>
      <li>
        A client on a mobile device where bandwidth and throughput are limited
        has a very large roster which cause connections to take an unacceptable
        amount of time. With entity versioning, connections after the first
        connection do not take as long, and use less bandwidth.
      </li>
      <li>
        A client often wants to view the list of multi-user chat rooms
        available on a servers MUC service. However, the list is very long and
        takes a long time to download. After enabling entity versioning the
        client can fetch the list, and then poll for changes at a later date
        without re-requesting the entire list.
      </li>
      <li>
        A client wishes to cache the features supported by servers of the
        contacts in their roster since their disco items is not likely to
        change often.
      </li>
    </ul>
  </section2>
  <section2 topic='Servers' anchor='server_use_cases'>
    <ul>
      <li>
        A server is running in an environment where storing multiple versions
        of each users roster may put too much pressure on the storage backend.
        After enabling entity versioning, they only have to store a small token
        per user and can calculate the diffs to send to the client afterwards.
      </li>
      <li>
        A server maintains an out-of-band HTTP API for fetching information
        about MUC rooms to display on their web page. They wish to use a
        reverse proxy to cache API requests based on etags. Instead of
        attempting to check if the backend page has changed and generate etags,
        the room's entity version token is used as a weakly-validated ETag.
      </li>
    </ul>
  </section2>
</section1>

<section1 topic='Entity Versioning Profiles' anchor='profiles'>
  <p>
    Because entity versioning is designed to be a generic system for syncing
    any sort of list in XMPP, and the format and requirements of various entity
    lists may vary greatly, no specific wire format is defined in this
    specification.  Instead, the specifics for various lists will be left up to
    separate XEPs which will define entity versioning "profiles" which must be
    registered with the XMPP registrar. These profiles will define exactly how
    version tokens are represented in the specific list format for which they
    wish to use entity versioning. The rest of this document will provide
    details about entity versioning which will be common to all entity
    versioning profiles and do not need to be redefined in EV profile XEPs. It
    will also define an EV profile for fetching the roster.
  </p>
  <p>
    The roster entity versioning profile which is used as an example throughout
    this document will use the namespace 'urn:xmpp:entityver:profile:roster:0'
    as described in the <link url="#registrar">XMPP Registrar
    Considerations</link> section of this document.
  </p>
</section1>

<section1 topic='Discovering Support' anchor='disco'>
  <p>
    If a server supports entity versioning, it MUST inform the connecting
    client when returning stream features during the stream negotiation
    process. This is done by including a &lt;ver/&gt; element, qualified by the
    'urn:xmpp:entityver:0' namespace with child &lt;profile&gt; nodes for each
    supported entity versioning profile. At the latest, this SHOULD be done
    when informing a client that resource binding is required. For example if
    the server only supports versioning of rosters it might return:
  </p>
  <example caption="Stream Features"><![CDATA[
<stream:features>
  <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
    <required/>
  </bind>
  <ver xmlns='urn:xmpp:entityver:0'>
    <profile xmlns='urn:xmpp:entityver:profile:roster:0'/>
  </ver>
</stream:features>
  ]]></example>
  <p>
    The entity versioning stream feature is merely informative and therefore
    is never mandatory-to-negotiate.
  </p>
  <p>
    Clients, servers, and other entities that support &xep0030; and entity
    versioning must respond to service discovery requests with a feature of
    'urn:xmpp:entityver:0' and with a feature for each EV profile supported by
    the responding entity as described in the relavant specifications. Eg. a
    response from a server that supports roster versioning for the requesting
    entity might look like the following:
    <example caption="Service discovery information response"><![CDATA[
<iq from='shakespeare.lit'
    id='ku6e51v3'
    to='kingclaudius@shakespeare.lit/castle'
    type='result'>
  <query xmlns='http://jabber.org/protocol/disco#info'>
    <feature var='urn:xmpp:entityver:0'/>
    <feature var='urn:xmpp:entityver:profile:roster:0'/>
  </query>
</iq>
    ]]></example>
  </p>
</section1>

<section1 topic='Entity Sync' anchor='entity_sync'>
  <section2 topic='Version Tokens' anchor='version_tokens'>
    <p>
      Version tokens are short case-sensitive strings which are generated by
      the server. Their format is not defined in this spec, but a
      recommendation may be found in the <link url="#impl">Implementation
      Notes</link>. Version tokens are akin to a weakly-validated etag for the
      entity in question.
    </p>
    <p>
      Servers that implement this protocol must assign such a version token to
      each entity that is controlled by the server. The server SHOULD then
      update this version every time any mutable property of the entity changes
      (eg. when the subscription status of a user changes). The server MAY
      choose to update this token at any time (to force the clients to
      invalidate their cached representation fo the object). This version token
      MUST then be included with every object representation of that entity
      sent down in the stream. This is done by including a sub-node called
      "version" qualified by the entity versioning XML namespace defined in
      this document.  Similarly, clients MAY also add version nodes for each
      version token they possess to the request for a list (not specifying a
      version token will force the server to send information on that entity to
      the client). If a server sends up a list of version tokens, the server
      MUST then check to see if those tokens correspond to any entity which it
      knows about, and not send down any entities with matching version tokens
      in the response.
    </p>
    <p>
      For example, a versioned roster request might look like this:
      <example caption="Roster Request"><![CDATA[
<!-- Client -->
<iq from='romeo@montague.lit/home' id='56' to='romeo@montague.lit' type='get'>
  <query xmlns='jabber:iq:roster'>
    <item jid='bill@shakespeare.lit'>
      <version xmlns='urn:xmpp:entityver:0'>25P2A7H8</version>
    </item>
    <item jid='anne@shakespeare.lit'>
      <version xmlns='urn:xmpp:entityver:0'>VIZSVF0D</version>
    </item>
  </query>
</iq>

<!-- Server -->
<iq from='romeo@montague.lit' id='56' to='romeo@montague.lit/home' type='result>
  <query xmlns='jabber:iq:roster'>
    <item subscription='both' jid='bill@shakespeare.lit'>
      <version xmlns='urn:xmpp:entityver:0'>9ZFZXVP9</version>
    </item>
  </query>
</iq>
        ]]></example>
    </p>
    <p>
      Note that in this case there may be three roster items total (and the
      client only knows about two of them), or there may be two total roster
      items and the server is informing the client about a change to
      "bill@shakespeare.lit". Version tokens MUST also be present in roster
      pushes:
      <example caption="Roster Push"><![CDATA[
<iq from='romeo@montague.lit' id='ah382g678jka7' to='romeo@montague.lit/home' type='set'>
  <query xmlns='jabber:iq:roster' ver='ver34'>
    <item jid='tybalt@shakespeare.lit' subscription='remove'>
      <version xmlns='urn:xmpp:entityver:0'>XWE4MUUP</version>
    </item>
  </query>
</iq>
      ]]></example>
    </p>
  </section2>
  <section2 topic='Cache Invalidation' anchor='deletes'>
    <p>
      When a client syncs with the server and indicates that it has a version
      token in its cache that does not match any entity on the server (or when
      the server wants to remove an entity from the clients cache for any other
      reason), the server MUST reply with an empty &lt;version/&gt; node. When
      the client receives such an empty version node it SHOULD purge the entity
      from its cache. For example, the following would remove the roster item
      'bill@shakespeare.lit' from the cache:
    <example caption="Cache invalidation"><![CDATA[
<iq from='romeo@montague.lit' id='56' to='romeo@montague.lit/home' type='result>
  <query xmlns='jabber:iq:roster'>
    <item subscription='both' jid='bill@shakespeare.lit'>
      <version xmlns='urn:xmpp:entityver:0'/>
    </item>
  </query>
</iq>
    ]]></example>
    </p>
    <p>
      Roster pushes that indicate a deleted item MUST also remove the version
      from the cache (and need not contain an empty &lt;version/&gt; element).
    </p>
  </section2>
  <section2 topic='Partial sync'>
    <p>
      For very large groups fetching an entire list may not be practical or
      necessary. For example, one might imagine a large corporation with a
      shared roster that is too large for its version tokens to be sent up to
      the server on every sync, or even to download fully the first time. To
      solve this, servers MAY choose to send down only a part of an entity list
      in response to a query (unless the individual EV profile forbids partial
      list sync). How servers choose what items to return is an implementation
      detail that is out of the scope of this document. Some suggestions may be
      found in the <link url="#impl">Implementation Notes</link>. On subsequent
      requests for the entity list, the server MAY choose to return more
      entities (eg. based on changes in its internal selection criteria),
      however it MUST NOT invalidate cached entities unless they have actually
      been removed from the list.
    </p>
    <p>
      XEPs defining entity versioning profiles MUST include a section to
      indicate if partial sync is supported, and if so, how it will be
      indicated to the client (and how the client can request a full list). If
      no mechanism is specified, this is done by adding a boolean "full_list"
      attribute to the request, eg. a roster request for a partial list looks
      like:
      <example caption="Roster Request"><![CDATA[
<!-- Client -->
<iq from='romeo@montague.lit/home'
     id='56'
     to='romeo@montague.lit'
     type='get'>
  <query xmlns='jabber:iq:roster' full_list='false'>
    <item jid='bill@shakespeare.lit'>
      <version xmlns='urn:xmpp:entityver:0'>25P2A7H8</version>
    </item>
    <item jid='anne@shakespeare.lit'>
      <version xmlns='urn:xmpp:entityver:0'>VIZSVF0D</version>
    </item>
  </query>
</iq>

<!-- Server -->
<iq from='romeo@montague.lit' id='56' to='romeo@montague.lit/home' type='result'>
  <query xmlns='jabber:iq:roster' full_list='false'>
    <item subscription='both' jid='bill@shakespeare.lit'>
      <version xmlns='urn:xmpp:entityver:0'>9ZFZXVP9</version>
    </item>
  </query>
</iq>
    ]]></example>
    </p>
    <p>
      When making a request for a partial list, clients do not need to send up
      every entity in their cache. Instead they MAY send up just those entities
      for which they wish to check for updates. The server MUST then respond
      with any updates for those entities, and MAY also add other entities to
      the list if desired. If the client requests a partial list but does not
      indicate that it has anything in its cache, what entities to return (if
      any) is left up to the server implementation.
    </p>
  </section2>
  <section2 topic='List search'>
    <p>
      When a client has an incomplete versioned list, it may be beneficial to
      download more of the list without requesting the full list. To do this,
      servers which support entity versioning MUST supply a "search" IQ which
      can be used to discover list items matching a certain criteria. What data
      to match on (JID, metadata, associated vcards, etc.), and what type of
      search are left up to the server implementation and MAY be different
      between profiles.
    </p>
    <p>
      Search queries are qualified by the 'urn:xmpp:entityver:0:search'
      namespace and MUST have a 'profile' attribute set to the namespace for
      which the search is being performed. For instance, searching the roster
      looks like the following:
      <example caption="Roster search"><![CDATA[
<!-- Client -->
<iq from='romeo@montague.lit/home'
     id='564'
     to='romeo@montague.lit'
     type='get'>
  <query xmlns="urn:xmpp:entityver:0:search" profile='urn:xmpp:entityver:profile:roster:0'>
    Search term
  </query>
</iq>

<!-- Server -->
<iq from='romeo@montague.lit' id='564' to='romeo@montague.lit/home' type='result'>
  <query xmlns="urn:xmpp:entityver:0:search" profile='urn:xmpp:entityver:profile:roster:0' type='result'>
    <item subscription='both' jid='matching_search@term.lit'>
      <version xmlns='urn:xmpp:entityver:0'>4YAZ7Y38</version>
    </item>
  </query>
</iq>
    ]]></example>
    </p>
    <p>
      Search results SHOULD be added to the given list's cache. In this way,
      the full list does not need to be known.
    </p>
  </section2>
  <section2 topic='Aggregate Tokens' anchor='agg_tokens'>
    <p>
      While the version token approach to caching does not require a great deal
      of state to be stored on the client or the server, it does require a lot
      more information to be sent by the client when requesting a list of
      entities. For a very large list which is not likely to have changed, it
      may be useful know in advance if the roster has changed or not (so that
      we can avoid sending the large request entirely). To do this, we can
      request an aggregate version token from the server. This aggregate token
      is calculated by constructing a string of comma separated "ID:version"
      pairs sorted in byte-wise order (because the ID:version pair is
      constructed before sorting, if two items in the list have the same ID
      they can still be sorted by the version token), and taking the MD5 hash
      of the constructed string. The ID in the pair is any ID or key that
      identifies the entity as defined in its profile (eg. a JID for roster
      items and most other entities). For example, if the server is calculating
      the aggregate version token for a roster, it might end up with the
      following string:
    </p>
    <example caption="Aggregate token list"><![CDATA[
anne@shakespeare.lit:VIZSVF0D,bill@shakespeare.lit:25P2A7H8
    ]]></example>
    <p>
      Which results in the aggregate token:
    </p>
    <example caption="Aggregate token"><![CDATA[
0514fc90e6c7981b06bbb2173bb8ef03
    ]]></example>
    <p>
      The actual request is an IQ sent to the server, or entity handling the
      versioned list which contains a query that specifies the namespace of the
      list we want to fetch. Eg. to fetch the aggregate token for the roster
      one would query the server with the query's XMLNS set to
      'urn:xmpp:entityver:profile:roster:0':
    </p>
    <example caption="Roster aggregate token request"><![CDATA[
<!-- Client -->
<iq to='bill@shakespeare.lit' type='get' id='bill1'>
  <query xmlns='urn:xmpp:entityver:profile:roster:0'/>
</iq>

<!-- Server -->
<iq to='bill@shakespeare.lit/home' type='result' id='bill1'>
  <query xmlns='urn:xmpp:entityver:profile:roster:0'>
    0514fc90e6c7981b06bbb2173bb8ef03
  </query>
</iq>
    ]]></example>
    <p>
      Because aggregate tokens are OPTIONAL to implement, clients MUST fall
      back to making a normal list request if any error is returned in response
      to an aggregate token IQ.
    </p>
    <p>
      If an aggregate token is requested for a list that may contain more than
      one type of entity (eg. MUC rooms and pubsub nodes that live on the same
      component), then the server MUST return the aggregate token constructed
      with the entire list (rooms and pubsub nodes).
    </p>
    <p>
      Because aggregate tokens are calculated for the entire list as seen by
      the client or server, they will never match if partial lists have been
      downloaded by the client.
    </p>
    <p>
      Clients are also NOT REQUIRED to check aggregate tokens. However, clients
      MAY wish to check aggregate tokens before making a roster or MUC request
      when the cached roster or MUC list is very large. When to check aggregate
      tokens (if at all) is left up to the implementation.
    </p>
  </section2>
</section1>
<section1 topic='Implementation Notes' anchor='impl'>
  <p>
    Version tokens may not provide enough collision resistance across versioned
    entities (hereafter simply called "entities"), and may vary from server to
    server, and therefore they MUST NOT be used as an entity identifier.
  </p>
  <p>
    Version tokens SHOULD always be considered opaque to the client (eg. even
    if the version token is a derivable and consistent hash on the server side,
    clients should not need to know how the server is calculating the token).
  </p>
  <p>
    The author RECOMMENDS using 8 character (32-bit) random alphanumeric ASCII
    strings (eg. AABd7z9T) for version tokens.
  </p>
  <p>
    If a server which supports this XEP provides an HTTP API which can be used
    to fetch information about entities (eg. for listing information about MUC
    rooms that a server provides on the providers web page), the entities
    version token MAY be used as a weakly validated ETag for any API requests
    for that entity.
  </p>
  <p>
    Servers following this specification may choose to send down partial entity
    lists in response to queries. For the case of rosters one or more of the
    following may be returned to the requesting entity during the initial
    roster sync:
    <ul>
      <li>
        Users that are grouped with the requester in some way. Eg. for a
        company with a large shared roster which places the requesting client
        in the "Marketing Department" group, the server may wish to return
        roster items that also share that group.
      </li>
      <li>Users whom the requester has contacted recently or frequently.</li>
      <li>Users that should always be returned as part of server policy.</li>
    </ul>
  </p>
</section1>

<section1 topic='Security Considerations' anchor='security'>
  <p>
    Client-side caching of entity information across sessions (rather than
    holding them in memory only for the life of a session) could pose a privacy
    risk, especially on shared systems. Implementations SHOULD protect cached
    entity data with strong encryption or other appropriate means.
  </p>
</section1>

<section1 topic='IANA Considerations' anchor='iana'>
  <p>This document requires no interaction with &IANA;.</p> 
</section1>

<section1 topic='XMPP Registrar Considerations' anchor='registrar'>
  <section2 topic='Protocol Namespaces' anchor='registrar-ns'>
    <p>This specification defines the following XML namespace:</p>
    <ul>
      <li>urn:xmpp:entityver:0</li>
      <li>urn:xmpp:entityver:0:search</li>
    </ul>
    <p>
      Upon advancement of this specification from a status of Experimental to a
      status of Draft, the &REGISTRAR; shall add the foregoing namespace to the
      registry located at &STREAMFEATURES;, as described in Section 4 of
      &xep0053;.
    </p>
  </section2>
  <section2 topic='Namespace Versioning' anchor='registrar-versioning'>
    &NSVER;
  </section2>
  <section2 topic='Entity Versioning Profiles Registry' anchor='registrar-ev'>
    <p>
      The XMPP Registrar shall maintain a registry of entity versioning
      profiles. All EV profile registrations shall be defined in separate
      specifications (not in this document). Application types defined within
      the XEP series MUST be registered with the XMPP Registrar, resulting in
      protocol URNs of the form "urn:xmpp:entityver:profile:name:X" (where
      "name" is the registered name of the profile and "X" is a non-negative
      integer).
    </p>
    &REGPROCESS;
    <code><![CDATA[
<profile>
  <name>The name of the entity versioning profile.</name>
  <desc>A natural-language summary of the profile.</desc>
  <listdef>
    The document in which the original list definition is specified.
  </listdef>
  <doc>
    The document in which the EV profile for the list is specified (may be the
    same as <listdev/>).
  </doc>
</profile>
    ]]></code>
  </section2>
  <section2 topic='Entity Versioning Profiles' anchor='registrar-evprofile'>
    <p>This specification defines the following entity versioning profile:</p>
    <ul>
      <li>urn:xmpp:entityver:profile:roster:0</li>
    </ul>
    <p>
      Upon advancement of this specification from a status of Experimental to a
      status of Draft, the &REGISTRAR; shall add the following definition to
      the entity versioning profiles registry, as described in this document:
      <code><![CDATA[
<profile>
  <name>Roster entity versioning</name>
  <desc>Allows versioning of entities in an XMPP roster.</desc>
  <listdef>RFC 6121</listdef>
  <doc>TODO: Insert this document once it is assigned a number</doc>
</profile>
      ]]></code>
    </p>
  </section2>
</section1>

<section1 topic='XML Schema' anchor='schema'>
TODO
</section1>

<section1 topic='Acknowledgements' anchor='ack'>
  <p>
    The original entity versioning proposal was engineered and written by
    HipChat's Doug Keen.
  </p>
</section1>

</xep>
