XEP-0395: Atomically Compare-And-Publish PubSub Items

WARNING: This Standards-Track document is Experimental. Publication as an XMPP Extension Protocol does not imply approval of this proposal by the XMPP Standards Foundation. Implementation of the protocol described herein is encouraged in exploratory implementations, but production systems are advised to carefully consider whether it is appropriate to deploy implementations of this protocol before it advances to a status of Draft.

Table of Contents

1. Introduction

This specification provides a mechanism to atomically publish items to a PubSub node depending on the item ID of the node's latest item. This allows to prevent race conditions and avoids data loss in certain situations.

2. Discoverying Support

If an entity supports the Compared-And-Publish feature it MUST advertise the fact by returning a <feature/> with the 'var' attribute set to 'urn:xmpp:pubsub:cap:0' in response to a Service Discovery (XEP-0030) [1] query for information.

3. Compare-And-Publish PubSub Items

<iq type='result'
    from='pubsub.shakespeare.lit'
    to='francisco@denmark.lit/barracks'
    id='items1'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <items node='princely_musings'>
      <item id='368866411b877c30064a5f62b917cffe'>
        <entry xmlns='http://www.w3.org/2005/Atom'>
          <title>The Uses of This World</title>
          <summary>
O, that this too too solid flesh would melt
Thaw and resolve itself into a dew!
          </summary>
        </entry>
      </item>
      <item id='3300659945416e274474e469a1f0154c'>
        <entry xmlns='http://www.w3.org/2005/Atom'>
          <title>Ghostly Encounters</title>
          <summary>
O all you host of heaven! O earth! what else?
And shall I couple hell? O, fie! Hold, hold, my heart;
And you, my sinews, grow not instant old,
But bear me stiffly up. Remember thee!
          </summary>
        </entry>
      </item>
      <item id='4e30f35051b7b8b42abe083742187228'>
        <entry xmlns='http://www.w3.org/2005/Atom'>
          <title>Alone</title>
          <summary>
Now I am alone.
O, what a rogue and peasant slave am I!
          </summary>
        </entry>
      </item>
      <cap-v-map xmlns='urn:xmpp:pubsub:cap:0'>
        <cap-value-map-entry
          item-id='368866411b877c30064a5f62b917cffe'
          cap-value='35a204c2-5d6c-43a2-8e0d-a235a627b04a'/>
        <cap-value-map-entry
          item-id='3300659945416e274474e469a1f0154c'
          cap-value='166b7c04-ed4d-4872-aa56-a58268da84e2'/>
        <cap-value-map-entry
          item-id='4e30f35051b7b8b42abe083742187228'
          cap-value='67f7f792-f2ee-4918-8894-36a3c4a6dd5f'/>
      </cap-v-map>
    </items>
  </pubsub>
</iq>

<iq type='set'
    from='hamlet@denmark.lit/blogbot'
    to='pubsub.shakespeare.lit'
    id='pub1'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <publish node='princely_musings'>
      <item id='2'>
        <entry xmlns='https://example.org'>
          <title>Soliloquy</title>
          <summary>
To be, or not to be: that is the question:
Whether 'tis nobler in the mind to suffer
The slings and arrows of outrageous fortune,
Or to take arms against a sea of troubles,
And by opposing end them?
          </summary>
        </entry>
      </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#prev_item_cap_value'>
          <value>1</value>
        </field>
      </x>
    </publish-options>
  </pubsub>
</iq>

<iq type='error'
    from='pubsub.shakespeare.lit'
    to='hamlet@denmark.lit/elsinore'
    id='retract1'>
  <error type='modify'>
    <conflict xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
    <precondition-not-met xmlns='http://jabber.org/protocol/pubsub#errors'/>
    <compare-and-publish-failed xmlns='urn:xmpp:pubsub:cap:0' cap-id='2'/>
  </error>
</iq>

4. Rationale

Unfortunately it was not possible to re-use the PubSub item ID for the "Atomically Compare-And-Publish" purpose. This is mostly due XEP-0060 § 12.8 stating that:

"If a publisher publishes an item and the ItemID matches that of an existing item, the pubsub service MUST overwrite the existing item and generate a new event notification."

Which means that the content of an item could change without its ID, rendering the item ID unusable for CAP.

Injecting a "cap"-namespaced attribute carrying the item's CAP-V into PubSub's <item/> would be a very elegant approach to assign CAP-Vs to PubSub items (and the favored one of the XEP's author). But the usage of namespaces attributes within XMPP is controversial. Therefore this XEP resorts to using the <cap-v-map/> approach for now.

5. Security Considerations

This extension protocol does not add any further security considerations to the ones mentioned in XEP-0060 § 14..

6. IANA Considerations

This document requires no interaction with the Internet Assigned Numbers Authority (IANA).

7. XMPP Registrar Considerations

This specification defines the following XML namespaces:

• urn:xmpp:pubsub:cap:0

Registry Submission

<var>
    <name>urn:xmpp:pubsub:cap:0</name>
    <desc>Indicates support for Compare-And-Publish</desc>
    <doc>XEP-XXXX</doc>
</var>

This specification defines the following <publish-options/> fields:

• pubsub#prev_item_cap_value

Registry Submission

<field var='pubsub#prev_item_cap_value'
         type='text-single'
         label='Precondition: The assumed value of the latest item&apos; CAP-V of the node'/>

8. XML Schema

TODO: Add after the XEP leaves the 'experimental' state.

9. Acknowledgements

Thanks to Kim Alvefur and Dave Cridland for their feedback.

Appendices

Appendix A: Document Information

Series: XEP
Number: 0395
Publisher: XMPP Standards Foundation
Status: Experimental
Type: Standards Track
Version: 0.1
Last Updated: 2017-11-29
Approving Body: XMPP Council
Dependencies: XEP-0030, XEP-0060
Supersedes: None
Superseded By: None
Short Name: cap
Source Control: HTML
This document in other formats: XML  PDF

Appendix B: Author Information

Appendix C: Legal Notices

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.

Appendix E: Discussion Venue

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 <http://xmpp.org/about/discuss.shtml> for a complete list.

Errata can be sent to <editor@xmpp.org>.

Appendix F: Requirements Conformance

The following requirements keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".

Appendix G: Notes

Appendix H: Revision History

Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/

END