&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.

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 Discover Nodes section of this document.

This section provides background information about collection nodes, with insights from graph theory. See http://en.wikipedia.org/wiki/Graph_(mathematics). The intended result is a clearer vocabulary about particular deployment scenarios. The terminology introduced in this section is used mainly in the discussion of collection node deletion.

In terms of graph theory, the set of nodes hosted at a pubsub service is a directed acyclic graph. See http://en.wikipedia.org/wiki/Directed_acyclic_graph. The particular graph types can be further described as follows:

1. If there are no collection nodes, we say that the graph is simply a flat set 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).
2. If there may be multiple paths between between any two given nodes (where the path may include intermediate collection nodes), the graph is a Directed Acyclic Graph or "DAG" See http://en.wikipedia.org/wiki/Directed_acyclic_graph. because a given node may be the child of multiple parents.
3. If there is only one path between any two given nodes (where the path may include intermediate collection nodes), the graph is a Tree See http://en.wikipedia.org/wiki/Tree_(graph_theory). because a given node may be the child of only one collection node.
4. If there is a root collection node but there are no internal collection nodes, we say informally that the graph has a depth 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.
5. If there is a root collection node and there are internal collection nodes, we say that the graph has infinite depth 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.
6. 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 dependency on its parent and that the tree is a Strict Hierarchy. (This is similar to a strictly hierarchical file system, in which deletion of a directory results in deletion of all its file and subdirectories.)
7. 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 Loose Hierarchy. (This is similar to a loosely hierarchical file system that is mostly hierarchical but that allows multiple soft links.)
8. 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 Dag Set (i.e., a set of directed acyclic graphs).
9. If a graph is made up of trees but there is no single root collection node for all the trees, the graph is a Forest (i.e., a set of trees).
10. If each tree in a forest is a Strict Hierarchy, we say that the graph is a Strict Hierarchy Set.
11. If each DAG in a set is a Loose Hierarchy, we say that the graph is a Loose Hierarchy Set.

Finally, in XMPP pubsub, all graphs are oriented 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).

This terminology is summarized in the following table.

A service that implements collection nodes SHOULD allow entities to subscribe to collection nodes (subject to access models and local security policies).

In addition to the subscription configuration options already defined, there are two subscription configuration options specific to collection nodes:

• pubsub#subscription_type

  This subscription option enables the subscriber to subscribe either to items or to nodes.

  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.

  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.

  The default value of this subscription option MUST be "nodes".

• pubsub#subscription_depth

  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").

  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.

  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.

  The default value of this subscription option MUST be "1".

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).

The subscriber will now receive notification of new first-level nodes created within the "blogs" collection.

The subscriber will now receive item notifications from nodes at any depth within the "blogs" collection.

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.

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.

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").

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.

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.

The notification event MAY also include the node meta-data, formatted using the Data Forms protocol.

To create a new collection node, the requesting entity MUST include a Data Form containing a 'pubsub#node_type' field whose <value/> is "collection".

In addition to the errors already defined for leaf node creation, there are several reasons why the collection node creation request might fail:

1. The service does not support collection nodes.
2. The service does not support creation of collection nodes.
3. The requesting entity does not have sufficient privileges to create collection nodes.

These error cases are described more fully in the following sections.

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".

If the service supports collection nodes but does not allow new collection nodes to be created, it MUST respond with a ¬allowed; error.

If the requesting entity has insufficient privileges to create new collections, the service MUST respond with a &forbidden; error.

A service MAY offer some node configuration options that are specific to collection nodes and not provided in configuration forms related to leaf nodes. The following are RECOMMENDED:

• pubsub#children_association_policy -- the policy regarding who may associate child nodes with the collection (values: all, owner, whitelist).
• pubsub#children_association_whitelist -- the whitelist of entities that may associate child nodes with the collection.
• pubsub#children_max -- the maximum number of child nodes that may be associated with a collection.

Deletion of a collection node can introduce a large number of changes to the system, depending on the node relationship model of the deployed system. This section describes recommended handling of deletion requests in the context of collection nodes.

Although a node can be associated with a collection when it is created (as described above), it can also be associated with a collection after it has been created. This can be done in two ways:

• By modifying the node's "pubsub#collection" configuration field.
• By modifying the collection node's "pubsub#children" configuration field.

These methods are described below.

In order to modify the (child) node's "pubsub#collection" configuration field, the owner of the node shall submit a request to edit the node's configuration, receive a configuration form from the service, and then submit a modified configuration form:

Note: To associate a node with the root collection node, the node owner MUST submit an empty <value/> element within the 'pubsub#collection' field.

In order to modify the (parent) node's "pubsub#children" configuration field, the owner of the node shall submit a request to edit the node's configuration, receive a configuration form from the service, and then submit a modified configuration form:

If the 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; this element in turn contains an <associate/> element whose 'node' attribute specifies the NodeID of node that has been associated with the collection.

If an entity attempts to associate a node with a collection in a way that would violate the node relationship model (e.g., adding a second parent to a node in a Tree or Strict Hierarchy or making a child a new parent of its existing parent or other predecessor and thus violating the orientation of the graph), the service MUST return a &conflict; error.

A node can be disassociated from a collection after it has been associated (whether at creation time or afterward). This can be done in two ways:

• By modifying the node's "pubsub#collection" configuration field.
• By modifying the collection node's "pubsub#children" configuration field.

These methods are described below.

In order to modify the (child) node's "pubsub#collection" configuration field, the owner of the node shall submit a request to edit the node's configuration, receive a configuration form from the service, and then submit a modified configuration form:

Note: To disassociate the node from all collection nodes, the node owner MUST submit an empty <field/> element for the 'pubsub#collection' field as shown in the foregoing example.

Note: To disassociate the node from the root collection node, the node owner MUST submit an empty <value/> element within the 'pubsub#collection' field as shown in the foregoing example.

In order to modify the (parent) node's "pubsub#children" configuration field, the owner of the node shall submit a request to edit the node's configuration, receive a configuration form from the service, and then submit a modified configuration form:

If the 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; this element in turn contains a <disassociate/> element whose 'node' attribute specifies the NodeID of node that has been disassociated from the collection.

If a node is disassociated from a collection node and a new association is not formed, the implementation MAY associate the node with the root collection node or associate it with no collection node.

Note: The combination of associating a node with one collection and disassociating the same node from another collection can be used to move a node from one collection to another.

The default configuration options can be different for a collection node vs. a leaf node. In order to specifically request the default configuration options for collection nodes, an entity MUST include a Data Form with a 'pubsub#node_type' field whose value is "collection" in the request (since the default value for the 'pubsub#node_type' field is "leaf").

If the service does not support collection nodes, it MUST return a &feature; error, specifying a pubsub-specific error condition of <unsupported/> and a feature of "collections".

If an item is published to a node which is also included by a collection, and an entity is subscribed to that collection with a subscription type of "items", then the notifications generated by the service MUST contain additional information. The &ITEMS; element contained in the notification message MUST specify the node identifier of the node that generated the notification (not the collection) and the &ITEM; element MUST contain a SHIM header that specifies the node identifier of the collection.

Note: The delivery options (such as "pubsub#deliver_payloads") are determined by the publishing leaf node, not by the aggregating collection node. If the owner of a collection node sets delivery options for a collection node, the service SHOULD ignore those options and apply the options set for the leaf node that publishes an item.

REQUIRED.

REQUIRED.

REQUIRED.

REQUIRED for protocol specifications.