Network Working Group P. Saint-Andre
Internet-Draft Jabber Software Foundation
Expires: February 9, 2006 J. Hildebrand
Jabber, Inc.
B. Wyman
PubSub.com
August 8, 2005
Transporting Atom Notifications over the Extensible Messaging and
Presence Protocol (XMPP)
draft-saintandre-atompub-notify-03
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Copyright Notice
Copyright (C) The Internet Society (2005).
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-
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subscribe functionality.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Basic Process Flows . . . . . . . . . . . . . . . . . . . . . 4
2.1 Notification of Entry Creation . . . . . . . . . . . . . . 5
2.2 Notification of Entry Modification . . . . . . . . . . . . 7
2.3 Notification of Entry Deletion . . . . . . . . . . . . . . 9
3. Implementation Notes . . . . . . . . . . . . . . . . . . . . . 10
3.1 Generation of ItemIDs . . . . . . . . . . . . . . . . . . 10
3.2 Handling of Duplicate Entries . . . . . . . . . . . . . . 10
3.3 Notifications Matching Multiple Subscriptions . . . . . . 10
4. Security Considerations . . . . . . . . . . . . . . . . . . . 11
5. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.1 Normative References . . . . . . . . . . . . . . . . . . . 12
5.2 Informative References . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . 14
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1. Introduction
1.1 Overview
The Atom Publishing Format and Protocol Working Group has developed
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" software 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], and it is
not envisioned that this will change since [ATOM-PROTOCOL] 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] 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] 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].
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
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content syndication. This extension is specified in [XMPP-PUBSUB]
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], [ATOM-
PROTOCOL], [XMPP-CORE], and [XMPP-PUBSUB].
The capitalized key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
"SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
interpreted as described in RFC 2119 [TERMS].
2. Basic 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, a 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]; 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:
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+-----------+
| HTTP User |
+-----------+
|
| [Atom API]
|
+--------------+
| HTTP Service |
+--------------+
|
| [XMPP Pubsub]
|
+---------------------+
| XMPP Pubsub Service |
+---------------------+
|
| [XMPP Pubsub]
|
+-----------+
| XMPP User |
+-----------+
2.1 Notification of Entry Creation
An implementation MUST support notifications related to creation of
an entry.
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:
o Author publishes a new entry to the HTTP service via the Atom API.
o 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. (Note: If the entry may be copied from one feed to
another, e.g., in the generation of "synthetic" feeds, the entry
SHOULD contain an atom:source element to ensure consistent
metadata.)
o 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]. The XMPP protocols for the last two
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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 xmlns='http://www.w3.org/2005/Atom'>
<source>
<title>Example Feed</title>
<link href="http://example.org/"/>
<updated>2003-12-13T18:30:02Z</updated>
<author>
<name>John Doe</name>
</author>
</source>
<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:entry-32397</id>
<issued>2003-12-13T18:30:02Z</issued>
<modified>2003-12-13T18:30:02Z</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).
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<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 xmlns='http://www.w3.org/2005/Atom'>
<source>
<title>Example Feed</title>
<link href="http://example.org/"/>
<updated>2003-12-13T18:30:02Z</updated>
<author>
<name>John Doe</name>
</author>
</source>
<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:entry-32397</id>
<issued>2003-12-13T18:30:02Z</issued>
<modified>2003-12-13T18:30:02Z</modified>
</entry>
</item>
</items>
</event>
</message>
2.2 Notification of Entry Modification
An implementation SHOULD support notifications related to
modification of an entry.
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:
o Author updates an existing entry at the HTTP service via the Atom
API.
o 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.
(Note: If the entry may be copied from one feed to another, e.g.,
in the generation of "synthetic" feeds, the entry SHOULD contain
an atom:source element to ensure consistent metadata.)
o The XMPP pubsub service sends an XMPP message notification to each
XMPP entity that is subscribed to the pubsub node.
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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 xmlns='http://www.w3.org/2005/Atom'>
<source>
<title>Example Feed</title>
<link href="http://example.org/"/>
<updated>2003-12-13T18:30:02Z</updated>
<author>
<name>John Doe</name>
</author>
</source>
<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:entry-32397</id>
<issued>2003-12-13T18:30:02Z</issued>
<modified>2003-12-13T18:31:03Z</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.
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<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 xmlns='http://www.w3.org/2005/Atom'>
<source>
<title>Example Feed</title>
<link href="http://example.org/"/>
<updated>2003-12-13T18:30:02Z</updated>
<author>
<name>John Doe</name>
</author>
</source>
<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:entry-32397</a>
<issued>2003-12-13T18:30:02Z</issued>
<modified>2003-12-13T18:31:03Z</modified>
</entry>
</item>
</items>
</event>
</message>
2.3 Notification of Entry Deletion
An implementation MAY support notifications related to deletion of an
entry.
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:
o Author deletes an existing entry at the HTTP service via the Atom
API.
o 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.
o 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:
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<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
3.1 Generation of ItemIDs
The pubsub ItemIDs MUST conform to the rules defined in [XMPP-
PUBSUB]. 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] algorithm.
3.2 Handling of Duplicate Entries
It is the responsibility of the receiving application to remove or
ignore duplicate entries that might be received from multiple feeds.
3.3 Notifications Matching Multiple Subscriptions
An XMPP entity may subscribe to a publish-subscribe node multiple
times (e.g., once for each of several keywords), in which case a
single notification may match one or more subscriptions. In order to
specify which of one or more subscriptions are matched, the
notification message SHOULD specify the subscription IDs using the
header syntax defined in [XMPP-SHIM] and the "pubsub#subid" header
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defined in [XMPP-PUBSUB], as shown at the end of the following
example.
<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 xmlns='http://www.w3.org/2005/Atom'>
<source>
<title>Example Feed</title>
<link href="http://example.org/"/>
<updated>2003-12-13T18:30:02Z</updated>
<author>
<name>John Doe</name>
</author>
</source>
<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:entry-32397</id>
<issued>2003-12-13T18:30:02Z</issued>
<modified>2003-12-13T18:30:02Z</modified>
</entry>
</item>
</items>
</event>
<headers xmlns='http://jabber.org/protocol/shim'>
<header name="pubsub#subid">123-abc</header>
<header name="pubsub#subid">004-yyy</header>
</headers>
</message>
4. Security Considerations
Detailed security considerations for the relevant protocols profiled
in this memo are given in [ATOM-FORMAT], [ATOM-PROTOCOL], [XMPP-
CORE], and [XMPP-PUBSUB]; this memo introduces no new security
concerns above and beyond those described in the foregoing
specifications.
5. References
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5.1 Normative References
[ATOM-FORMAT]
Sayre, R. and M. Nottingham, "The Atom Syndication
Format", draft-ietf-atompub-format-10 (work in progress),
July 2005.
[ATOM-PROTOCOL]
Sayre, R. and J. Gregorio, "The Atom Publishing Protocol",
draft-ietf-atompub-protocol-04 (work in progress),
May 2005.
[TERMS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[XMPP-CORE]
Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 3920, October 2004.
[XMPP-PUBSUB]
Millard, P., Saint-Andre, P., and R. Meijer, "Publish-
Subscribe", JSF JEP 0060, March 2005.
5.2 Informative References
[HTTP] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[SHA1] Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1
(SHA1)", RFC 3174, September 2001.
[SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
[XMPP-SHIM]
Saint-Andre, P. and J. Hildebrand, "Stanza Headers and
Internet Metadata (SHIM)", JSF JEP 0131, October 2004.
Authors' Addresses
Peter Saint-Andre
Jabber Software Foundation
Email: stpeter@jabber.org
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Joe Hildebrand
Jabber, Inc.
Email: jhildebrand@jabber.com
Bob Wyman
PubSub.com
Email: bob@wyman.us
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