Publish-Subscribe (XEP-0060) [1] defines an XMPP protocol extension for generic publish-subscribe features. However, it only allows notifications from nodes to which an entity is directly subscribed. It is useful in some circumstances to describe a relationship between nodes so that a publish on one node may be delivered via another node. For instance, if an entity is interested in notifications from a set of nodes the entity would discover each node somehow and then subscribe to them. With collection nodes, the entity would subscribe only to the collection which links the desired nodes, simplifying the subscription process.
In addition to simplifying the subscriber's usage, collection nodes also allow the owner to describe almost any type of relationship between nodes. Using various access models on different nodes the owner can also create almost any desired authorization semantics on a set of leaf nodes.
Note: Any use cases not described herein are described in Publish-Subscribe (XEP-0060) [1]. Also, this document does not show error flows related to the generic publish-subscribe use cases referenced herein, since they are exhaustively defined in XEP-0060. The reader is referred to XEP-0060 for all relevant protocol details related to the XMPP publish-subscribe extension.
This documents addresses the common requirements regarding configuration, publishing, subscribing, and notification semantics of collection nodes.
The following terms are used in this document to refer to collection node-specific features.
Note: some of these terms are specified in greater detail within the body of this document.
Collection nodes link nodes together to unify notifications from a set of collection or leaf nodes. An entity can subscribe to the collection and receive notifications of any associated leaf nodes.
A collection node can link with any other node in order to create a directed acyclic graph (DAG). Collection nodes MUST NOT be linked in such a way as to produce a cyclic graph (i.e., they cannot link to nodes that eventually link back to the initial node).
Collection nodes only contain other nodes and MUST NOT contain published items (therefore a collection MUST NOT support the "publish" feature or related features such as "persistent-items").
An entity might wish to discover if a service implements collection nodes; in order to do so, it sends a service discovery information ("disco#info") query to the component's JID using Service Discovery (XEP-0030) [2]. If a service supports collection nodes it MUST return a "pubsub#collections" feature. In addition, if the service supports associating a node with more than one collection it MUST return a feature of "pubsub#multi-collections".
<iq type='get'
from='francisco@denmark.lit/barracks'
to='pubsub.shakespeare.lit'
id='info1'>
<query xmlns='http://jabber.org/protocol/disco#info'/>
</iq>
<iq type='result'
from='pubsub.shakespeare.lit'
to='francisco@denmark.lit/barracks'
id='info1'>
<query xmlns='http://jabber.org/protocol/disco#info'>
...
<feature var='http://jabber.org/protocol/pubsub#collections'/>
<feature var='http://jabber.org/protocol/pubsub#multi-collections'/>
...
</query>
</iq>
If a service implements a hierarchy of nodes, it MUST also enable entities to discover the nodes in that hierarchy by means of the Service Discovery protocol, subject to the recommendations in Service Discovery (XEP-0030) [2] regarding large result sets (for which Jabber Search (XEP-0055) [3] or some other protocol SHOULD be used). The service discovery items ("disco#items") protocol enables an entity to query a service for a list of associated items, which, in the case of collection nodes would consist of the children associated with a given node. The following examples show the use of service discovery in discovering the nodes available at a hierarchical pubsub service.
In the first example, an entity sends a service discovery items ("disco#items") request to the root node (i.e., the service itself):
<iq type='get'
from='francisco@denmark.lit/barracks'
to='pubsub.shakespeare.lit'
id='nodes1'>
<query xmlns='http://jabber.org/protocol/disco#items'/>
</iq>
<iq type='result'
from='pubsub.shakespeare.lit'
to='francisco@denmark.lit/barracks'
id='nodes1'>
<query xmlns='http://jabber.org/protocol/disco#items'>
<item jid='pubsub.shakespeare.lit'
node='blogs'
name='Weblog updates'/>
<item jid='pubsub.shakespeare.lit'
node='news'
name='News and announcements'/>
</query>
</iq>
In the second example, an entity sends a disco#items request to one of the first-level nodes, which is also a collection node:
<iq type='get'
from='francisco@denmark.lit/barracks'
to='pubsub.shakespeare.lit'
id='nodes2'>
<query xmlns='http://jabber.org/protocol/disco#items'
node='blogs'/>
</iq>
<iq type='result'
from='pubsub.shakespeare.lit'
to='francisco@denmark.lit/barracks'
id='nodes2'>
<query xmlns='http://jabber.org/protocol/disco#items'
node='blogs'>
<item jid='pubsub.shakespeare.lit'
node='princely_musings'/>
<item jid='pubsub.shakespeare.lit'
node='kingly_ravings'/>
<item jid='pubsub.shakespeare.lit'
node='starcrossed_stories'/>
<item jid='pubsub.shakespeare.lit'
node='moorish_meanderings'/>
</query>
</iq>
If a notification on a child node is created and then delivered via the collection then the notifications generated by the service MUST contain additional information. The 'node' attribute of the <item/> or <node/> element contained in the notification message MUST specify the node identifier of the node that generated the notification (not the collection) and the <message/> stanza MUST contain Stanza Headers and Internet Metadata (XEP-0131) [4] 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 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.
Item notifications are notifications about the contents of a leaf node, and are generated by a publish, retract, or purge request.
<message to='francisco@denmark.lit' from='pubsub.shakespeare.lit'>
<event xmlns='http://jabber.org/protocol/pubsub#event'>
<items node='princely_musings'>
<item id='ae890ac52d0df67ed7cfdf51b644e901'>
...
</item>
</items>
</event>
<headers xmlns='http://jabber.org/protocol/shim'>
<header name='Collection'>blogs</header>
</headers>
</message>
Node notifications are notifications about nodes themselves, and are generated by a create, delete, or configure request.
<message to='francisco@denmark.lit' from='pubsub.shakespeare.lit'>
<event xmlns='http://jabber.org/protocol/pubsub#event'>
<create node='princely_musings'/>
</event>
<headers xmlns='http://jabber.org/protocol/shim'>
<header name='Collection'>blogs</header>
</headers>
</message>
If a 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/> or <dissociate/> element whose 'node' attribute specifies the NodeID of node that has been associated with the collection.
<message from='pubsub.shakespeare.lit'
to='francisco@denmark.lit'
id='newnode1'>
<event xmlns='http://jabber.org/protocol/pubsub#event'>
<collection node='some-collection'>
<associate node='new-node-id'>
</collection>
</event>
</message>
<message from='pubsub.shakespeare.lit'
to='francisco@denmark.lit'
id='newnode1'>
<event xmlns='http://jabber.org/protocol/pubsub#event'>
<collection node='some-collection'>
<dissociate node='old-node-id'>
</collection>
</event>
</message>
The notification event MAY also include the node metadata, formatted using the Data Forms (XEP-0004) [5] protocol.
<message from='pubsub.shakespeare.lit'
to='francisco@denmark.lit'
id='newnode2'>
<event xmlns='http://jabber.org/protocol/pubsub#event'>
<collection node='some-collection'>
<associate node='new-node-id'>
<x xmlns='jabber:x:data' type='result'>
<field var='FORM_TYPE' type='hidden'>
<value>http://jabber.org/protocol/pubsub#meta-data</value>
</field>
<field var='pubsub#creation_date'>
<value>2003-07-29T22:56Z</value>
</field>
<field var='pubsub#creator'>
<value>hamlet@denmark.lit</value>
</field>
<field var='pubsub#description'>
<value>Atom feed for my blog.</value>
</field>
<field var='pubsub#language'>
<value>en</value>
</field>
<field var='pubsub#contact'>
<value>bard@shakespeare.lit</value>
</field>
<field var='pubsub#owner'>
<value>hamlet@denmark.lit</value>
</field>
<field var='pubsub#title'>
<value>Princely Musings (Atom).</value>
</field>
<field var='pubsub#type'
><value>http://www.w3.org/2005/Atom</value>
</field>
</x>
</node>
</collection>
</event>
</message>
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 in Publish-Subscribe (XEP-0060) [1], there are two subscription configuration options specific to collection nodes:
pubsub#subscription_type
This subscription option enables the subscriber to subscribe either to notifications about items or notifications about nodes.
If the subscription type is "items", the subscriber shall be notified whenever any node contained in the collection has an item published to it, retracted from it, or the node is purged, 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, removed from the collection, or the configuration of a node within the collection has changed, as modified by the value of the "pubsub#subscription_depth" option.
If the subscription type is "all", the subscriber shall be notified about both "items" and "nodes" types of events, as modified by the value of the "pubsub#subscription_depth" option.
The default value of this subscription option SHOULD be "nodes".
pubsub#subscription_depth
This subscription option enables the subscriber to specify how far to traverse the node graph when determining whether a notification will be sent. It may be any integer value, 0 or greater, or the value "all" which means that any node within the collection will generate a notification.
The default value of this subscription option SHOULD 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).
<iq type='set'
from='francisco@denmark.lit/barracks'
to='pubsub.shakespeare.lit'
id='collsub1'>
<pubsub xmlns='http://jabber.org/protocol/pubsub'>
<subscribe jid='francisco@denmark.lit'
node='blogs'/>
</pubsub>
</iq>
The subscriber will now receive notification of new first-level nodes created within the "blogs" collection.
<iq type='set'
from='francisco@denmark.lit/barracks'
to='pubsub.shakespeare.lit'
id='collsub1'>
<pubsub xmlns='http://jabber.org/protocol/pubsub'>
<subscribe jid='francisco@denmark.lit'
node='blogs'/>
<options>
<x xmlns='jabber:x:data' type='submit'>
<field var='FORM_TYPE' type='hidden'>
<value>http://jabber.org/protocol/pubsub#subscribe_options</value>
</field>
<field var='pubsub#subscription_type'>
<value>items</value>
</field>
<field var='pubsub#subscription_depth'>
<value>all</value>
</field>
</x>
</options>
</pubsub>
</iq>
If the service allows the subscription it MUST inform the requesting entity that it is now subscribed.
<iq type='result'
from='pubsub.shakespeare.lit'
to='francisco@denmark.lit/barracks'
id='collsub1'>
<pubsub xmlns='http://jabber.org/protocol/pubsub'>
<subscription node='blogs'
jid='francisco@denmark.lit'
subscription='subscribed'/>
</pubsub>
</iq>
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.
<iq type='error'
from='pubsub.shakespeare.lit'
to='francisco@denmark.lit/barracks'
id='collsub1'>
<error type='cancel'>
<conflict xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
Depending on the nature of the node graph, 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 <not-allowed/> error to the requesting entity.
<iq type='error'
from='pubsub.shakespeare.lit'
to='francisco@denmark.lit/barracks'
id='collsub1'>
<error type='cancel'>
<not-allowed xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
When an entity requests items on a collection node the service SHOULD return the items on any leaf nodes associated with it subject to the access model of the collection node.
<iq type='get'
from='francisco@denmark.lit/barracks'
to='pubsub.shakespeare.lit'
id='items1'>
<pubsub xmlns='http://jabber.org/protocol/pubsub'>
<items node='blogs'/>
</pubsub>
</iq>
When a collection contains multiple nodes with items it MUST return multiple <items/> elements, one per node.
<iq type='result'
from='pubsub.shakespeare.lit'
to='francisco@denmark.lit/barracks'
id='items1'>
<pubsub xmlns='http://jabber.org/protocol/pubsub'>
<items node='Romeoance'>
<item id='368866411b877c30064a5f62b917cffe'>
...
</item>
</items>
<items node='Julliennui'>
<item id='3300659945416e274474e469a1f0154c'>
...
</item>
</items>
</pubsub>
</iq>
Depending on the nature of the node graph it may be expensive to allow item retrieval from a collection node. Therefore the service MAY disallow item retrieval via collection nodes, in which case it MUST return a <feature-not-implemented/> error to the requesting entity.
<iq type='error'
from='pubsub.shakespeare.lit'
to='francisco@denmark.lit/barracks'
id='items1'>
<error type='cancel'>
<feature-not-implemented xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
As specified in Publish-Subscribe (XEP-0060) [1] the default value for "pubsub#node_type" SHOULD be "leaf". To create a new collection node, the requesting entity MUST include a Data Form containing a "pubsub#node_type" field whose <value/> element contains "collection".
<iq type='set'
from='francisco@denmark.lit/barracks'
to='pubsub.shakespeare.lit'
id='create3'>
<pubsub xmlns='http://jabber.org/protocol/pubsub'>
<create node='announcements'/>
<configure>
<x xmlns='jabber:x:data' type='submit'>
<field var='FORM_TYPE' type='hidden'>
<value>http://jabber.org/protocol/pubsub#node_config</value>
</field>
<field var='pubsub#node_type'><value>collection</value></field>
</x>
</configure>
</pubsub>
</iq>
<iq type='result'
from='pubsub.shakespeare.lit'
to='francisco@denmark.lit/barracks'
id='create3'/>
In addition to the errors already defined for leaf node creation, there are several reasons why the collection node creation request might fail:
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-not-implemented/> error, specifying a pubsub-specific error condition of <unsupported/> and a feature of "collections".
<iq type='error'
from='pubsub.shakespeare.lit'
to='francisco@denmark.lit/barracks'
id='create3'>
<error type='cancel'>
<feature-not-implemented xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
<unsupported xmlns='http://jabber.org/protocol/pubsub#errors'
feature='collections'/>
</error>
</iq>
If the service supports collection nodes but does not allow new collection nodes to be created, it MUST respond with a <not-allowed/> error.
<iq type='error'
from='hamlet@denmark.lit/elsinore'
to='pubsub.shakespeare.lit'
id='create3'>
<error type='cancel'>
<not-allowed xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
If the requesting entity has insufficient privileges to create new collections, the service MUST respond with a <forbidden/> error.
<iq type='error'
from='pubsub.shakespeare.lit'
to='francisco@denmark.lit/barracks'
id='create3'>
<error type='auth'>
<forbidden xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
In addition to the node configuration options specified in Publish-Subscribe (XEP-0060) [1], there are three additional node configuration options that a service which supports collection nodes MUST supply.
Whether this is a "leaf" or "collection" node.
The parents of this node.
The children of this node.
To associate the root node to the collection the <value/> element MUST be empty.
A service MUST support the "pubsub#notify_config" option (specified in Publish-Subscribe (XEP-0060) [1]) for collection nodes.
A service MAY offer some node configuration options that are specific to collection nodes and SHOULD NOT be provided in configuration forms related to leaf nodes. The following are RECOMMENDED:
The policy regarding who may associate child nodes with the collection (values: all, owner, whitelist).
The whitelist of entities that may associate child nodes with the collection.
The maximum number of child nodes that may be associated with a collection.
<iq type='set'
from='francisco@denmark.lit/barracks'
to='pubsub.shakespeare.lit'
id='config1'>
<pubsub xmlns='http://jabber.org/protocol/pubsub#owner'>
<configure node='blogs'>
<x xmlns='jabber:x:data' type='submit'>
<field var='FORM_TYPE' type='hidden'>
<value>http://jabber.org/protocol/pubsub#node_config</value>
</field>
<field var='pubsub#node_type'>
<value>collection</value>
</field>
<field var='pubsub#children'>
<value>Romeoance</value>
<value>Julliennui</value>
</field>
<field var='pubsub#collection'>
<value/>
</field>
</x>
</configure>
</pubsub>
</iq>
<iq type='result'
from='pubsub.shakespeare.lit'
to='francisco@denmark.lit/barracks'
id='config1'/>
Leaf nodes only contain published items and MUST NOT have any children. If an entity attempts to add children to a leaf node (either via "pubsub#children" on the leaf node or "pubsub#collection" on another node) the service MUST return a <not-allowed/> error with a pubsub-specific error condition of <invalid-options/>.
<iq type='error'
from='pubsub.shakespeare.lit'
to='francisco@denmark.lit/barracks'
id='config1'>
<error type='cancel'>
<not-allowed xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
<invalid-options xmlns='http://jabber.org/protocol/pubsub#errors'/>
</error>
</iq>
If the requesting entity is not authorized to add the node to a collection then the service MUST return a <forbidden/> error.
<iq type='error'
from='pubsub.shakespeare.lit'
to='francisco@denmark.lit/barracks'
id='config1'>
<error type='cancel'>
<forbidden xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
If the configuration would exceed the maximum number of children allowed on a node, either because the node's "pubsub#children" exceeds its own "pubsub#children_max" value or because adding this node to a parent via "pubsub#collection" would exceed the parent's "pubsub#children_max" value, the service MUST return a <not-allowed/> error with a pubsub-specific error condition of <max-nodes-exceeded/>.
<iq type='error'
from='pubsub.shakespeare.lit'
to='francisco@denmark.lit/barracks'
id='config1'>
<error type='cancel'>
<not-allowed xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
<max-nodes-exceeded xmlns='http://jabber.org/protocol/pubsub#errors'/>
</error>
</iq>
The service MUST NOT allow the node type to be changed from collection to leaf. If it is attempted the service MUST return a <not-allowed/> error, specifying a pubsub-specific error condition of <invalid-options/>
<iq type='error'
from='pubsub.shakespeare.lit'
to='francisco@denmark.lit/barracks'
id='config1'>
<error type='cancel'>
<not-allowed xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
<invalid-options xmlns='http://jabber.org/protocol/pubsub#errors'/>
</error>
</iq>
Note: Changing the node type leaf to collection is permissible under conditions specified in Changing Node Type to Collection.
The service MUST NOT allow a cycle to be created in the node graph (e.g., node A to B to C to A). If an entity attempts to submit a configuration that would create a cycle the service MUST return a <not-allowed/> error, specifying a pubsub-specific error condition of <invalid-options/>.
<iq type='error'
from='pubsub.shakespeare.lit'
to='francisco@denmark.lit/barracks'
id='config1'>
<error type='cancel'>
<not-allowed xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
<invalid-options xmlns='http://jabber.org/protocol/pubsub#errors'/>
</error>
</iq>
A node type can be changed from leaf to collection (but not vice versa). If the node type is changed from leaf to collection and there are items associated with the node, the service MUST purge the node of all items (with or without notifying the subscribers).
As specified in Publish-Subscribe (XEP-0060) [1] a service can support retrieval of default node configuration, allowing an entity to request information about the default node configuration.
To get the node options for a collection node, the entity MUST send an empty <default/> element to the service with no NodeID. The entity SHOULD include the optional 'type' attribute with value 'collection'. In response, the service SHOULD return the default node options.
<iq type='get'
from='hamlet@denmark.lit/elsinore'
to='pubsub.shakespeare.lit'
id='def1'>
<pubsub xmlns='http://jabber.org/protocol/pubsub#owner'>
<default type='collection'/>
</pubsub>
</iq>
If no error occurs, the service MUST return the default node configuration options.
<iq type='result'
from='pubsub.shakespeare.lit'
to='hamlet@denmark.lit/elsinore'
id='def1'>
<pubsub xmlns='http://jabber.org/protocol/pubsub#owner'>
<default>
<x xmlns='jabber:x:data' type='form'>
<field var='FORM_TYPE' type='hidden'>
<value>http://jabber.org/protocol/pubsub#node_config</value>
</field>
<field var='pubsub#title' type='text-single'
label='A friendly name for the node'/>
<field var='pubsub#description' type='text-single'
label='A description of the node'/>
<field var='pubsub#node_type'
type='list-single'
label='Whether the node is a leaf (default) or a collection'>
<option label='The node is a leaf node (default)'>
<value>leaf</value>
</option>
<option label='The node is a collection node'>
<value>collection</value>
</option>
<value>collection</value>
</field>
<field var='pubsub#collection'
type='text-multi'
label='The collections of which this node is a child'/>
<field var='pubsub#children'
type='text-multi'
label='The nodes of which this node is a parent'/>
<field var='pubsub#children_association_policy'
type='list-single'>
<option label='Only the owners of this node may associate other nodes to this collection'>
owners
</option>
<option label='Only those on the children association whitelist may associate other nodes to this collection'>
whitelist
</option>
<option label='Anyone may associate nodes with this collection'>
all
</option>
<value>owner</value>
</field>
<field var='pubsub#children_association_whitelist'
type='jid-multi'
label='JIDs who can associate nodes to this collection'/>
<field var='pubsub#children_max'
type='text-single'
label='The maximum number of children for this collection'/>
<field var='pubsub#deliver_notifications' type='boolean'
label='Deliver event notifications'>
<value>true</value>
</field>
<field var='pubsub#notify_config' type='boolean'
label='Notify subscribers when the node configuration changes'>
<value>0</value>
</field>
<field var='pubsub#notify_delete' type='boolean'
label='Notify subscribers when the node is deleted'>
<value>0</value>
</field>
<field var='pubsub#notify_sub' type='boolean'
label='Notify owners about new subscribers and unsubscribes'>
<value>0</value>
</field>
<field var='pubsub#subscribe' type='boolean'
label='Whether to allow subscriptions'>
<value>1</value>
</field>
<field var='pubsub#access_model' type='list-single'
label='Specify the subscriber model'>
<option><value>authorize</value></option>
<option><value>open</value></option>
<option><value>presence</value></option>
<option><value>roster</value></option>
<option><value>whitelist</value></option>
<value>open</value>
</field>
</x>
</default>
</pubsub>
</iq>
There are several reasons why the default node configuration options request might fail. These cases are defined in Publish-Subscribe (XEP-0060) [1].
If a service supports collection node creation it MUST support collection node deletion.
<iq type='set'
from='francisco@denmark.lit/barracks'
to='pubsub.shakespeare.lit'
id='delete1'>
<pubsub xmlns='http://jabber.org/protocol/pubsub#owner'>
<delete node='blogs'/>
</pubsub>
</iq>
If no error occurs, the service MUST inform the owner of success.
<iq type='result'
from='pubsub.shakespeare.lit'
to='francisco@denmark.lit/barracks'
id='delete1'/>
If the requesting entity attempts to delete the root node, the service MUST return a <not-allowed/> error.
<iq type='error'
from='pubsub.shakespeare.lit'
to='francisco@denmark.lit/barracks'
id='delete1'>
<error type='cancel'>
<not-allowed xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
A service MAY allow collection nodes to have children associated with them without changing the rest of the configuration. If the service allows this an entity can send and <associate/> element with a 'node' attribute that contains the child node within a <collection/> element that posesses a 'node' attribute containing the parent node to the service.
<iq type='set'
from='francisco@denmark.lit/barracks'
to='pubsub.shakespeare.lit'
id='assoc1'>
<pubsub xmlns='http://jabber.org/protocol/pubsub#owner'>
<collection node='some-collection'>
<associate node='new-child-node'/>
</collection>
</pubsub>
</iq>
If the service allows the node association then it MUST confirm the association with an empty result.
<iq type='result'
from='pubsub.shakespeare.lit'
to='francisco@denmark.lit'
id='assoc1'/>
<iq type='error'
from='pubsub.shakespeare.lit'
to='francisco@denmark.lit/barracks'
id='assoc1'>
<error type='cancel'>
<forbidden xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
If the configuration would exceed the maximum number of children allowed on a node, either because the node's "pubsub#children" exceeds its own "pubsub#children_max" value or because adding this node to a parent via "pubsub#collection" would exceed the parent's "pubsub#children_max" value, the service MUST return a <not-allowed/> error with a pubsub-specific error condition of <max-nodes-exceeded/>.
<iq type='error'
from='pubsub.shakespeare.lit'
to='francisco@denmark.lit/barracks'
id='assoc1'>
<error type='cancel'>
<not-allowed xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
<max-nodes-exceeded xmlns='http://jabber.org/protocol/pubsub#errors'/>
</error>
</iq>
The service MUST NOT allow a cycle to be created in the node graph (e.g., node A to B to C to A). If an entity attempts to submit a configuration that would create a cycle the service MUST return a <not-allowed/> error, specifying a pubsub-specific error condition of <invalid-options/>.
<iq type='error'
from='pubsub.shakespeare.lit'
to='francisco@denmark.lit/barracks'
id='assoc1'>
<error type='cancel'>
<not-allowed xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
<invalid-options xmlns='http://jabber.org/protocol/pubsub#errors'/>
</error>
</iq>
A service MAY allow collection nodes to have children dissociated from them without changing the rest of the configuration. If the service allows this an entity can send and <dissociate/> element with a 'node' attribute that contains the child node within a <collection/> element that posesses a 'node' attribute containing the parent node to the service.
<iq type='set'
from='francisco@denmark.lit/barracks'
to='pubsub.shakespeare.lit'
id='dissoc1'>
<pubsub xmlns='http://jabber.org/protocol/pubsub#owner'>
<collection node='some-collection'>
<dissociate node='old-child-node'/>
</collection>
</pubsub>
</iq>
If the service allows the node dissociation then it MUST confirm the association with an empty result.
<iq type='result'
from='pubsub.shakespeare.lit'
to='francisco@denmark.lit'
id='dissoc1'/>
If a dissociation is requested between two nodes that are not already associated then the service MUST return a <bad-request/> error.
<iq type='error'
from='pubsub.shakespeare.lit'
to='francisco@denmark.lit'
id='dissoc1'>
<error type='modify'>
<bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
<iq type='error'
from='pubsub.shakespeare.lit'
to='francisco@denmark.lit/barracks'
id='dissoc1'>
<error type='cancel'>
<forbidden xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
To provide a starting point for service discovery a service SHOULD support a root node. A root node represents the node belonging to a given service and MUST be identified by the lack of a node identifier (i.e., the address of the pubsub service itself, such as "pubsub.shakespeare.lit"). Because the root node is owned by the service itself an entity SHOULD NOT be allowed create, delete, or configure the root node.
If a node is created or configured without any parents specified, a service MAY automatically associate otherwise orphaned nodes directly to the root node. If a service automatically associates a node with the root it MUST reflect that in the node configuration data form.
<iq type='set'
from='francisco@denmark.lit/barracks'
to='pubsub.shakespeare.lit'
id='root1'>
<pubsub xmlns='http://jabber.org/protocol/pubsub'>
<subscribe jid='francisco@denmark.lit'/>
</pubsub>
</iq>
Deletion of collection nodes can have a number of side effects due to implementation. Depending on the nature of the collection any of the following MAY happen when a collection node is deleted:
Node configuration MUST always reflect the current state of the node graph. Because node configuration contains both a pointer to its parents as well as its children an update to a primary node's "pubsub#collection" value will change the value of the secondary node's "pubsub#children" value, and vice-versa. A service MAY send a notification of the configuration change on the secondary node to subscribers if "pubsub#notify_config" is enabled on the secondary node.
This section summarizes the features described herein, specifies the appropriate requirements level for each feature (REQUIRED, RECOMMENDED, or OPTIONAL), and provides cross-references to the section of this document in which each feature is described.
Note: The feature names are all of the form "http://jabber.org/protocol/pubsub#name", where "name" is the text specified in the first column below.
| Name | Description | Support | Section |
|---|---|---|---|
| collections | Collection nodes are supported. | OPTIONAL | |
| multi-collection | A single leaf node can be associated with multiple collections. | OPTIONAL |
Collection nodes can be used to associate almost any node within the service, but only the access model of the collection node itself is used to determine what an entity is allowed to see. Therefore care should be taken that nodes are not linked in such a way as to leak private data (e.g., from a "closed" leaf node through an "open" collection) unless that behavior is specifically desired.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [6].
Note: These options are in addition to the standard options described in Publish-Subscribe (XEP-0060) [1] and related XEPs.
<var> <name>pubsub#collections</name> <desc>Support for collection nodes</desc> <doc>XEP-0248</doc> </var> <var> <name>pubsub#multi-collections</name> <desc>Support for multiple collections on a node</desc> <doc>XEP-0248</doc> </var>
<form_type>
<name>http://jabber.org/protocol/pubsub#subscribe_options</name>
<doc>XEP-0248</doc>
<desc>Options for collection node subscription</desc>
<field
var='pubsub#subscription_depth'
type='text-single'>
label='How far to traverse the node graph for notifications'/>
<field
var='pubsub#subscription_type'
type='list-single'>
<option label='Receive notification of items only'>
items
</option>
<option label='Receive notification of nodes only'>
nodes
</option>
<option label='Receive notification of items and nodes'>
all
</option>
</field>
</form_type>
<form_type>
<name>http://jabber.org/protocol/pubsub#node_config</name>
<doc>XEP-0248</doc>
<desc>Options for collection node configuration</desc>
<field
var='pubsub#node_type'
type='list-single'>
<option label='The node contains items'>
leaf
</option>
<option label='The node contains other nodes'>
collection
</option>
</field>
<field
var='pubsub#collection'
type='text-multi'
label='The collections of which this node is a child'/>
<field
var='pubsub#children'
type='text-multi'
label='The nodes of which this node is a parent'/>
<field
var='pubsub#children_association_policy'
type='list-single'>
<option label='Only the owners of this node may associate other nodes to this collection'>
owners
</option>
<option label='Only those on the children association whitelist may associate other nodes to this collection'>
whitelist
</option>
<option label='Anyone may associate nodes with this collection'>
all
</option>
</field>
<field
var='pubsub#children_association_whitelist'
type='jid-multi'
label='JIDs who can associate nodes to this collection'/>
<field
var='pubsub#children_max'
type='text-single'
label='The maximum number of children for this collection'/>
</form_type>
The XMPP Registrar includes "Collection" in its registry of SHIM headers (see <https://xmpp.org/registrar/shim.html>). The registry submission is as follows:
<header> <name>Collection</name> <desc>The node of subscription that sent a notification</desc> <doc>XEP-0248</doc> </header>
In its registry of Service Discovery identities (see <https://xmpp.org/registrar/disco-categories.html>) the XMPP Registrar includes a specific types within the "pubsub" category:
| collection | A pubsub node of the "collection" type as described in XEP-0248. |
The registry submission is as follows:
<category>
<name>pubsub</name>
<desc>Services and nodes that adhere to XEP-0060.</desc>
<type>
<name>collection</name>
<desc>A pubsub node of the "collection" type.</desc>
<doc>XEP-0248</doc>
</type>
</category>
REQUIRED.
Many thanks to Dave Cridland for his feedback and advice.
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The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) specifications contributed by the XMPP Standards Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.
The primary venue for discussion of XMPP Extension Protocols is the <standards@xmpp.org> discussion list.
Discussion on other xmpp.org discussion lists might also be appropriate; see <https://xmpp.org/community/> for a complete list.
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
1. XEP-0060: Publish-Subscribe <https://xmpp.org/extensions/xep-0060.html>.
2. XEP-0030: Service Discovery <https://xmpp.org/extensions/xep-0030.html>.
3. XEP-0055: Jabber Search <https://xmpp.org/extensions/xep-0055.html>.
4. XEP-0131: Stanza Headers and Internet Metadata <https://xmpp.org/extensions/xep-0131.html>.
5. XEP-0004: Data Forms <https://xmpp.org/extensions/xep-0004.html>.
6. The Internet Assigned Numbers Authority (IANA) is the central coordinator for the assignment of unique parameter values for Internet protocols, such as port numbers and URI schemes. For further information, see <http://www.iana.org/>.
Note: Older versions of this specification might be available at https://xmpp.org/extensions/attic/
Add section on retrieving default node configuration that's specific to a particular node type
Completely reworked from initial version. To wit:
Initial version, split from XEP-0060.
@report{saint-andre2008xep0248,
title = {PubSub Collection Nodes},
author = {Saint-Andre, Peter and Meijer, Ralph and Cully, Brian},
type = {XEP},
number = {0248},
version = {0.5.0},
institution = {XMPP Standards Foundation},
url = {https://xmpp.org/extensions/xep-0248.html},
date = {2008-08-11/2025-09-11},
}END