Publish-Subscribe (XEP-0060)  allows Jabber entities to subscribe to various kinds of information, but provides no way of remembering which nodes a user has subscribed to. Other protocols (e.g. User Geolocation (XEP-0080) , User Avatar (XEP-0084) ) allow information about a certain entity to be published to a Pubsub node. These protocols use Service Discovery (XEP-0030)  to allow other entities to find the pubsub node used by a certain entity, but provide no way of performing the opposite mapping, from pubsub node to information source. This document attempts to fill that void, using Private XML Storage (XEP-0049)  for storing information about subscriptions.
This protocol enables Jabber clients to do the following:
The <subscriptions/> element qualified by the 'storage:pubsubs' namespace is the root element used in the jabber:iq:private transactions. It has zero or more <subscription/> child elements, each of which MUST possess the following attributes:
Additionally, the <subscription/> element MAY possess these attributes:
In this example, the user already has a subscription to Juliet's geolocation, possibly established through another client.
Due to the nature of XEP-0049, incremental updates are not possible; a client MUST send the entire <subscriptions/> node for each update. Before performing the update, the client SHOULD retrieve the stored subscriptions, and incorporate any changes.
In this example, the user has just subscribed to Romeo's tune (see User Tune (XEP-0118) ). Assuming that retrieving happened as in the previous use case, updating the subscriptions proceeds as follows:
Having recorded the retrieved mappings, the client is now prepared to identify incoming pubsub events. Assume that the following event arrives:
The client now knows that this information comes from email@example.com.
Pubsub events offer an opportunity to spoof sender addresses e.g. through 'replyto' data (as specified by the Extended Stanza Addressing (XEP-0033)  protocol). This protocol attempts to close that hole. It does so by the following rules and assumptions:
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) .
No namespaces or parameters need to be registered with the XMPP Registrar  as a result of this document.
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Initial version; changed title to Pubsub Subscription Storage; changed namespace to storage:pubsubs for consistency with other storage XEPs.
Add resource attribute.
Add subscription attribute.
Fix typo and schema.