XEP-0222: Persistent Storage of Public Data via PubSub

1.2 How It Works¶

Imagine that you are a Shakespearean character named Juliet and that you want to persistently store information such as your public keys (see User Profile (XEP-0154) [4]).

We assume that you have three contacts with the following relationship to you:

[email protected], who has no subscription to your presence
[email protected], who has a bidirectional subscription to your presence and who is in your "Servants" roster group
[email protected], who has a bidirectional subscription to your presence and who is in your "Friends" roster group

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.

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 <[email protected]>.

Example 1. Publishing a key¶

<iq from='[email protected]/balcony' type='set' id='pub1'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <publish node='urn:xmpp:tmp:pubkey'>
      <item id='julietRSAkey1hash'>
        <key xmlns='urn:xmpp:tmp:pubkey'>
          <x509cert>
            der-encoded-cert
          </x509cert>
        </key>
      </item>
    </publish>
    <publish-options>
      <x xmlns='jabber:x:data' type='submit'>
        <field var='FORM_TYPE' type='hidden'>
          <value>http://jabber.org/protocol/pubsub#publish-options</value>
        </field>
        <field var='pubsub#persist_items'>
          <value>true</value>
        </field>
        <field var='pubsub#send_last_published_item'>
          <value>never</value>
        </field>
        <field var='pubsub#access_model'>
          <value>roster</value>
        </field>
        <field var='pubsub#roster_groups_allowed'>
          <value>Friends</value>
        </field>
      </x>
    </publish-options>
  </pubsub>
</iq>

Your publish request is a standard pubsub request except that:

The item is persisted (pubsub#persist_items is true).
The last published item is never sent (pubsub#send_last_published_item is set to "never" so that items are pushed out only when modified).

(In this case, access is limited to people in your Friends roster group.)

If all goes well (see Publishing an Item), 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 Entity Capabilities (XEP-0115) [5] data indicated that he is interested in the "urn:xmpp:tmp:pubkey" payload type.

Example 2. Appropriate entities receive event notifications¶

<message from='[email protected]'
         to='[email protected]/orchard'
         type='headline'
         id='keyfoo1'>
  <event xmlns='http://jabber.org/protocol/pubsub#event'>
    <items node='urn:xmpp:tmp:pubkey'>
      <item id='julietRSAkey1hash'>
        <key xmlns='urn:xmpp:tmp:pubkey'>
          <x509cert>
            der-encoded-cert
          </x509cert>
        </key>
      </item>
    </items>
  </event>
</message>

Because PEP services must send notifications to the account owner, you too receive the notification at each of your resources (here "balcony" and "chamber").

Example 3. Publisher receives event notification¶

<message from='[email protected]'
         to='[email protected]/balcony'
         type='headline'
         id='keyfoo2'>
  <event xmlns='http://jabber.org/protocol/pubsub#event'>
    <items node='urn:xmpp:tmp:pubkey'>
      <item id='julietRSAkey1hash'>
        <key xmlns='urn:xmpp:tmp:pubkey'>
          <x509cert>
            der-encoded-cert
          </x509cert>
        </key>
      </item>
    </items>
  </event>
</message>

<message from='[email protected]'
         to='[email protected]/chamber'
         type='headline'
         id='keyfoo3'>
  <event xmlns='http://jabber.org/protocol/pubsub#event'>
    <items node='urn:xmpp:tmp:pubkey'>
      <item id='julietRSAkey1hash'>
        <key xmlns='urn:xmpp:tmp:pubkey'>
          <x509cert>
            der-encoded-cert
          </x509cert>
        </key>
      </item>
    </items>
  </event>
</message>

So that's the general idea.

2. Concepts and Approach¶

The best practices described herein re-use the concepts already defined in Publish-Subscribe (XEP-0060) [2] and XEP-0163. In order to optimize for object persistence instead of transient event notifications, a node MUST be configured as follows:

3. Publishing an Item¶

An account owner publishes an item to a node by following the protocol specified in Publish-Subscribe (XEP-0060) [2]:

If the node does not already exist, the virtual pubsub service MUST create the node. As described in Personal Eventing Protocol (XEP-0163) [1], this "auto-create" feature (defined in XEP-0060) MUST be supported by a PEP service. (Naturally, the account owner's client MAY follow the node creation use case specified in XEP-0060 before attempting to publish an item.)

In order for the client to reliably persist objects, the virtual pubsub service must also support the "publish-options" feature defined in Publish-Subscribe (XEP-0060) [2]. Typically, a client will publish with options so that the object is properly persisted.

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 Personal Eventing Protocol (XEP-0163) [1].

4. Composition¶

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).

5. Determining Support¶

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 Service Discovery (XEP-0030) [6] information request to the server (or cache Entity Capabilities information received from the server).

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:

6. Security Considerations¶

This document introduces no security considerations above and beyond those specified in Publish-Subscribe (XEP-0060) [2] and XEP-0163.

7. IANA Considerations¶

8. XMPP Registrar Considerations¶

Appendices

Appendix A: Document Information¶

Appendix D: Relation to XMPP¶

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.