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Add initial draft of Entity Versioning.

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<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE xep SYSTEM 'xep.dtd' [
<!ENTITY % ents SYSTEM 'xep.ent'>
%ents;
]>
<?xml-stylesheet type='text/xsl' href='xep.xsl'?>
<xep>
<header>
<title>Entity Versioning</title>
<abstract>
A method by which rosters and disco 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 during session initialization with minimal state being
stored by the server and client.
</abstract>
&LEGALNOTICE;
<number>xxxx</number>
<status>ProtoXEP</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>0.0.1</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) 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 (eg. MUC
disco#items). Furthermore, roster versioning requires that the server
maintain a great deal of state (multiple versions of the roster) which can
be difficult to implement in a large, distributed system. This XEP defines
a method by which entities other than the roster can be versioned and
cached.
</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 generally 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>
</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='Protocol' anchor='proto'>
<section2 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:features:entityver:0' namespace. At the latest, this SHOULD
be done when informing a client that resource binding is required. For
example:
</p>
<example caption="Stream Features"><![CDATA[
<stream:features>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
<required/>
</bind>
<ver xmlns='urn:xmpp:features:entityver:0'/>
</stream:features>
]]></example>
<p>
The entity versioning stream feature is merely informative and therefore
is never mandatory-to-negotiate.
</p>
</section2>
<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 Implementation Notes. 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 MUST 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 roster request might look like:</p>
<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:features:entityver:0'>25P2A7H8</version>
</item>
<item jid='anne@shakespeare.lit'>
<version xmlns='urn:xmpp:features: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:features:entityver:0'>9ZFZXVP9</version>
</item>
</query>
</iq>
]]></example>
<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:
</p>
<example caption="Roster Push"><![CDATA[
<!-- Server -->
<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:features:entityver:0'>XWE4MUUP</version>
</item>
</query>
</iq>
]]></example>
<p>A disco request for rooms (as defined in &xep0045;) might look like:</p>
<example caption="MUC Disco"><![CDATA[
<!-- Client -->
<iq from='hag66@shakespeare.lit/phone'
id='zb8q41fas6yn4'
to='chat.shakespeare.lit'
type='get'>
<query xmlns='http://jabber.org/protocol/disco#items>
<item jid='coven@chat.shakespeare.lit'>
<version xmlns='urn:xmpp:features:entityver:0'>25P2A7H8</version>
</item>
<item jid='inverness@chat.shakespeare.lit'>
<version xmlns='urn:xmpp:features:entityver:0'>4OLGSVNY</version>
</item>
</query>
</iq>
<!-- Server -->
<iq from='chat.shakespeare.lit'
id='zb8q41fas6yn4'
to='hag66@shakespeare.lit/phone'
type='result'>
<query xmlns='http://jabber.org/protocol/disco#items'>
<item jid='coven@chat.shakespeare.lit'
name='A Dark Cave'>
<version xmlns='urn:xmpp:features:entityver:0'>VIZSVF0D</version>
</item>
</query>
</iq>
]]></example>
<p>
In this example coven@chat.shakespeare.lit has been modified (eg. the
room name might have been changed), but inverness@chat.shakespeare.lit
has not changed, therefore no update is sent down.
</p>
<p>
Clients that implement this protocol SHOULD then cache the entity in
question when a version token is received.
</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 "bare JID:version" pairs sorted
in byte-wise order, and taking the MD5 hash of the constructed string. 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:
</p>
<example caption="Roster aggregate token request"><![CDATA[
<!-- Client -->
<iq to='bill@shakespeare.lit' type='get' id='bill1'>
<query xmlns='urn:xmpp:features:entityver:0' type='jabber:iq:roster' />
</iq>
<!-- Server -->
<iq to='bill@shakespeare.lit/home' type='result' id='bill1'>
<query xmlns='urn:xmpp:features:entityver:0' type='jabber:iq:roster'>
0514fc90e6c7981b06bbb2173bb8ef03
</query>
</iq>
]]></example>
<p>
Similarly, to fetch the aggregate token for a list of MUC rooms, one would
query the MUC component directly:
</p>
<example caption="MUC rooms aggregate token request"><![CDATA[
<!-- Client -->
<iq to='chat.shakespeare.lit' type='get' id='bill2'>
<query xmlns='urn:xmpp:features:entityver:0' type='http://jabber.org/protocol/disco#items' />
</iq>
<!-- Server -->
<iq to='bill@shakespeare.lit/home' type='result' id='bill2'>
<query xmlns='urn:xmpp:features:entityver:0' type='http://jabber.org/protocol/disco#items'>
32151d1d01440d5536a7f106afd3f4d8
</query>
</iq>
]]></example>
<p>
Because aggregate tokens are OPTIONAL to implement, clients MUST fall back
to a normal request if any error is returned in response to an aggregate
token IQ.
</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 is left up to the clients.
</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 consistant 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>
</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:features:entityver:0</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>
</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>