ResourceSync Framework Specification - Framework Notification

18 Jan 2017

This version:
http://www.openarchives.org/rs/notification/1.0/framework_notification
Latest version:
http://www.openarchives.org/rs/framework_notification
Previous version:
http://www.openarchives.org/rs/notification/0.9/notification
Editors:
Martin Klein, Herbert Van de Sompel - Los Alamos National Laboratory
Simeon Warner - Cornell University
Graham Klyne - University of Oxford
Bernhard Haslhofer - University of Vienna
Michael Nelson - Old Dominion University
Carl Lagoze - University of Michigan
Robert Sanderson - The Getty

Abstract

The ResourceSync core specification introduces a pull-based synchronization framework for the web that consists of various capabilities that a Source can implement to allow Destinations to remain synchronized with its evolving resources. This ResourceSync Framework Notification specification describes an additional, push-based, capability that a Source can support. It is aimed at informing Destinations about changes to a Source's ResourceSync implementation; it entails the Source sending notifications to subscribing Destinations.

Status of this Document

This specification is one of several documents comprising the ResourceSync Framework Specifications. Feedback is most welcome on the ResourceSync Google Group.

Table of Contents

1. Introduction
    1.1 Motivating Example
    1.2 Notational Conventions
2. Framework Notification Channels
3. Framework Notification
4. Transport Protocol: PubSubHubbub
    4.1 Source Submits Notifications to Hub
    4.2 Destination Subscribes to Hub to Receive Notifications
    4.3 Dub Delivers Notifications to Destination
    4.4 Destination Unsubscribes from Hub
5. Advertising Framework Notification Channels
6. References

Appendices

A. Acknowledgements
B. Change Log

1. Introduction

This specification describes a Framework Notification capability defined for the ResourceSync framework. The push-based notification capability consists of a Source sending out notifications about changes to its implementation of the ResourceSync framework, for example the publication of a new Resource List or the updating of a Change List. Another specification describes a Change Notification capability that consists of a Source sending notifications about changes to its resources in order to decrease the synchronization latency between a Source and a Destination that is inherent in the pull-based capabilities defined in the ResourceSync core specification.

1.1. Motivating Example

The pull-based capabilities specified in the ResourceSync core specification allow Destinations to remain informed about the evolving state of a Source's resources. However, they do leave the question open as to when a Destination should check whether, for example, a Source has published a new Resource List or has updated a Change List. A pragmatic solution is for Destinations to recurrently poll a Source at a frequency that is based on experience with the pace of prior updates. The Framework Notification capability is about informing Destinations about changes to a Source's ResourceSync environment, thereby providing an explicit trigger to poll a Source, and in doing so removing uncertainty and optimizing the synchronization process. The efficiency gain of this approach is particularly significant in the case of a Source with infrequent changes where Destinations nonetheless require low latency updates.

1.2. Definitions and Namespace Prefix Bindings

This specification uses the terms "resource", "representation", "request", "response", "content negotiation", "client", and "server" as described in Architecture of the World Wide Web.

Throughout this document, the following namespace prefix bindings are used:

PrefixNamespace URIDescription
nonehttp://www.sitemaps.org/schemas/sitemap/0.9 Sitemap XML elements defined in the Sitemap protocol
rshttp://www.openarchives.org/rs/terms/ Namespace for elements and attributes introduced in this specification

Table 1: Namespace prefix bindings used in this document

2. Framework Notification Channels

Framework Notifications are sent to inform Destinations about changes to capabilities of a Source's implementation of the ResourceSync framework, for example, if a Source's Change List or Capability List was created, updated, or deleted. The payload for these notifications is described in Section 3. Notifications are sent from Source to Destination on one or more channels provided by a push technology discussed in Section 4.

Figure 1 displays the structure of the ResourceSync framework for a Source that has a single set of resources, showing the Source Description and the Capability List at the top. The Capability List advertises four distinct capabilities: a Resource List, a Change List, a Resource Dump, and a Change Dump. The figure also shows a Framework Notification channel (red hexagon) and indicates it is used to send information about changes in the various capability documents (e.g. Resource List, Change List, etc.), as well as in the Capability List and the Source Description. Changes to all these documents are communicated as framework notifications via the Framework Notification channel.

Figure 1 also shows that framework notifications are not sent for changes at the level of an index document. For example, if new Resource Lists are created that reside under a Resource List Index then the framework notification is only sent about the creation of one of the Resource Lists and not about the creation or update of the encompassing Resource List Index. It is the Source's responsibility to ensure that the Resource List Index points to all new component Resource Lists at the time of the notification. Also, a framework notification sent about a change to a document that resides under an index must contain a link with the relation type index pointing at that index. This allows Destinations to navigate towards the index and detect further changes there. For example, the framework notification about the creation of a new Resource List must contain an index link pointing at the Resource List Index.

A Framework Notification channel

Figure 1: A Framework Notification channel in the ResourceSync framework structure

The ResourceSync framework allows a Source to offer multiple sets of resources in which case the Source Description points to multiple Capability Lists, one for each set of resources. A dedicated Framework Notification channel must be provided for each distinct set of resources for which Framework Notification is supported. This means that each set of resources has its own Framework Notification channel through which notifications about changes to capability documents and the Capability List associated with the set of resources are sent. However, notifications about changes to the Source Description (e.g., if a new Capability List was created) are sent via all Framework Notification channels. This way a Source can make sure that Destinations remain informed about changes to the overall organization of the Source's ResourceSync implementation regardless of the Framework Notification channel they subscribe to.

Figure 2 depicts a scenario where a Source offers multiple sets of resources and its Source Description therefore points to multiple Capability Lists, one for each set of resources, in this case Capability List 1 and Capability List 2. Figure 2 shows that each set of resources has a designated Framework Notification channel: Framework Notification Channel 2 is used to send notifications about changes to the capability documents advertised by Capability List 2 and about changes to Capability List 2 itself. Notifications about changes to the Source Description are sent via Framework Notification Channel 1 and Framework Notification Channel 2.

Framework Notification channels for multiple Capability Lists

Figure 2: Framework Notification channels for multiple sets of resources

2.1. Notification Change Types

The following table provides an overview of the possible change types that Framework Notifications inform about within the ResourceSync framework.

CapabilityChange Type
 CreateUpdateDelete
Framework Notification
    Resource ListXX
    Resource DumpX
    Change ListXX
    Change DumpXX
    Capability ListXXX
    Source DescriptionXXX

Table 2: Framework Notification Change Types

Note that the creation and deletion of Framework Notification channels is reflected in updated Capability Lists (see Section 5). This specification does not define a separate notification about notification channels.

3. Framework Notification

A framework notification is sent on the appropriate Framework Notification channel, as described in Section 2, if a Source wishes to notify a Destination about changes to Resource Lists, Change Lists, Resource Dumps, Change Dumps, Capability Lists, and Source Descriptions. By subscribing to a Framework Notification channel, Destinations can refrain from periodically pulling these documents to determine whether they changed.

The format of a framework notification is very similar to the Change List format introduced in Section 12 of the core specification. It is based on the <urlset> document format introduced by the Sitemap protocol. It has the <urlset> root element and the following structure:

Framework notifications do not use the <sitemapindex> document format introduced by the Sitemap protocol.

Example 1 shows the payload of a framework notification informing the Destination about the availability of a new Resource List.

<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
        xmlns:rs="http://www.openarchives.org/rs/terms/">
  <rs:ln rel="up"
         href="http://example.com/dataset1/capabilitylist.xml"/>
  <rs:md capability="framework-notification"/>
   <url>
      <loc>http://example.com/resourceset1/resourcelist.xml</loc>
      <rs:md change="created" datetime="2013-01-03T00:07:22Z"  
             capability="resourcelist"/>
   </url>
</urlset>

Example 1: The payload of a framework notification that informs about the availability of a new Resource List

As shown in Figure 1 and Figure 2, framework notifications are never sent at the index level. If the Source sends a framework notification about the change to a document (e.g., a Resource List) that resides under an index, it must provide a <rs:ln> child element to the <url> element in which that change is communicated. The relation type of that link must be index, and the target of the link must be the index (e.g., the Resource List Index) that the changed document resides under.

It is likely that framework notifications only contain information about a single change to the framework. However, multiple such changes can be aggregated into a single framework notification. Example 2 shows the payload of a framework notification informing the Destination about a new Resource List, a new Resource Dump, and about an updated Change List. The Resource List resides under an index and hence the corresponding <url> element has a <rs:ln> child element with the relation type index. Note that the framework notification only contains one entry for one new Resource List that resides under an index even though the index likely points to other new Resource Lists.

<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
        xmlns:rs="http://www.openarchives.org/rs/terms/">
  <rs:md capability="framework-notification"/>
   <url>
      <loc>http://example.com/dataset1/resourcelist.xml</loc>
      <rs:md change="created"
             capability="resourcelist"/>
      <rs:ln rel="index"
             href="http://example.com/dataset1/resourcelist-index.xml"/>
   </url>
   <url>
      <loc>http://example.com/dataset1/resourcedump.xml</loc>
      <rs:md change="created"
             capability="resourcedump"/>
   </url>
   <url>
      <loc>http://example.com/dataset1/changelist.xml</loc>
      <rs:md change="updated"
             capability="changelist"/>
   </url>
</urlset>

Example 2: A framework notification informing about multiple framework changes

4. Transport Protocol: PubSubHubbub

In order to bootstrap the notification capabilities of the ResourceSync framework, a single transport protocol is chosen: PubSubHubbub. PubSubHubbub is a simple, HTTP-based publish/subscribe protocol that is expected to perform well for use cases that do not require notifications to be sent at a very high frequency.

Table 3 maps terminology used in ResourceSync and PubSubHubbub. In order to implement the publish/subscribe paradigm, PubSubHubbub introduces a hub that acts as a conduit between Source and Destination. A hub can be operated by the Source itself or by a third party. It is uniquely identified by the hub URI. PubSubHubbub's topic corresponds with the notion of channel used in this specification. A topic is uniquely identified by its topic URI. Hence, per set of resources, the Source has a dedicated topic (and hence topic URI) for framework notifications.

ResourceSyncPubSubHubbub
Source Publisher
Destination Subscriber
Channel Topic
Notification Notification
Hub

Table 3: Mapping of terminologies between ResourceSync and PubSubHubbub

The remainder of this section describes the use of PubSubHubbub in ResourceSync. It only provides the information about the PubSubHubbub protocol that is essential to gain an adequate understanding of the overall mechanism. Details about the PubSubHubbub protocol are available in the PubSubHubbub specification. Figure 3 shows an overview of HTTP interactions between Source, Hub, and Destination. They will be detailed in the remainder of this section.

HTTP interactions between Source, Hub, and Destination

Figure 3: HTTP interactions between Source, Hub, and Destination

4.1. Source Submits Notifications to Hub

The PubSubHubbub protocol provides no specific guidelines regarding the way in which a Source should communicate notifications to a hub. The mechanism for ResourceSync framework notifications is as follows:

Example 3 shows the HTTP POST issued by the Source against its hub to submit the framework notification payload of Example 1. For brevity, the payload is not shown in its entirety. The third party hub URI is http://hub.example.org/pubsubhubbub/ and the Source's topic URI (channel) for framework notifications pertaining to dataset1 is http://example.com/dataset1/framework/.

POST /pubsubhubbub/ HTTP/1.1
Host: http://hub.example.org
Content-Type: application/xml
Link: <http://example.com/dataset1/framework/> ; rel="self",
 <http://hub.example.org/pubsubhubbub/> ; rel="hub", 
 <http://www.example.com/dataset1/capabilitylist.xml> ; rel="resourcesync"
Content-Length: 849

<?xml version="1.0" encoding="UTF-8"?>
<urlset ...

Example 3: The HTTP POST used by a Source to submit a framework notification payload to its hub

4.2. Destination Subscribes to Hub to Receive Notifications

A Destination subscribes to a Source's topic using the process described in the section "Subscribing and Unsubscribing" of PubSubHubbub. The process consists of mandatory subscription request and subscription verification phases:

Example 4 shows the HTTP POST issued by a Destination against the hub URI http://hub.example.org/pubsubhubbub/ requesting a subscription to the Source's topic URI (channel) http://example.com/dataset1/framework/ as a means to receive framework notifications pertaining to dataset1 at its callback URI http://destination.example.net/callback/.

POST /pubsubhubbub/ HTTP/1.1
Host: http://hub.example.org
Content-Type: application/x-www-form-urlencoded
Content-Length: 144

hub.mode=subscribe&hub.topic=http%3A%2F%2FAexample.com%2Fdataset1%2Fframework%2F
&hub.callback=http%3A%2F%2Fdestination.example.net%2Fcallback%2F&hub.lease_seconds=3600

Example 4: A Destination's request to a hub to subscribe to a Source's framework notification channel

Example 5 shows the HTTP GET issued by the hub against the Destination's callback URI to verify that it was the Destination's intent to subscribe.

GET /callback/?hub.mode=subscribe&hub.topic=http%3A%2F%2FAexample.com%2Fdataset1%2Fframework%2F
&hub.challenge=c0cc4630-5116-11e3-8f96-0800200c9a66&hub.lease_seconds=2400 HTTP/1.1
Host: http://destination.example.net
Connection: Close

Example 5: A hub's request to verify a Destination's intent to subscribe

Example 6 shows the response by a Destination to the hub's subscription verification request of Example 5. It indicates that the Destination wants the subscription.

HTTP/1.1 200 OK
Date: Tue, 19 Nov 2013 12:42::13 GMT
Content-Type: text/plain; charset=UTF-8
Content-Length: 36
Connection: Close

c0cc4630-5116-11e3-8f96-0800200c9a66

Example 6: A hub's request to verify a Destination's intent to subscribe

4.3. Hub Delivers Notifications to Destination

When the hub receives a framework notification from the Source, it passes it on to the subscribing Destination(s). The process, shown as "Hub notifies Destination" in Figure 3 , is as follows:

Example 7 shows the HTTP POST that the hub issues against the Destination's callback URI to relay the notification it received from the Source in Example 3. For brevity, the payload is not shown in its entirety.

POST /callback/ HTTP/1.1
Host: http://destination.example.net
Content-Type: application/xml
Link: <http://example.com/dataset1/framework/> ; rel="self",
 <http://hub.example.org/pubsubhubbub/> ; rel="hub", 
 <http://www.example.com/dataset1/capabilitylist.xml> ; rel="resourcesync"
Content-Length: 849

<?xml version="1.0" encoding="UTF-8"?>
<urlset ...

Example 7: The HTTP POST used by a hub to submit a Source's framework notification payload to a Destination

4.4. Destination Unsubscribes from Hub

The mechanism by which a Destination unsubscribes from a Source's topic URI is as described in Section 4.1 but uses unsubscribe as the value of hub.mode instead of subscribe.

5. Advertising Framework Notification Channels: PubSubHubbub

Framework Notification capabilities are advertised via Capability Lists, as is the case with the capabilities defined in the core ResourceSync specification. As each set of resources has its dedicated Framework Notification channel, that channel is advertised in the Capability List that corresponds with the respective set of resources.

Figure 4 displays a Framework Notification channel advertised in a Capability List. The figure shows a structure with only one Capability List that advertises its designated Framework Notification channel. Other Capability Lists, each of which pertain to a different set of resources, would advertise their respective notification channels. In addition to Framework Notifications, the Capability List can advertise other capabilities such as a Resource List and Change List as introduced in the core specification, and archive capabilities as introduced in the archiving specification.

Framework Notification channel discovery

Figure 4: Framework Notification channel discovery

Example 8 shows the Capability List from Example 13 of the core specification with discovery links for a Framework Notification channel added. The PubSubHubbub topic URI is provided in the <loc> element, whereas the hub URI is provided using a <rs:ln> child element of <loc>. That <rs:ln> must have hub as the value of the rel attribute and the hub URI as the value of the href attribute. Note the introduction of the framework-notification values for the capability attribute to indicate the Framework Notification capability.

<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
        xmlns:rs="http://www.openarchives.org/rs/terms/">
   <rs:ln rel="describedby"
          href="http://example.com/info_about_set1_of_resources.xml"/>
   <rs:ln rel="up"
          href="http://example.com/source_description.xml"/>
   <rs:md capability="capabilitylist"/>
   <url>
       <loc>http://example.com/dataset1/resourcelist.xml</loc>
       <rs:md capability="resourcelist"/>
   </url>
   <url>
       <loc>http://example.com/dataset1/resourcedump.xml</loc>
       <rs:md capability="resourcedump"/>
   </url>
   <url>
       <loc>http://example.com/dataset1/changelist.xml</loc>
       <rs:md capability="changelist"/>
   </url>
   <url>
       <loc>http://example.com/dataset1/changedump.xml</loc>
       <rs:md capability="changedump"/>
   </url>
   <url>
      <loc>http://example.com/dataset1/framework/</loc>
      <rs:ln rel="hub" href="http://hub.example.org/pubsubhubbub/"/>
      <rs:md capability="framework-notification"/>
   </url>
</urlset>

Example 8: A Capability List with an entry to discover a PubSubHubbub framework notification channel

6. References

[PubSubHubbub]
IETF PubSubHubbub Core, Fitzpatrick, B., Slatkin, B., Atkins, M., Genestoux, J., June 2013
[Sitemaps]
Sitemaps XML Format. sitemaps.org, last updated February 27, 2008. Available at: http://www.sitemaps.org/protocol.html
[Web Architecture]
Jacobs, Ian, and Norman Walsh, eds. Architecture of the World Wide Web, Volume One. W3C Recommendation. World Wide Web Consortium, December 15, 2004. Available at: http://www.w3.org/TR/webarch/

A. Acknowledgements

This specification is the collaborative work of NISO and the Open Archives Initiative. Initial funding for ResourceSync was provided by the Alfred P. Sloan Foundation. UK participation was supported by Jisc.

B. Change Log

Date Editor Description
2017-01-18 herbert, simeon link to Internet Archive copy of PubSubHubbub, no change to content
2016-08-10 herbert, martin, simeon version 1.0, created separate Framework Notification spec, made updates related to Core Framework changes
2014-03-24 graham, herbert version 0.9, removed ResourceSync-specific requirements from communication between Source and hub
2013-12-18 herbert, martin, rob, simeon version 0.8.1, using PubSubHubbub
2013-11-12 martin, herbert, rob, simeon version 0.8, using WebSockets

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