<title>Best Practices for Persistent Storage of Public Data via Publish-Subscribe</title>
<abstract>This specification defines best practices for using the XMPP publish-subscribe extension to persistently store semi-public data objects such as public keys and personal profiles.</abstract>
<p>&xep0163; introduced the idea of a virtual &xep0060; service associated with an IM user's bare JID &LOCALBARE;. However, the default node configuration options associated with PEP nodes are not optimized for the persistent storage of semi-public data objects such as public keys or user profiles. Therefore this document defines a set of best practices that enable IM users to persistently store semi-public data objects at their virtual pubsub service; in effect, we "sub-class" PEP by showing how a particular pubsub node can be configured for persisting objects.</p>
<p>Imagine that you are a Shakespearean character named Juliet and that you want to persistently store information such as your public keys (see &xep0189;) and user profile (see &xep0154;).</p>
<p>We assume that you have three contacts with the following relationship to you:</p>
<olstart='1'>
<li>benvolio@montague.lit, who has no subscription to your presence</li>
<li>nurse@capulet.lit, who has a bidirectional subscription to your presence and who is in your "Servants" roster group</li>
<li>romeo@montague.lit, who has a bidirectional subscription to your presence and who is in your "Friends" roster group</li>
</ol>
<p>We also assume that your server (capulet.lit) supports PEP along with the "publish-options" feature, and that your client discovered that support when you logged in.</p>
<p>Because you want to keep your communications with Romeo confidential, you decide to start encrypting your messages. Therefore you reconfigure your client, which generates an RSA key that it publishes to the virtual pubsub service hosted at your bare JID <juliet@capulet.lit>.</p>
<p>(In this case, access is limited to people in your Friends roster group.)</p>
<p>If all goes well (see <linkurl='#publish'>Publishing an Item</link>), your key will be pushed out to all appropriate individuals (in this case only Romeo). In particular, Romeo receives your key because he has auto-subscribed to the virtual pubsub service at your bare JID via a presence subscription and because his &xep0115; data indicated that he is interested in the "urn:xmpp:tmp:pubkey" payload type.</p>
<p>Because PEP services must send notifications to the account owner, you too receive the notification at each of your resources (here "balcony" and "chamber").</p>
<p>The best practices described herein re-use the concepts already defined in <cite>XEP-0060</cite> and <cite>XEP-0163</cite>. In order to optimize for object persistence instead of transient event notifications, a node MUST be configured as follows:</p>
<p>If the node does not already exist, the virtual pubsub service MUST create the node. As described in <cite>XEP-0163</cite>, this "auto-create" feature (defined in <cite>XEP-0060</cite>) MUST be supported by a PEP service. (Naturally, the account owner's client MAY follow the node creation use case specified in <cite>XEP-0060</cite> before attempting to publish an item.)</p>
<p>In order for the client to reliably persist objects, the virtual pubsub service must also support the "publish-options" feature defined in <cite>XEP-0060</cite>. Typically, a client will publish with options so that the object is properly persisted.</p>
<p>If the publication logic dictates that event notifications shall be sent, the account owner's server generates notifications and sends them to all appropriate entities as described in the Receiving Event Notifications section of <cite>XEP-0163</cite>.</p>
<p>Each item published to the node is a logically separate instance of the data to be stored. It is the responsibility of the publishing and receiving entities to construct a complete view of all such items, if desired. For example, each bookmark published to a private data node is a separate piece of data, whereas the history of all items published to the node provides a complete list of the user's bookmarks. This history may include items that are republished with an existing ItemID (thus overwriting the previous version of that item).</p>
<p>Before an account owner attempts to complete the use cases defined herein, its client SHOULD verify that the account owner's server supports both PEP and the "publish-options" feature; to do so, it MUST send a &xep0030; information request to the server (or cache <cite>Entity Capabilities</cite> information received from the server).</p>
<p>The server MUST return an identity of "pubsub/pep" and include the "publish-options" feauture in the list of the namespaces and other features it supports:</p>
