<remark><p>Initial version, split from XEP-0060.</p></remark>
</revision>
</header>
<section1topic='Introduction'anchor='intro'>
<p>&xep0060; defines an XMPP protocol extension for generic publish-subscribe features. This specification defines the nature and handling of collection nodes in XMPP-based publish-subscribe technologies.</p>
<p>A pubsub service MAY support collection nodes as well as leaf nodes. Collections enable nodes to be grouped together in many ways. A collection node MUST contain only leaf nodes and/or other collection nodes (similar to the way in which a file system directory can contain both files and subdirectories) and MUST NOT contain published items (therefore a collection MUST NOT support the "publish" feature or related features such as "persistent-items"). If collections are supported, a service MUST advertise that fact in its "disco#info" responses by including a feature of "pubsub#collections" and MUST support service discovery of child nodes as described in the <linkurl='#entity-nodes'>Discover Nodes</link> section of this document.</p>
<p>This section provides background information about collection nodes, with insights from graph theory. <note>See <linkurl='http://en.wikipedia.org/wiki/Graph_(mathematics)'>http://en.wikipedia.org/wiki/Graph_(mathematics)</link>.</note> The intended result is a clearer vocabulary about particular deployment scenarios. The terminology introduced in this section is used mainly in the discussion of <linkurl='delete'>collection node deletion</link>.</p>
<p>In terms of graph theory, the set of nodes hosted at a pubsub service is a directed acyclic graph. <note>See <linkurl='http://en.wikipedia.org/wiki/Directed_acyclic_graph'>http://en.wikipedia.org/wiki/Directed_acyclic_graph</link>.</note> The particular graph types can be further described as follows:</p>
<li>If there are no collection nodes, we say that the graph is simply a <strong>flat set</strong> of nodes without connections because there are no arcs between nodes, i.e., no node is the direct predecessor of another node (here we use the less formal phrase that no node is the parent of any other child node).</li>
<li>If there may be multiple paths between any two given nodes (where the path may include intermediate collection nodes), the graph is a <strong>Directed Acyclic Graph</strong> or "DAG" <note>See <linkurl='http://en.wikipedia.org/wiki/Directed_acyclic_graph'>http://en.wikipedia.org/wiki/Directed_acyclic_graph</link>.</note> because a given node may be the child of multiple parents.</li>
<li>If there is only one path between any two given nodes (where the path may include intermediate collection nodes), the graph is a <strong>Tree</strong><note>See <linkurl='http://en.wikipedia.org/wiki/Tree_(graph_theory)'>http://en.wikipedia.org/wiki/Tree_(graph_theory)</link>.</note> because a given node may be the child of only one collection node.</li>
<li>If there is a root collection node but there are no internal collection nodes, we say informally that the graph has a <strong>depth</strong> of 1 because all of the connections from leaf nodes to the root collection node are direct (i.e., each connection is an arc); this case is equivalent to a flat set with a root collection node and is typically uninteresting.</li>
<li>If there is a root collection node and there are internal collection nodes, we say that the graph has <strong>infinite depth</strong> because there is an unbounded number of arcs between each leaf node and the root collection node; this case is more interesting than a graph of depth=1 since it enables a wide range of trees and hierarchies.</li>
<li>In a tree with collection nodes, deletion of a collection node automatically results in destruction of the arcs to that collection node from leaf nodes or other collection nodes because a child can have only one parent; in this case we say that a child node has a <strong>dependency</strong> on its parent and that the tree is a <strong>Strict Hierarchy</strong>. (This is similar to a strictly hierarchical file system, in which deletion of a directory results in deletion of all its file and subdirectories.)</li>
<li>In a DAG with collection nodes, deletion of a collection node does not automatically result in destruction of the arcs to that collection node from leaf nodes or other collection nodes because a child can have multiple parents (but if the last parent is deleted, the last remaining arc is destroyed); in this case we say that the tree is a <strong>Loose Hierarchy</strong>. (This is similar to a loosely hierarchical file system that is mostly hierarchical but that allows multiple soft links.)</li>
<li>If a graph is made up of directed acyclic graphs but there is no single root collection node for all the DAGs, we say that the graph is a <strong>Dag Set</strong> (i.e., a set of directed acyclic graphs).</li>
<li>If a graph is made up of trees but there is no single root collection node for all the trees, the graph is a <strong>Forest</strong> (i.e., a set of trees).</li>
<li>If each tree in a forest is a Strict Hierarchy, we say that the graph is a <strong>Strict Hierarchy Set</strong>.</li>
<li>If each DAG in a set is a Loose Hierarchy, we say that the graph is a <strong>Loose Hierarchy Set</strong>.</li>
</ol>
<p>Finally, in XMPP pubsub, all graphs are <strong>oriented</strong> because any two collection nodes cannot have a bidirectional relationship (i.e., if collection node #1 is a direct predecessor of collection node #2 then #2 cannot also be a direct predecessor of #1).</p>
<p>This terminology is summarized in the following table.</p>
<tablecaption='Node Relationship Models'>
<tr>
<th>Model</th>
<th>Description</th>
<th>Root Node</th>
<th>Multiple Parents</th>
<th>Node Dependency</th>
<th>Depth</th>
</tr>
<tr>
<td>Flat Set</td>
<td>A set of nodes with no parent-child relationships (i.e., there are no collection nodes).</td>
<td>No</td>
<td>N/A</td>
<td>No</td>
<td>0 (zero)</td>
</tr>
<tr>
<td>Directed Acyclic Graph (DAG)</td>
<td>A set of nodes with parent-child relationships, where a child node can have more than one parent.</td>
<td>Yes</td>
<td>Yes</td>
<td>No</td>
<td>1 or infinite</td>
</tr>
<tr>
<td>Dag Set</td>
<td>A set of DAGs with no root node.</td>
<td>No</td>
<td>Yes</td>
<td>No</td>
<td>1 or infinite</td>
</tr>
<tr>
<td>Tree</td>
<td>A set of nodes with parent-child relationships, where a node can be the child of only one parent.</td>
<td>Yes</td>
<td>No</td>
<td>No</td>
<td>1 or infinite</td>
</tr>
<tr>
<td>Forest</td>
<td>A set of trees with no root node.</td>
<td>No</td>
<td>No</td>
<td>No</td>
<td>1 or infinite</td>
</tr>
<tr>
<td>Strict Hierarchy</td>
<td>An infinite tree in which a child node can have only one parent and is dependent on its parent.</td>
<td>Yes</td>
<td>No</td>
<td>Yes</td>
<td>Infinite</td>
</tr>
<tr>
<td>Strict Hierarchy Set</td>
<td>A set of strict hierarchies with no root node.</td>
<td>No</td>
<td>No</td>
<td>Yes</td>
<td>Infinite</td>
</tr>
<tr>
<td>Loose Hierarchy</td>
<td>An infinite DAG in which a child node can have multiple parents but cannot exist without at least one parent.</td>
<td>Yes</td>
<td>Yes</td>
<td>Yes</td>
<td>Infinite</td>
</tr>
<tr>
<td>Loose Hierarchy Set</td>
<td>A set of loose hierarchies with no root node.</td>
<td>No</td>
<td>Yes</td>
<td>Yes</td>
<td>Infinite</td>
</tr>
</table>
</section1>
<section1topic='Subscribe to a Collection Node'anchor='subscribe'>
<p>A service that implements collection nodes SHOULD allow entities to subscribe to collection nodes (subject to access models and local security policies).</p>
<p>In addition to the subscription configuration options already defined, there are two subscription configuration options specific to collection nodes:</p>
<ul>
<li>
<p><strong>pubsub#subscription_type</strong></p>
<p>This subscription option enables the subscriber to subscribe either to items or to nodes.</p>
<p>If the subscription type is "items", the subscriber shall be notified whenever any node contained in the collection generates a notification (e.g., when an item is published or deleted), as modified by the value of the "pubsub#subscription_depth" option.</p>
<p>If the subscription type is "nodes", the subscriber shall be notified whenever a new node is added to the collection, as modified by the value of the "pubsub#subscription_depth" option.</p>
<p>The default value of this subscription option MUST be "nodes".</p>
</li>
<li>
<p><strong>pubsub#subscription_depth</strong></p>
<p>This subscription option enables the subscriber to specify whether it wants to receive notifications only from first-level children of the collection (a value of "1") or from all descendents (a value of "all").</p>
<p>For subscriptions of type "items", this enables the subscriber to be informed only when an item is published to a leaf node that is a direct child of the collection node to which it has subscribed, or to be informed whenever an item is published to any leaf node in the "tree" that begins at the level of the collection to which it has subscribed.</p>
<p>For subscriptions of type "nodes", this enables the subscriber to be informed only when a new node is added in the specific collection to which it has subscribed, or to be informed whenever a node is added anywhere in the "tree" that begins at the level of the collection to which it has subscribed.</p>
<p>The default value of this subscription option MUST be "1".</p>
</li>
</ul>
<p>In order to subscribe to a collection node, an entity MUST send a subscription request to the node; the subscription request MAY include subscription options, but this is not strictly necessary (especially if the entity does not wish to override the default settings for the "pubsub#subscription_type" and "pubsub#subscription_depth" options).</p>
<examplecaption='Entity subscribes to a collection node (no configuration)'><![CDATA[
<iqtype='set'
from='francisco@denmark.lit/barracks'
to='pubsub.shakespeare.lit'
id='collsub1'>
<pubsubxmlns='http://jabber.org/protocol/pubsub'>
<subscribejid='francisco@denmark.lit'
node='blogs'/>
</pubsub>
</iq>
]]></example>
<p>The subscriber will now receive notification of new first-level nodes created within the "blogs" collection.</p>
<examplecaption='Entity subscribes to a collection node (with configuration)'><![CDATA[
<p>The subscriber will now receive item notifications from nodes at any depth within the "blogs" collection.</p>
<p>Depending on the nature of the node "tree", a subscription type of "items" and depth of "all" may result in an extremely large number of notifications. Therefore, a service MAY disallow such a combination of subscription options, in which case it MUST return a ¬allowed; error to the requesting entity.</p>
<p>A service MAY allow an entity to subscribe to a collection node in two ways, once with a subscription of type "nodes" (to receive notification of any new nodes added to the collection or the entire tree) and once with a subscription of type "items" (to receive all items published within the tree). However, a service SHOULD NOT allow an entity to subscribe twice to a collection node (once with a subscription depth of "1" and once with a subscription depth of "all") for the same subscription type, since two such subscriptions are unnecessary (a depth of "all" includes by definition a depth of "1"); in this case the service SHOULD return a &conflict; error to the requesting entity.</p>
<p>A service that implements collections SHOULD support a root collection. The root collection shall be identified by the lack of a node identifier (i.e., the address of the pubsub service itself, such as "pubsub.shakespeare.lit").</p>
<p>Subscribing to this node with a subscription of type "nodes" and a depth of "1" enables an entity to be notified whenever a new first-level node is created at the pubsub service. Subscribing to this node with a subscription of type "nodes" and a depth of "all" enables an entity to be notified whenever a new node is created anywhere at the pubsub service.</p>
<examplecaption='Entity subscribes to the root collection node'><![CDATA[
<iqtype='set'
from='francisco@denmark.lit/barracks'
to='pubsub.shakespeare.lit'
id='root1'>
<pubsubxmlns='http://jabber.org/protocol/pubsub'>
<subscribejid='francisco@denmark.lit'/>
</pubsub>
</iq>
]]></example>
<p>If the root collection node is configured to send notification of node associations and disassociations, the service shall send an event that contains a <collection/> element whose 'node' attribute specifies the NodeID of the collection (in this case the NodeID is empty to signify that the collection is the root collection); this element in turn contains an <associate/> element whose 'node' attribute specifies the NodeID of node that has been associated with the collection.</p>
<examplecaption='Notification of node association'><![CDATA[
<section1topic='Create a New Collection Node'anchor='createnode'>
<p>To create a new collection node, the requesting entity MUST include a Data Form containing a 'pubsub#node_type' field whose <value/> is "collection".</p>
<examplecaption='Entity requests a new collection node'><![CDATA[
<examplecaption='Service responds with success'><![CDATA[
<iqtype='result'
from='pubsub.shakespeare.lit'
to='bard@shakespeare.lit/globe'
id='create3'/>
]]></example>
<p>In addition to the errors already defined for leaf node creation, there are several reasons why the collection node creation request might fail:</p>
<ol>
<li>The service does not support collection nodes.</li>
<li>The service does not support creation of collection nodes.</li>
<li>The requesting entity does not have sufficient privileges to create collection nodes.</li>
</ol>
<p>These error cases are described more fully in the following sections.</p>
<p>If the service does not support collection nodes, it MUST respond with a &feature; error, specifying a pubsub-specific error condition of <unsupported/> and a feature of "collections".</p>
<examplecaption='Service does not support collection nodes'><