<abstract>This document defines methods for distributing Multi-User Chat (MUC) rooms across multiple chat services. Note: This document has been retracted by the author in favor of XEP-0289.</abstract>
<remark><p>Added protocol flows for finding and joining shadow rooms, thus removing dependency on communication with firsthost; changed examples to mimic XEP-0045.</p></remark>
<p>&xep0045; defines a full-featured technology for multi-user text conferencing in XMPP. By design, <cite>XEP-0045</cite> assumes that a conference room is hosted at a single service, which can be accessed from any point on the network. However, this assumption introduces a single point of failure for the conference room, since if occupants at a using domain lose connectivity to the hosting domain then they also lose connectivity to the room. In some deployment scenarios (and even on the open Internet) this behavior is suboptimal. Therefore, this document attempts to define a technology for distributing MUC rooms across multiple services.</p>
<p>The basic approach to distribution of MUC rooms is as follows:</p>
<olstart="1">
<li>A user creates a room on a service and configures it as "distributed" (or the service assumes that the room is distributed based on local service policy); this first instance of the room is called a SOURCE and the service on which it is created is called a FIRSTHOST.</li>
<li>The firsthost can immediately request that other services (called PEERHOSTS) replicate the room by creating their own local instances (called SHADOWS); alternatively, the firsthost can wait to send the replication request until users from the peerhost have joined the room.</li>
<li>If a user from the peerhost attempts to join the source room after replication is established, the firsthost invites the user to join the shadow rather than the source by sending a direct invitation to the user.</li>
<li>As long as the peerhost and firsthost have connectivity, they share room messages, room rosters, and room configuration changes in real time. If any conflict arises, the firsthost's information rules since it is "first among equals".</li>
<li>If the peerhost loses connectivity to the firsthost, it maintains the shadow, including local room history, room roster, and room configuration, and if possible also maintains connectivity with other peerhosts.</li>
<li>Upon reconnecting to the firsthost, a peerhost exchanges room history and room rosters with the firsthost and receives room configuration data (if modified).</li>
</ol>
<p>The room IDs of source rooms SHOULD be opaque to users and unique across all possible peerhosts, for example by generating a UUID in accordance with &rfc4122; or by hashing the human-readable name of the room using the SHA-256 algorithm in accordance with &nistfips180-2;.</p>
<p>When the original room owner creates the room (or subsequently configures the room), the service MAY simply default all rooms to "distributed".</p>
<p>Alternatively, the server MAY offer the option of making the room a "distributed room". This is done by including the "muc#roomconfig_distributed" feature in the room configuration form:</p>
<p>Alternatively, the firsthost can choose to perform room distribution in the background, rather than exposing the 'muc#roomconfig_distributed' option to the user.</p>
<p>Several error cases are possible (the peerhost is resource constrained, the firsthost is forbidden to peer with the peerhost, etc.); these will be specified more fully in a future version of this specification.</p>
<p>Once the peerhost acknowledges that it is willing and able to replicate the room, four things happen:</p>
<olstart="1">
<li>The source sends the room configuration to the shadow.</li>
<li>The source sends the room roster to the shadow.</li>
<li>The shadow optinoally requests the room history from the source.</li>
<li>The firsthost informs other peerhosts about the new peerhost.</li>
</ol>
<examplecaption='Source Sends Room Configuration to Shadow'><![CDATA[
<p>The new shadow SHOULD request the room history. This is done by sending an IQ-get from the shadow to the source, containing a <history/> element qualified by the 'http://jabber.org/protocol/muc' namespace (the syntax and semantics of this element are described in <cite>XEP-0045</cite>).</p>
<p>XEP-0045 specifies that rooms can be found on a chat service using &xep0030;.</p>
<p>A user at a peerhost could send a service discovery items ("disco#items") request to the firsthost in order to find the source room. However, this introduces a dependency on communication with the firsthost, and we have stipulated that such communication might not be available.</p>
<p>Alternatively, the peerhost could keep a list of the shadow rooms that it hosts. From the perspective of the user at the peerhost, these rooms are local in the sense that the user can join the shadow and thereby interact with the occupants of the shadow (and, if server-to-server communication is working, with the occupants of the source). This approach is RECOMMENDED.</p>
<p>Note: If the peerhost frequently loses communications with the firsthost and the list of rooms located at the firsthost is large, the peerhost will want to use a more efficient method of synchronizing its room list than sending disco#items requests and receiving large disco#items results. However, methods for optimizing this synchronization process are out of scope for this specification.</p>
<p>To find rooms on the peerhost, the local user sends a "disco#items" request to the peerhost.</p>
<examplecaption='User Queries Peerhost for Rooms'><![CDATA[
<p>The user can then send a disco#info request to each room. If the room is a shadow, the service MUST include extended information about the room using the &xep0128; format, specifically with a "http://jabber.org/protocol/muc#roominfo" FORM_TYPE and a (newly-defined) "dmuc-source" field.</p>
<examplecaption='Room Returns Extended Disco Info Results'><![CDATA[
<p>This informs the user that the source room is "coven@chat.shakespeare.lit"; as a result, the user's client can trap any outbound requests destined for the source room (service discovery, room join, etc.) and redirect them to the shadow.</p>
</section2>
<section2topic='Joining a Shadow'anchor='shadowjoin'>
<p>The process of joining a shadow is exactly as described in XEP-0045.</p>
<examplecaption='Local User Seeks to Join Shadow Room'><![CDATA[
<presence
from='somewitch@macbeth.lit/mobile'
to='coven@rooms.macbeth.lit/anotherwitch'>
<xxmlns='http://jabber.org/protocol/muc'/>
</presence>
]]></example>
<p>The shadow then informs the source (and any other shadows) of the user's presence; it does so by sending presence from the roomjid of the user at the shadow to a roomjid with the same roomnick at the source and shadow(s).</p>
<examplecaption='Shadow Informs Source of New Occupant'><![CDATA[
<presence
from='theroom@rooms.macbeth.lit/anotherwitch'
to='coven@chat.shakespeare.lit/anotherwitch'>
<xxmlns='http://jabber.org/protocol/muc#user'>
<itemaffiliation='none'role='participant'/>
</x>
</presence>
]]></example>
<p>The source then delivers that presence stanza to its local users. (Note: The shadow needs to send only one presence stanza to the source, thus reducing the number of stanzas sent over the server-to-server link between the peerhost and the firsthost.)</p>
</section2>
<section2topic='Joining a Room'anchor='sourcejoin'>
<p>If there is no local shadow available at the peerhost, or if the peerhost does not support extended service discovery information as described above, then the local user at the peerhost will end up sending a join request to the source room instead of the shadow room.</p>
<p>When this happens, the firsthost determines if it will invite the user to join a shadow at a peerhost instead. The process for determining when to send invitations is implementation specific and might be subject to configuration at the firsthost (e.g., the firsthost might send invitations only to users of a domain associated with the peerhost and only after a certain number of such users have joined the room at the firsthost).</p>
<p>The shadow then informs the source (and any other shadows) of the user's presence; it does so by sending presence from the roomjid of the user at the shadow to a roomjid with the same roomnick at the source and shadow(s).</p>
<examplecaption='Shadow Informs Source of New Occupant'><