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Copyright Notice

Copyright (C) The Internet Society (2004). All Rights Reserved.

Abstract

This memo describes a method for notifying interested parties about changes in syndicated information encapsulated in the Atom feed format, where such notifications are delivered via an extension to the Extensible Messaging and Presence Protocol (XMPP) for publish-subscribe functionality.

Table of Contents

1.  Introduction
    1.1  Overview
    1.2  Terminology
    1.3  Discussion Venue
2.  Process Flows
    2.1  Notification of Entry Creation
    2.2  Notification of Entry Modification
    2.3  Notification of Entry Deletion
3.  Implementation Notes
4.  IANA Considerations
5.  Security Considerations
§.  Normative References
§.  Informative References
§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1. Introduction

1.1 Overview

The Atom Publishing Format and Protocol Working Group has been chartered to develop two technologies relevant to content syndication:

1. A data format for syndication of information about periodically-updated resources (such as weblog entries and news stories) available on the World Wide Web.
2. A protocol for publishing, editing, deleting, and otherwise managing such resources.

Content syndication follows a classic "observer" or "publish-subscribe" design pattern: a person or application publishes information to a "channel", and an event notification (or the data itself) is broadcasted to all those who are interested in knowing when information is published or modified for that channel. On the Internet today, publication of periodically-updated resources is handled by means of standard technologies such as [HTTP]Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P. and T. Berners-Lee, Hypertext Transfer Protocol -- HTTP/1.1, June 1999., and it is not envisioned that this will change since [ATOM-PROTOCOL]Gregorio, J. and R. Sayre, The Atom Publishing Protocol, July 2004. specifies the use of HTTP for publication. However, existing methods for learning that a resource has been updated are currently limited to "polling" for changes via HTTP, which is inherently inefficient. What is needed is a technology that can be relied on to "push" information only when a resource undergoes a state change, and only to those who are interested in learning about such state changes.

One possible technology for doing so is email, since [SMTP]Klensin, J., Simple Mail Transfer Protocol, April 2001. provides a way to initiate the sending of information from "publishers" to "subscribers" (think, for example, of email lists such as those used to announce newly-published RFCs). While email is one possible solution, it is not necessarily the best solution for Atom; in particular, [ATOM-FORMAT]Nottingham, M., The Atom Syndication Format, July 2004. defines an XML data format for content syndication, which implies that it might be beneficial to use a native XML delivery mechanism rather than to attach a special XML media type to email messages. Thankfully, a specialized XML delivery protocol has been developed through the IETF: the Extensible Messaging and Presence Protocol (XMPP) as specified in [XMPP-CORE]Saint-Andre, P., Extensible Messaging and Presence Protocol (XMPP): Core, May 2004.. XMPP has the added benefit of being optimized for near-real-time data delivery, which may be important in applications of Atom that require subscribers to be notified about syndicated content in a highly timely manner.

While the semantics of a normal XMPP <message/> element may be suitable for Atom content notifications, there also exists an XMPP extension that provides more structured communications in the context of information "channels" or "nodes" of the kind that are used in content syndication. This extension is specified in [XMPP-PUBSUB]Millard, P., Publish-Subscribe, July 2004. and may be especially useful for delivering notifications related to changes in Atom resources. Therefore, this memo describes a method for notifying interested parties about changes in syndicated information encapsulated in the Atom feed format, where such notifications are delivered via the XMPP publish-subscribe extension.

1.2 Terminology

This document inherits terminology from [ATOM-FORMAT]Nottingham, M., The Atom Syndication Format, July 2004., [ATOM-PROTOCOL]Gregorio, J. and R. Sayre, The Atom Publishing Protocol, July 2004., [XMPP-CORE]Saint-Andre, P., Extensible Messaging and Presence Protocol (XMPP): Core, May 2004., and [XMPP-PUBSUB]Millard, P., Publish-Subscribe, July 2004..

The capitalized key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119Bradner, S., Key words for use in RFCs to Indicate Requirement Levels, March 1997.[TERMS].

1.3 Discussion Venue

The authors welcomes discussion and comments related to the topics presented in this document. The preferred forum is the <atom-syntax@imc.org> mailing list, for which subscription information and archive links are available at <http://www.ietf.org/html.charters/atompub-charter.html>.

2. Process Flows

The following process flows demonstrate how Atom-formatted data (specifically, feed entries) can be delivered using the XMPP pubsub extension. The actors in these process flows are an application and one or more XMPP users. The application acts as a translator between HTTP and XMPP, since it generates XMPP pubsub requests when certain events occur at an Atom-aware HTTP service (e.g., an HTTP POST to create a new dynamic resource). The XMPP pubsub service then translates those pubsub requests into notifications that are sent to a potentially large number of XMPP users who have subscribed to such events (e.g., who have asked to receive an XMPP notification whenever a new dynamic resource is created for a certain Atom "channel"). Of course, an XMPP user is not necessarily a human, and could represent another application on the XMPP network (e.g., a chatroom bot or a content management system).

Note well that an HTTP user (e.g., a weblog author) would still publish information using the methods defined in [ATOM-PROTOCOL]Gregorio, J. and R. Sayre, The Atom Publishing Protocol, July 2004.; the process flows described herein enable the HTTP service with which an HTTP user interacts to generate notifications that are delivered via an XMPP pubsub service to a potentially large number of XMPP users who want to receive such information.

We can visualize the architecture as follows:

       +-----------+
       | HTTP User |
       +-----------+
            |
            | [Atom API]
            |
      +--------------+
      | HTTP Service |
      +--------------+
            |
            | [XMPP Pubsub]
            |
   +---------------------+
   | XMPP Pubsub Service |
   +---------------------+
            |
            | [XMPP Pubsub]
            |
       +-----------+
       | XMPP User |
       +-----------+
        

2.1 Notification of Entry Creation

When a content author publishes a new dynamic resource, many entities may be interested in learning that the resource is now available. The process flow is as follows:

• Author publishes a new entry to the HTTP service via the Atom API.
• The HTTP service sends data for the new Atom entry in an XMPP pubsub "publish" request to a specific node at the XMPP pubsub service.
• The XMPP pubsub service sends an XMPP message notification to each XMPP entity that is subscribed to the pubsub node.

The result is that the XMPP subscribers will receive something close to real-time notification whenever a new feed entry has been published.

Obviously the first step is out of scope for this memo, since it is described in [ATOM-PROTOCOL]Gregorio, J. and R. Sayre, The Atom Publishing Protocol, July 2004.. The XMPP protocols for the last two steps are shown below.

First the HTTP service sends an XMPP pubsub "publish" request to the XMPP pubsub service:

<iq type='set'
    from='http-service.example.org'
    to='pubsub.example.org'
    id='publish1'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <publish node='http://example.org/feeds/atom-example.xml'>
      <item id='70b2a83be71dfca04df91133d953fb22b41b4267'>
        <entry version='draft-ietf-atompub-format-01: do not deploy'
        xmlns='http://purl.org/atom/ns#draft-ietf-atompub-format-01'>
          <title>Atom-Powered Robots Run Amuck</title>
          <link rel='alternate' 
                type='text/html'
                href='http://example.org/2003/12/13/atom03'/>
          <id>tag:example.org,2003-12-13:robots</id>
          <issued>2003-12-13T08:29:29-04:00</issued>
          <modified>2003-12-13T08:29:29-04:00</modified>
        </entry>
      </item>
    </publish>
  </pubsub>
</iq>
          

The XMPP pubsub service then sends a pubsub notification to each XMPP subscriber; depending on pubsub node configuration, the notification may or may not contain the Atom payload (we assume here that the payload will be delivered).

<message from='pubsub.example.org'
         to='alice@example.net'>
  <event xmlns='http://jabber.org/protocol/pubsub#event'>
    <items node='http://example.org/feeds/atom-example.xml'>
      <item id='70b2a83be71dfca04df91133d953fb22b41b4267'>
        <entry version='draft-ietf-atompub-format-01: do not deploy'
        xmlns='http://purl.org/atom/ns#draft-ietf-atompub-format-01'>
          <title>Atom-Powered Robots Run Amuck</title>
          <link rel='alternate' 
                type='text/html'
                href='http://example.org/2003/12/13/atom03'/>
          <id>tag:example.org,2003-12-13:robots</id>
          <issued>2003-12-13T08:29:29-04:00</issued>
          <modified>2003-12-13T08:29:29-04:00</modified>
        </entry>
      </item>
     </items>
  </event>
</message>
          

2.2 Notification of Entry Modification

When a content author updates an existing dynamic resource, many entities may be interested in learning that the resource has been modified. The process flow is as follows:

• Author updates an existing entry at the HTTP service via the Atom API.
• The HTTP service sends data for the updated Atom entry in an XMPP pubsub "publish" request to a specific node at the XMPP pubsub service, specifying the same Item ID as previously supplied.
• The XMPP pubsub service sends an XMPP message notification to each XMPP entity that is subscribed to the pubsub node.

First the HTTP service sends an XMPP pubsub "publish" request to the XMPP pubsub service (note the modified title and time):

<iq type='set'
    from='http-service.example.org'
    to='pubsub.example.org'
    id='publish2'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <publish node='http://example.org/feeds/atom-example.xml'>
      <item id='70b2a83be71dfca04df91133d953fb22b41b4267'>
        <entry version='draft-ietf-atompub-format-01: do not deploy'
        xmlns='http://purl.org/atom/ns#draft-ietf-atompub-format-01'>
          <title>Atom-Powered Robots Run Amok</title>
          <link rel='alternate' 
                type='text/html'
                href='http://example.org/2003/12/13/atom03'/>
          <id>tag:example.org,2003-12-13:robots</id>
          <issued>2003-12-13T08:29:29-04:00</issued>
          <modified>2003-12-13T08:41:41-04:00</modified>
        </entry>
      </item>
    </publish>
  </pubsub>
</iq>
          

Subject to node configuration and/or subscription options, each XMPP subscriber would then receive a pubsub notification, which may or may not contain the Atom payload.

<message from='pubsub.example.org'
         to='alice@example.net'>
  <event xmlns='http://jabber.org/protocol/pubsub#event'>
    <items node='http://example.org/feeds/atom-example.xml'>
      <item id='70b2a83be71dfca04df91133d953fb22b41b4267'>
        <entry version='draft-ietf-atompub-format-01: do not deploy'
        xmlns='http://purl.org/atom/ns#draft-ietf-atompub-format-01'>
          <title>Atom-Powered Robots Run Amok</title>
          <link rel='alternate' 
                type='text/html'
                href='http://example.org/2003/12/13/atom03'/>
          <id>tag:example.org,2003-12-13:robots</id>
          <issued>2003-12-13T08:29:29-04:00</issued>
          <modified>2003-12-13T08:41:41-04:00</modified>
        </entry>
      </item>
     </items>
  </event>
</message>
          

2.3 Notification of Entry Deletion

If a content author deletes an existing dynamic resource, many entities may be interested in learning that the resource is no longer available. The process flow is as follows:

• Author deletes an existing entry at the HTTP service via the Atom API.
• The HTTP service sends an XMPP pubsub "retract" request to a specific node at the XMPP pubsub service, specifying the same Item ID as previously supplied.
• The XMPP pubsub service sends an XMPP message notification to each XMPP entity that is subscribed to the pubsub node.

First the HTTP service sends an XMPP pubsub "retract" request to the XMPP pubsub service:

<iq type='set'
    from='http-service.example.org'
    to='pubsub.example.org'
    id='delete1'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <retract node='http://example.org/feeds/atom-example.xml'>
      <item id='70b2a83be71dfca04df91133d953fb22b41b4267'/>
    </retract>
  </pubsub>
</iq>
          

Subject to node configuration and/or subscription options, each XMPP subscriber would then receive a pubsub notification that the item was deleted.

<message from='pubsub.example.org'
         to='alice@example.net'>
  <event xmlns='http://jabber.org/protocol/pubsub#event'>
    <items node='http://example.org/feeds/atom-example.xml'>
      <retract id='70b2a83be71dfca04df91133d953fb22b41b4267'/>
     </items>
  </event>
</message>
          

3. Implementation Notes

The pubsub ItemIDs MUST conform to the rules defined in [XMPP-PUBSUB]Millard, P., Publish-Subscribe, July 2004.. One possible method for generating a unique ItemID is to concatenate the XMPP address of the pubsub service, the pubsub node to which the item is published, and the atom:id of the feed entry, then hash the resulting string using the [SHA1]Eastlake, D. and P. Jones, US Secure Hash Algorithm 1 (SHA1), September 2001. algorithm.

It is the responsibility of the receiving application to remove or ignore duplicate entries that might be received from multiple feeds.

4. IANA Considerations

This document requires no action on the part of the IANA.

5. Security Considerations

Detailed security considerations for the relevant protocols profiled in this memo are given in [ATOM-PROTOCOL]Gregorio, J. and R. Sayre, The Atom Publishing Protocol, July 2004., [ATOM-FORMAT]Nottingham, M., The Atom Syndication Format, July 2004., [XMPP-CORE]Saint-Andre, P., Extensible Messaging and Presence Protocol (XMPP): Core, May 2004., and [XMPP-PUBSUB]Millard, P., Publish-Subscribe, July 2004.; this memo introduces no new security concerns above and beyond those described in the foregoing specifications.

6. References

6.1 Normative References

6.2 Informative References

Authors' Addresses

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