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ResourceSync Framework Specification (ANSI/NISO Z39.99-2014)

21 April 2014

This version:
http://www.openarchives.org/rs/1.0/resourcesync
Latest version:
http://www.openarchives.org/rs/resourcesync
Previous version:
http://www.openarchives.org/rs/0.9.1/resourcesync

Abstract

This ResourceSync specification describes a synchronization framework for the web consisting of various capabilities that allow third-party systems to remain synchronized with a server's evolving resources. The capabilities may be combined in a modular manner to meet local or community requirements. This specification also describes how a server should advertise the synchronization capabilities it supports and how third-party systems may discover this information. The specification repurposes the document formats defined by the Sitemap protocol and introduces extensions for them.

Status of this Document

This document is an HTML version of ANSI/NISO Z39.99-2014, an American National Standard developed by the National Information Standards Organization. Approved by the American National Standards Institute (ANSI) April 21, 2014. The front matter, authorship and acknowledgements of the PDF version are provided in Appendix C.

This specification is one of several documents comprising the ResourceSync Framework Specifications.

Table of Contents

1. Introduction
    1.1 Purpose and Scope
    1.2 Motivating Examples
    1.3 Walkthrough
2. Normative References
3. Definitions
4. Namespace Prefix Bindings
5. Synchronization Processes
    5.1 Source Perspective
    5.2 Destination Perspective
    5.3 Summary
6. Framework Organization
    6.1 Structure
    6.2 Navigation
    6.3 Discovery
        6.3.1 Overview
        6.3.2 ResourceSync Well-Known URI
        6.3.3 Links
        6.3.4 robots.txt
7. Sitemap Document Formats
8. Describing the Source
9. Advertising Capabilities
10. Describing Resources
    10.1 Resource List
    10.2 Resource List Index
11. Packaging Resources
    11.1 Resource Dump
        11.1.1 Resource Dump Manifest
12. Describing Changes
    12.1 Change List
    12.2 Change List Index
13. Packaging Changes
    13.1 Change Dump
        13.1.1 Change Dump Manifest
14. Linking to Related Resources
    14.1 Overview
    14.2 Mirrored Content
    14.3 Alternate Representations
    14.4 Patching Content
    14.5 Resources and Metadata about Resources
    14.6 Prior Versions of Resources
    14.7 Collection Membership
    14.8 Republishing Resources
Appendix A. (normative) Time Attribute Requirements
Appendix B. Bibliography
Appendix C. Front Matter, Authorship, Acknowledgements
Appendix D. Change Log

1. Introduction

1.1 Purpose and Scope

The web is highly dynamic, with resources continuously being created, updated, and deleted. As a result, using resources from a remote server involves the challenge of remaining in step with its changing content. In many cases, there is no need to reflect a server's evolving content perfectly and, therefore, well-established resource discovery techniques, such as crawling, suffice as an updating mechanism. However, there are significant use cases that require low latency and high accuracy in reflecting a remote server's changing content. These requirements have typically been addressed by ad-hoc technical approaches implemented within a small group of collaborating systems. There have been no widely adopted, web-based approaches.

This ResourceSync specification introduces a range of easy to implement capabilities that a server may support in order to enable remote systems to remain more tightly in step with its evolving resources. It also describes how a server should advertise the capabilities it supports. Remote systems may inspect this information to determine how best to remain aligned with the evolving data.

Each capability provides a different synchronization functionality, such as a list of the server's resources or its recently changed resources, including what the nature of the change was: create, update, or delete. All capabilities are implemented on the basis of the document formats introduced by the Sitemap protocol. Capabilities may be combined to achieve varying levels of functionality and hence meet different local or community requirements. This modularity provides flexibility and makes ResourceSync suitable for a broad range of use cases.

1.2 Motivating Examples

Many projects and services have synchronization needs and have implemented ad hoc solutions. ResourceSync provides a standard synchronization method that will reduce implementation effort and facilitate easier reuse of resources. This section describes motivating examples with differing needs and complexities.

Consider first the case of a website for a small museum collection. The website may contain just a few dozen static webpages. The maintainer may create a Resource List of these webpages and expose it to services that leverage ResourceSync.

When building services over Linked Data, it is often desirable to maintain a local copy of data for improved access and availability. Harvesting may be enabled by publishing a Resource List for the dataset. In many cases, resource representations exposed as Linked Data are small and so retrieving them via individual HTTP GET requests is slow because of the large number of round trips for a small amount of content. Publishing a Resource Dump that points to content packaged and described in ZIP files makes this more efficient for the client and less burdensome for the server. Continued synchronization is enabled by recurrently publishing an up-to-date Resource List or Resource Dump, or, more efficiently, by publishing a Change List that provides information about resource changes only.

The arXiv.org collection of scientific articles propagates resource changes to a set of mirror sites and interacting services on a daily basis. As of July 2013, the collection contains about 2.6 million resources and there are about 1,600 changes (creates, updates) per day. The mirroring system operated since 1994 uses HTTP with custom change descriptions and occasionally rsync to verify the copies and to cope with any errors in the incremental updates. The approach assumes a tight connection between arXiv.org and its mirrors. It would be desirable to have a solution that allows any third-party system to accurately synchronize with arXiv.org using commodity software. arXiv.org could publish both metadata records and full-text content as separate web resources with their own URI. Use of ResourceSync capabilities including Resource Lists, Resource Dumps, Change Lists, and Change Dumps, allows both mirrors and new parties to remain accurately in sync with the collection. This would extend the openly available metadata sharing capabilities provided by arXiv.org, currently implemented via OAI-PMH, to full-text sharing in a web-friendly fashion.

1.3 Walkthrough

Let's assume a Source, http://example.com/, that exposes changing content that others would like to remain synchronized with. A first step towards making this easy for Destinations is for the Source to publish a Resource List that conveys the URIs of resources available for synchronization. This Resource List is expressed as a Sitemap. As shown in Example 1, the Source conveys the URI of each resource as the value of the <loc> child element of a <url> element. Note the <rs:md> child element of the <urlset> root element, which expresses that the Sitemap implements ResourceSync's Resource List capability. It also conveys that the Resource List reflects the state of the Source's resources at the datetime provided in the at attribute. This datetime allows a Destination to quickly determine whether it has previously processed this specific 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:md capability="resourcelist"
         at="2013-01-03T09:00:00Z"/>
  <url>
      <loc>http://example.com/res1</loc>
  </url>
  <url>
      <loc>http://example.com/res2</loc>
  </url>
</urlset>

Example 1: A Resource List

The Source can provide additional information in the Resource List to help the Destination optimize the process of collecting content and verifying its accuracy. For example, when the Source expresses the datetime of the most recent modification for a resource, a Destination can determine whether or not it already holds the current version, minimizing the number of HTTP requests it needs to issue in order to remain up-to-date. Example 2 shows this information conveyed using Sitemap's <lastmod> element. When the Source also conveys a hash for a specific bitstream, a Destination can verify whether the process of obtaining it was successful. The example shows this information conveyed using the hash attribute on the <rs:md> element. In addition, the Source can provide links to related resources using the <rs:ln> element. The example shows a link to a mirror copy of the second listed resource, indicating that the Source would prefer a Destination to obtain the resource from it.

<?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="resourcelist"
         at="2013-01-03T09:00:00Z"/>
  <url>
      <loc>http://example.com/res1</loc>
      <lastmod>2013-01-02T13:00:00Z</lastmod>
      <rs:md hash="md5:1584abdf8ebdc9802ac0c6a7402c03b6"/>
  </url>
  <url>
      <loc>http://example.com/res2</loc>
      <lastmod>2013-01-02T14:00:00Z</lastmod>
      <rs:md hash="md5:1e0d5cb8ef6ba40c99b14c0237be735e"/>
      <rs:ln rel="duplicate"
             href="http://mirror.example.com/res2"/>
  </url>
</urlset>

Example 2: A Resource List with additional information

In order to describe its changing content in a more timely manner, the Source can increase the frequency at which it publishes an up-to-date Resource List. However, changes may be so frequent or the size of the content collection so vast that regularly updating a complete Resource List may be impractical. In such cases, the Source can implement an additional capability that communicates information about changes only. To this end, ResourceSync introduces Change Lists. A Change List enumerates resource changes, along with the nature of the change (create, update, or delete) and the time that the change occurred. A Destination can recurrently obtain a Change List from the Source, inspect the listed changes to discover those it has already acted upon, and process the remaining ones. Changes in a Change List are provided in forward chronological order, making it straightforward for a Destination to determine which changes it already processed. In addition, a Change List also contains datetimes that convey the start time and the end time of the temporal interval covered by the Change List. These times convey that all resource changes that occurred during the interval are described in the Change List. (ResourceSync does not specify for how long change lists must continue to be available once they have been produced. The longer that Change Lists are maintained by the Source, the better the odds are for a Destination to catch up on changes it missed because it was offline, for example.)

Example 3 shows a Change List. The value of the capability attribute of the <rs:md> child element of <urlset> makes it clear that, this time, the Sitemap is a Change List and not a Resource List. The from and until attributes inform about the temporal interval covered by the Change List. The Change List shown below conveys two resource changes, one an update and the other a deletion, as can be seen from the value of the change attribute of the <rs:md> element. The example also shows the use of the <lastmod> element to convey the time of the changes. Note that these times are used to order the Change List chronologically.

<?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="changelist"
         from="2013-01-02T00:00:00Z"
         until="2013-01-03T00:00:00Z"/>
  <url>
      <loc>http://example.com/res2.pdf</loc>
      <lastmod>2013-01-02T13:00:00Z</lastmod>
      <rs:md change="updated"/>
  </url>
  <url>
      <loc>http://example.com/res3.tiff</loc>
      <lastmod>2013-01-02T18:00:00Z</lastmod>
      <rs:md change="deleted"/>
  </url>
</urlset>

Example 3: A Change List

A Destination can issue HTTP GET requests against each resource URI listed in a Resource List. For large Resource Lists, issuing all of these requests may be cumbersome. Therefore, ResourceSync introduces a capability that a Source can use to make packaged content available. A Resource Dump, implemented as a Sitemap, contains pointers to packaged content. Each content package referenced in a Resource Dump is a ZIP file that contains the Source's bitstreams along with a Resource Dump Manifest that describes each. The Resource Dump Manifest itself is also implemented as a Sitemap. A Destination can retrieve a Resource Dump, obtain content packages by dereferencing the contained pointers, and unpack the retrieved packages. Since the Resource Dump Manifest also lists the URI the Source associates with each bitstream, a Destination is able to achieve the same result as obtaining the data by dereferencing the URIs listed in a Resource List. Example 4 shows a Resource Dump that points at a single content package. Dereferencing the URI of that package leads to a ZIP file that contains the Resource Dump Manifest shown in Example 5. It indicates that the Source's ZIP file contains two bitstreams. The path attribute of the <rs:md> element conveys the file path of the bitstream in the ZIP file (the relative file system path where the bitstream would reside if the ZIP were unpacked), whereas the <loc> element conveys the URI associated with the bitstream at the Source.

An additional capability, the Change Dump, provides a functionality similar to a Resource Dump but pertains to packaging bitstreams of resources that have changed during a temporal interval, instead of packaging a snapshot of resource bitstreams at a specific moment in time.

<?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="resourcedump"
         at="2013-01-03T09:00:00Z"/>
  <url>
      <loc>http://example.com/resourcedump.zip</loc>
      <lastmod>2013-01-03T09:00:00Z</lastmod>
  </url>
</urlset>

Example 4: A Resource Dump

<?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="resourcedump-manifest"
         at="2013-01-03T09:00:00Z"/>
  <url>
      <loc>http://example.com/res1</loc>
      <lastmod>2013-01-03T03:00:00Z</lastmod>
      <rs:md hash="md5:1584abdf8ebdc9802ac0c6a7402c03b6"
             path="/resources/res1"/>
  </url>
  <url>
      <loc>http://example.com/res2</loc>
      <lastmod>2013-01-03T04:00:00Z</lastmod>
      <rs:md hash="md5:1e0d5cb8ef6ba40c99b14c0237be735e"
             path="/resources/res2"/>
  </url>
</urlset>

Example 5: A Resource Dump Manifest detailing the content of a ZIP file

ResourceSync also introduces a Capability List, which is a way for the Source to describe the capabilities it supports for one set of resources. Example 6 shows an example of such a description. It indicates that the Source supports the Resource List, Resource Dump, and Change List capabilities and it lists their respective URIs. Note the inclusion of an <rs:ln> child element of <urlset> that links by means of a describedby relation to a description of the set of resources covered by the Capability List. Because these capabilities are conveyed in the same Capability List, they uniformly apply to this set of resources. For example, if a given resource appears in the Resource List then it must also appear in a Resource Dump and changes to the resource must be reported in the Change 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="describedby"
         href="http://example.com/info_about_set1_of_resources.xml"/>
  <rs:ln rel="up"
         href="http://example.com/resourcesync_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>
</urlset>

Example 6: A Capability List enumerating the ResourceSync capabilities a Source supports for a set of its resources

There are three ways by which a Destination can discover whether and how a Source supports ResourceSync: a Source-wide approach, a resource-specific approach, and an approach that leverages existing practice for discovering Sitemaps. The Source-wide approach leverages the well-known URI specification and consists of the Source making a Source Description, like the one shown in Example 7, available at /.well-known/resourcesync. The Source Description enumerates the Capability Lists a Source offers, one Capability List per set of resources. If a Source only has one set of resources and hence only one Capability List, the mandatory Source Description contains only one pointer. The resource-specific discovery approach consists of a Source providing a link in an HTML document or in an HTTP Link header that points at a Capability List that covers the resource that provides the link. Note in Example 6, the inclusion of an <rs:ln> child element of <urlset> that links by means of an up relation to the Source Description, allowing for navigation from a Capability List to a Source Description. Yet another approach follows the established practice for discovering Sitemaps via a Source's robots.txt file. Since a Resource List is a Sitemap it can be made discoverable by including its URI in the robots.txt file as the value of the Sitemap directive. A navigational up link included in the Resource List allows discovery of a Capability List pertaining to the set of resources covered by that Resource List, and a further up link in the Capability List leads to the Source Description.

<?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="describedby"
         href="http://example.com/info-about-source.xml"/>
  <rs:md capability="description"/>
  <url>
      <loc>http://example.com/dataset1/capabilitylist.xml</loc>
      <rs:md capability="capabilitylist"/>
      <rs:ln rel="describedby"
             href="http://example.com/info_about_set1_of_resources.xml"/>
  </url>
</urlset>

Example 7: A Source Description with a pointer to the Capability List for the single set of resources offered by a Source

In some cases, there is a need to split the documents described so far into parts. For example, the Sitemap protocol currently prescribes a maximum of 50,000 resources per Sitemap and a Source may have more resources that are subject to synchronization. The ResourceSync framework follows these community defined limits and hence, in such cases, publishes multiple Resource Lists as well as a Resource List Index that points to each of them. The Resource List Index is expressed using Sitemap's <sitemapindex> document format. Example 8 shows a Resource List Index that points at two Resource Lists.

<?xml version="1.0" encoding="UTF-8"?>
<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
              xmlns:rs="http://www.openarchives.org/rs/terms/">
  <rs:md capability="resourcelist"
         at="2013-01-03T09:00:00Z"/>
  <sitemap>
      <loc>http://example.com/resourcelist-part1.xml</loc>
  </sitemap>
  <sitemap>
      <loc>http://example.com/resourcelist-part2.xml</loc>
  </sitemap>
</sitemapindex>

Example 8: A Resource List Index expressed using the <sitemapindex> document format

2. Normative References

The following documents contain provisions that are required for implementing this standard. All standards are subject to revision; the most current version of these standards should be used.

[ALE]
Snell, J. Atom Link Extensions. Internet Draft. Internet Engineering Task Force (IETF), June 8, 2012. Available at: http://tools.ietf.org/html/draft-snell-atompub-link-extensions-09
[IANA MIME]
MIME Media Types [registry website]. Internet Assigned Numbers Authority (IANA). Available at: http://www.iana.org/assignments/media-types
[IANA Relation]
Link Relations [registry website]. Internet Assigned Numbers Authority (IANA). Available at: http://www.iana.org/assignments/link-relations/link-relations.xml
[Memento]
Van de Sompel, H., M. L. Nelson, and R. D. Sanderson. HTTP framework for time-based access to resource states -- Memento. Internet Draft. Internet Engineering Task Force (IETF), October 1, 2013. Available at: http://tools.ietf.org/html/draft-vandesompel-memento-10
[RFC 2616]
Fielding, R. et al. Hypertext Transfer Protocol -- HTTP/1.1. RFC 2616. Internet Engineering Task Force (IETF), June 1999. Available at: http://www.ietf.org/rfc/rfc2616.txt
[RFC 4287]
Nottingham, M., and R. Sayre, eds. The Atom Syndication Format. RFC 4287. Internet Engineering Task Force (IETF), December 2005. Available at: http://www.ietf.org/rfc/rfc4287.txt
[RFC 5988]
Nottingham, M. Web Linking. RFC 5988. Internet Engineering Task Force (IETF), October 2010. Available at: http://www.ietf.org/rfc/rfc5988.txt
[RFC 6249]
Bryan, A. et al. Metalink/HTTP: Mirrors and Hashes. RFC 6249. Internet Engineering Task Force (IETF), June 2011. Available at: http://www.ietf.org/rfc/rfc6249.txt
[RFC 6906]
Wilde, E. The 'profile' Link Relation Type. RFC 6906. Internet Engineering Task Force (IETF), March 2013. Available at: http://www.ietf.org/rfc/rfc6906.txt
[Sitemaps]
Sitemaps XML Format. sitemaps.org, last updated February 27, 2008. Available at: http://www.sitemaps.org/protocol.html
[W3C Datetime]
Wolf, Misha, and Charles Wicksteed. Date and Time Formats. W3C Note. World Wide Web Consortium, August 27, 1998. Available at: http://www.w3.org/TR/1998/NOTE-datetime-19980827
[ZIP]
.ZIP File Format Specification. Application Note. Version 6.3.3. PKWARE Inc., September 1, 2012. Available at: http://www.pkware.com/documents/casestudies/APPNOTE.TXT

3. Definitions

The following terms, as used in this standard, have the meanings indicated.

TermDefinition
Source A server that hosts resources subject to synchronization
Destination A system that synchronizes itself with the Source's resources
set of resources A collection of resources that is made available for synchronization by a Source. A Source may expose one or more such collections and support distinct ResourceSync capabilities for each. Individual resources may be included in more than one set of resources

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

4. Namespace Prefix Bindings

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

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

5. Synchronization Processes

Section 1.3 provides a concrete walkthrough of some capabilities that a Source may implement and describes how a Destination may use those capabilities to remain synchronized with the Source's changing data. This section provides a high-level overview of the various ResourceSync capabilities and shows how these fit into processes at a Destination designed to keep it in step with changes.

5.1 Source Perspective

From the perspective of a Source, the ResourceSync capabilities that may be supported to enable Destination processes to remain in sync with its changing data are summarized as follows:

ResourceSync Source Perspective

Figure 1: ResourceSync Source perspective of resource description

Figure 2: ResourceSync Source perspective of change description

5.2 Destination Perspective

From the perspective of a Destination, three key processes are enabled by the ResourceSync capabilities; Figure 3 provides an overview:

ResourceSync Destination Perspective

Figure 3: ResourceSync Destination perspective

5.3 Summary

Table 1 provides a summary of Section 5. The table lists Destination processes as columns and Source capabilities as rows, with cells indicating the applicability of a capability for a given process.

Source CapabilitiesDestination Processes
 Baseline SynchronizationIncremental SynchronizationAudit
Describing the SourceXXX
Advertising CapabilitiesXXX
Describing Resources 
     Resource ListX X
Packaging Resources 
     Resource DumpX 
Describing Changes 
     Change List XX
Packaging Changes 
     Change Dump X 
Linking to Related Resources 
     Mirrored ContentXXX
     Alternate RepresentationsXXX
     Patching Content XX
     Resources and Metadata about ResourcesXXX
     Prior Versions of ResourcesXX
     Collection MembershipXXX
     Republishing ResourcesXXX

Table 1: Source capabilities versus Destination processes

6. Framework Organization

6.1 Structure

All capabilities in the ResourceSync framework are implemented on the basis of the <urlset> and <sitemapindex> Sitemap document formats. Figure 4: ResourceSync framework structure, depicts the overall structure of the set of documents that is used:

The Resource List branch of Figure 4 is fully compatible with the existing Sitemap specification, whereas the other branches are extensions introduced to support resource synchronization that leverage the Sitemap document formats.

ResourceSync Framework Structure

Figure 4: ResourceSync framework structure

6.2 Navigation

The following mechanisms are introduced to support navigating the document hierarchy described in Section 6.1; they are illustrated in Figure 5 and Figure 6:

ResourceSync Navigation Upwards

Figure 5: ResourceSync upwards navigation

ResourceSync Navigation Downwards

Figure 6: ResourceSync downwards navigation

6.3 Discovery

6.3.1 Overview

ResourceSync provides three ways for a Destination to discover whether and how a Source supports ResourceSync: a Source-wide approach detailed in Section 6.3.2, a resource-specific approach detailed in Section 6.3.3, and an approach that leverages the existing practice of Sitemap discovery via the robots.txt file described in Section 6.3.4. All approaches are summarized in Figure 7.

ResourceSync Discovery

Figure 7: Discovery of Source Description and Capability List

6.3.2 ResourceSync Well-Known URI

A Source must publish a Source Description, such as the one shown in Example 7, and it should be published at the well-known URI [RFC 5785] /.well-known/resourcesync as defined here. The Source Description document enumerates a Source's Capability Lists and as such is an appropriate entry point for Destinations interested in understanding a Source's capabilities.

6.3.3 Links

A Capability List may be made discoverable by means of links provided either in an HTML document [HTML Links, XHTML Links] or in an HTTP Link header [RFC 5988].

In order to include a discovery link in an HTML document, a <link> element is introduced in the <head> of the document that points to a Capability List. This <link> must have a rel attribute with a value of resourcesync. The Capability List that is made discoverable in this way must pertain to the resource that provides the link. This means that the resource must be covered by the capabilities listed in the linked Capability List. Example 9 shows the structure of a webpage that contains a link to a Capability List.

As shown in Example 6 the Source Description can be discovered from the Capability List by following the link provided in the <rs:ln> element with the relation type up.

<html>
  <head>
    <link rel="resourcesync"
          href="http://www.example.com/dataset1/capabilitylist.xml"/>
    ...
  </head>
  <body>...</body>
</html>

Example 9: Discovery by means of an HTML link

A Capability List may also be made discoverable by means of an HTTP Link header that may be included with a representation of a resource of any content-type. In order to do so, a link is introduced in the HTTP Link header. The target of this link is the URI of a Capability List and the value of its rel attribute is resourcesync. The Capability List that is made discoverable in this way must pertain to the resource that provides the link. This means that the resource must be covered by the capabilities listed in the linked Capability List. Example 10 shows an excerpt of an HTTP response header that illustrates this approach.

As shown in Example 6 the Source Description can be discovered from the Capability List by following the link provided in the <rs:ln> element with the relation type up.

HTTP/1.1 200 OK
  Date: Thu, 21 Jan 2010 00:02:12 GMT
  Server: Apache
  Link: <http://www.example.com/dataset1/capabilitylist.xml>;
         rel="resourcesync"
  ...

Example 10: Discovery by means of an HTTP link

6.3.4 robots.txt

A Resource List is a Sitemap and hence may be made discoverable via the established approach of adding a Sitemap directive to a Source's robots.txt file that has the URI of the Resource List as its value. If a Source supports multiple sets of resources, multiple directives may be added, one for each Resource List associated with a specific set of resources. In case a Source supports both regular Sitemaps and ResourceSync Sitemaps (Resource Lists) they may be made discoverable, again, by including multiple Sitemap directives as shown in Example 11.

Once a Resource List for a set of resources has been discovered in this manner, the corresponding Capability List can be discovered by following a link with the up relation type provided in the Resource List. Next, the Source Description can be discovered by following yet another link with the up relation type provided in the Capability List.

User-agent: *
Disallow: /cgi-bin/
Disallow: /tmp/
Sitemap: http://example.com/dataset1/resourcelist.xml

Example 11: A robots.txt file that points at a Resource List

7. Sitemap Document Formats

In order to convey information pertaining to resources in the ResourceSync framework, the Sitemap (root element <urlset>) and Sitemap index (root element <sitemapindex>) document formats introduced by the Sitemap protocol are used for a variety of purposes. The <sitemapindex> document format is used when it is necessary to group multiple documents of the <urlset> format. The ResourceSync framework follows community-defined limits for when to publish multiple documents of the <urlset> format. At time of publication of this specification, the limit is 50,000 items per document and a document size of 50 MB.

The document formats, as well as their ResourceSync extension elements, are shown in Table 2. The <rs:md> and <rs:ln> elements are introduced to express metadata and links, respectively. Both are in the ResourceSync XML Namespace and may have attributes. The attributes of these elements defined by ResourceSync are listed in Table 3 and detailed below. As shown in the examples, these attributes must not have an XML Namespace prefix. The <rs:ln> element as well as several of the ResourceSync attributes are based upon other specifications and in those cases inherit the semantics defined there; the "Specification" column of Table 3 refers to those specifications. Communities may introduce additional attributes when needed but must use an XML Namespace other than that of ResourceSync and must appropriately use namespace prefixes for those attributes.

SitemapSitemap Index
<?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 />
  <rs:ln />
  <url>
      <loc />
      <lastmod />
      <rs:md />
      <rs:ln />
  </url>
  <url>
      ...
  </url>
</urlset>
  
<?xml version="1.0" encoding="UTF-8"?>
<sitemapindex xmlns="..."
              xmlns:rs="...">
  <rs:md />
  <rs:ln />
  <sitemap>
      <loc />
      <lastmod />
      <rs:md />
      <rs:ln />
  </sitemap>
  <sitemap>
      ...
  </sitemap>
</sitemapindex>
  

Table 2: The Sitemap document formats including the ResourceSync extensions

The overall structure of the ResourceSync documents is as follows:

<urlset> or <sitemapindex> -- These elements are the root elements of ResourceSync documents; this specification adds one mandatory and one optional child element to the child elements of the Sitemap document formats:

Table 3 lists the elements used in ResourceSync documents and for each shows the attributes defined by ResourceSync that may be used with them. The "Specification" column refers to the specification where elements or attributes were introduced that ResourceSync equivalents are based upon and inherit their semantics from. A mark in the "Representation" column for an attribute indicates that it should only be used when a specific representation of a resource is concerned, whereas a mark in the "Resource" column indicates it is usable for a resource in general. A W3C XML Schema (available at http://www.niso.org/schemas/resourcesync) is provided to validate the elements introduced by ResourceSync.

Relation types other than the ones listed above may be used in the ResourceSync Framework. Valid relation types must be registered in the IANA Link Relation Type Registry or expressed as URIs as specified in RFC 5988, Sec. 4.2. The document Relation Types Used in the ResourceSync Framework attempts to provide an up-to-date overview.

Element/AttributeSpecificationResourceRepresentation
<urlset> or <sitemapindex>Sitemap protocol
    <rs:md>This specification
        atThis specification
        capabilityThis specification
        completedThis specification
        fromThis specification
        untilThis specification
    <rs:ln>RFC4287
        hrefRFC4287
        relRFC4287
    <url> or <sitemap>Sitemap protocol
        <loc>Sitemap protocol
        <lastmod>Sitemap protocol
        <changefreq>Sitemap protocol
        <rs:md>This specification
            atThis specification
            capabilityThis specification
            changeThis specificationXX
            completedThis specification
            encodingRFC2616X
            fromThis specification
            hashAtom Link ExtensionsX
            lengthRFC4287X
            pathThis specificationX
            typeRFC4287X
            untilThis specification
        <rs:ln>This specification
            encodingRFC2616X
            hashAtom Link ExtensionsX
            hrefRFC4287XX
            lengthRFC4287X
            modifiedAtom Link ExtensionsXX
            pathThis specificationX
            priRFC6249XX
            relRFC4287XX
            typeRFC4287X

Table 3: Elements and associated attributes defined for the ResourceSync documents

8. Describing the Source

A Source Description is a mandatory document that enumerates the Capability Lists offered by a Source. Since a Source has one Capability List per set of resources that it distinguishes, the Source Description will enumerate as many Capability Lists as the Source has distinct sets of resources.

The Source Description is based on the <urlset> format. It has the <urlset> root element and the following structure:

The <lastmod> elements should be omitted from the Source Description unless the Source updates the Source Description every time it updates one of the Capability Lists.

Example 12 shows a Source Description where the Source offers three Capability 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:ln rel="describedby"
         href="http://example.com/info_about_source.xml"/>
  <rs:md capability="description"/>
  <url>
      <loc>http://example.com/capabilitylist1.xml</loc>
      <rs:md capability="capabilitylist"/>
      <rs:ln rel="describedby"
             href="http://example.com/info_about_set1_of_resources.xml"/>
  </url>
  <url>
      <loc>http://example.com/capabilitylist2.xml</loc>
      <rs:md capability="capabilitylist"/>
      <rs:ln rel="describedby"
             href="http://example.com/info_about_set2_of_resources.xml"/>
  </url>
  <url>
      <loc>http://example.com/capabilitylist3.xml</loc>
      <rs:md capability="capabilitylist"/>
      <rs:ln rel="describedby"
             href="http://example.com/info_about_set3_of_resources.xml"/>
  </url>
</urlset>

Example 12: A Source Description

If a Source needs to or chooses to publish multiple Source Descriptions, it must group them by means of a Source Description Index.

9. Advertising Capabilities

A Capability List is a document that enumerates all capabilities supported by a Source for a specific set of resources. The Source defines which resources are part of the set of resources described by the Capability List. If there is more than one such set, then the Source must distinguish them with different capability lists. The choice of which resources are part of which set may derive from a variety of criteria, including media type, collection membership, change frequency, subject of the resource and many others.

A Capability List points at the capability documents for its set of resources: Resource List (Section 10.1), Resource Dump (Section 11.1), Change List (Section 12.1), and Change Dump (Section 13.1). A Capability List must only contain one entry per capability.

Capabilities that are conveyed in the same Capability List uniformly apply to the set of resources covered by that Capability List. For example, if a Capability List enumerates a Resource List, a Resource Dump, and a Change List, then a given resource that appears in a Resource List must also appear in a Resource Dump, and changes to the resource must be conveyed in the Change List.

The Capability List is based on the <urlset> format. It has the <urlset> root element and the following structure:

The <lastmod> elements should be omitted from the Capability List unless the Source updates the Capability List every time it updates one of the capability documents.

Example 13 shows a Capability List where the Source offers four capabilities: a Resource List, a Resource Dump, a Change List, and a Change Dump. A Destination cannot determine from the Capability List whether a Source provides, for example, a Resource List Index or a single Resource List. The capability document must be downloaded to make this determination: a document with a <sitemapindex> root element is an index, a document with a <urlset> root element is not.

<?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="describedby"
         href="http://example.com/info_about_set1_of_resources.xml"/>
  <rs:ln rel="up"
         href="http://example.com/resourcesync_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>
</urlset>

Example 13: A Capability List

ResourceSync defines only a small number of capabilities, and enumerating those does not approach the limits of a single Capability List. Extensions or revisions of this specification may introduce the use of Capability List Indexes, but Sources should not generate such structures for the features introduced in this version of the ResourceSync specification.

10. Describing Resources

A Source may publish a description of the resources it makes available for synchronization. This information enables a Destination to make an initial copy of some or all of those resources, or to update a local copy to remain synchronized with changes.

10.1 Resource List

A Resource List is introduced to list and describe the resources that a Source makes available for synchronization. It presents a snapshot of a Source's resources at a particular point in time.

A Resource List is based on the <urlset> document format introduced by the Sitemap protocol. It has the <urlset> root element and the following structure:

Example 14 shows a Resource List with two resources. The at attribute allows a Destination to determine that neither of the listed resources have undergone a change between their respective last modification datetimes, 2013-01-02T13:00:00Z and 2013-01-02T14:00:00Z, and the datetime that is the value of the at attribute, 2013-01-03T09:00:00Z.

<?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="resourcelist"
         at="2013-01-03T09:00:00Z"
         completed="2013-01-03T09:01:00Z"/>
  <url>
      <loc>http://example.com/res1</loc>
      <lastmod>2013-01-02T13:00:00Z</lastmod>
      <rs:md hash="md5:1584abdf8ebdc9802ac0c6a7402c03b6"
             length="8876"
             type="text/html"/>
  </url>
  <url>
      <loc>http://example.com/res2</loc>
      <lastmod>2013-01-02T14:00:00Z</lastmod>
      <rs:md hash="md5:1e0d5cb8ef6ba40c99b14c0237be735e 
                   sha-256:854f61290e2e197a11bc91063afce22e43f8ccc655237050ace766adc68dc784"
             length="14599"
             type="application/pdf"/>
  </url>
</urlset>

Example 14: A Resource List

10.2 Resource List Index

The ResourceSync framework adopts the community-defined limits for publishing documents of the <urlset> format and introduces a Resource List Index for grouping multiple Resource Lists. The union of the Resource Lists referred to in the Resource List Index represents the entire set of resources that a Source makes available for synchronization. This set of resources, regardless of whether it is conveyed in a single Resource List or in multiple Resource Lists via a Resource List Index, represents the state of the Source's data at a point in time.

A Resource List Index is based on the <sitemapindex> document format introduced by the Sitemap protocol. It has the <sitemapindex> root element and the following structure:

The Destination can determine whether it has reached a Resource List or a Resource List Index based on whether the root element is <urlset> or <sitemapindex> respectively. A Resource List Index that points to three Resource Lists is shown in Example 15.

<?xml version="1.0" encoding="UTF-8"?>
<sitemapindex 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="resourcelist"
         at="2013-01-03T09:00:00Z"
         completed="2013-01-03T09:10:00Z"/>
  <sitemap>
      <loc>http://example.com/resourcelist1.xml</loc>
      <rs:md at="2013-01-03T09:00:00Z"/>
  </sitemap>
  <sitemap>
      <loc>http://example.com/resourcelist2.xml</loc>
      <rs:md at="2013-01-03T09:03:00Z"/>
  </sitemap>
  <sitemap>
      <loc>http://example.com/resourcelist3.xml</loc>
      <rs:md at="2013-01-03T09:07:00Z"/>
  </sitemap>
</sitemapindex>

Example 15: A Resource List Index

Example 16 shows the content of the Resource List identified by the URI http://example.com/resourcelist1.xml. Structurally, it is identical to the Resource List shown in Example 14 but it contains an additional <rs:ln> child element of <urlset> that provides a navigational link with the relation type index to the parent Resource List Index shown in Example 15. This link is meant to ease navigation for Destinations and their adoption is therefore recommended.

<?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:ln rel="index"
         href="http://example.com/dataset1/resourcelist-index.xml"/>
  <rs:md capability="resourcelist"
         at="2013-01-03T09:00:00Z"/>
  <url>
      <loc>http://example.com/res3</loc>
      <lastmod>2013-01-02T13:00:00Z</lastmod>
      <rs:md hash="md5:1584abdf8ebdc9802ac0c6a7402c8753"
             length="4385"
             type="application/pdf"/>
  </url>
  <url>
      <loc>http://example.com/res4</loc>
      <lastmod>2013-01-02T14:00:00Z</lastmod>
      <rs:md hash="md5:4556abdf8ebdc9802ac0c6a7402c9881"
             length="883"
             type="image/png"/>
  </url>
</urlset>

Example 16: A Resource List with a navigational link to its parent Resource List Index

11. Packaging Resources

In order to provide Destinations with an efficient way to copy a Source's data using a small number of HTTP requests, a Source may provide packaged bitstreams for its resources.

11.1 Resource Dump

A Source may publish a Resource Dump, which provides links to packages of the resources' bitstreams. The Resource Dump represents the Source's state at a point in time. It may be used to transfer resources from the Source in bulk, rather than the Destination having to make many separate requests.

The ResourceSync framework specifies the use of the ZIP file format as the packaging format. Communities may define their own packaging format. A Resource Dump should only point to packages of the same format.

A Resource Dump is based on the <urlset> document format introduced by the Sitemap protocol. It has the <urlset> root element and the following structure:

Example 17 shows a Resource Dump that points to three ZIP files. Included in each <url> element is a pointer to the Resource Dump Manifest associated with the package. While this pointer is optional and intended for the Destination's convenience, if provided, the Source needs to ensure that the referred Manifest corresponds with the Manifest included in the bitstream package.

<?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="resourcedump"
         at="2013-01-03T09:00:00Z"
         completed="2013-01-03T09:04:00Z"/>
  <url>
      <loc>http://example.com/resourcedump-part1.zip</loc>
      <rs:md type="application/zip"
             length="4765"
             at="2013-01-03T09:00:00Z"
             completed="2013-01-03T09:02:00Z"/>
      <rs:ln rel="contents"
             href="http://example.com/resourcedump_manifest-part1.xml"
             type="application/xml"/>
  </url>
  <url>
      <loc>http://example.com/resourcedump-part2.zip</loc>
      <rs:md type="application/zip"
             length="9875"
             at="2013-01-03T09:01:00Z"
             completed="2013-01-03T09:03:00Z"/>
      <rs:ln rel="contents"
             href="http://example.com/resourcedump_manifest-part2.xml"
             type="application/xml"/>
  </url>
  <url>
      <loc>http://example.com/resourcedump-part3.zip</loc>
      <rs:md type="application/zip"
             length="2298"
             at="2013-01-03T09:03:00Z"
             completed="2013-01-03T09:04:00Z"/>
      <rs:ln rel="contents"
             href="http://example.com/resourcedump_manifest-part3.xml"
             type="application/xml"/>
  </url>
</urlset>

Example 17: A Resource Dump

If a Source needs to or chooses to publish multiple Resource Dumps, it must group them using a Resource Dump Index, in a manner that is similar to what was described in Section 11.2.

11.2 Resource Dump Manifest

Each ZIP package referred to from a Resource Dump must contain a Resource Dump Manifest file that describes the package's constituent bitstreams. The file must be named manifest.xml and must be located at the top level of the ZIP package.

The Resource Dump Manifest is based on the <urlset> format. It has the <urlset> root element and the following structure:

Example 18 shows a Resource Dump Manifest for a ZIP file that contains two bitstreams.

<?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="resourcedump-manifest"
         at="2013-01-03T09:00:00Z"
         completed="2013-01-03T09:02:00Z"/>
  <url>
      <loc>http://example.com/res1</loc>
      <lastmod>2013-01-02T13:00:00Z</lastmod>
      <rs:md hash="md5:1584abdf8ebdc9802ac0c6a7402c03b6"
             length="8876"
             type="text/html"
             path="/resources/res1"/>
  </url>
  <url>
      <loc>http://example.com/res2</loc>
      <lastmod>2013-01-02T14:00:00Z</lastmod>
      <rs:md hash="md5:1e0d5cb8ef6ba40c99b14c0237be735e
                   sha-256:854f61290e2e197a11bc91063afce22e43f8ccc655237050ace766adc68dc784"
             length="14599"
             type="application/pdf"
             path="/resources/res2"/>
  </url>
</urlset>

Example 18: A Resource Dump Manifest

12. Describing Changes

A Source may publish a record of the changes to its resources. This enables Destinations to efficiently learn about those changes and hence to synchronize incrementally.

12.1 Change List

A Change List is a document that contains a description of changes to a Source's resources. It is up to the Source to determine the publication frequency of Change Lists, as well as the temporal interval they cover. For example, a Source may choose to publish a fixed number of changes per Change List, or all the changes in a period of fixed length, such as an hour, a day, or a week. All entries in a Change List must be provided in forward chronological order: the least recently changed resource must be listed at the beginning of the Change List, while the most recently changed resource must be listed at the end of the document. If a resource underwent multiple changes in the period covered by a Change List, then it will be listed multiple times, once per change.

A Change List is based on the <urlset> document format introduced by the Sitemap protocol. It has the <urlset> root element and the following structure:

The temporal interval covered by a Change List is conveyed by means of the from and until attributes of the <rs:md> child element of the <urlset> root element. The from attribute indicates that the Change List includes all changes that occurred to the set of resources at the Source since the datetime expressed as the value of the attribute. If it exists, the until attribute indicates that the Change List includes all changes that occurred to the set of resources at the Source up until the datetime expressed as the value of the attribute. Its use is optional for Change Lists:

The from and until attributes help a Destination to determine whether it has or has not fully processed a Change List. The forward chronological order of changes in a Change List, the datetime of a resource change, and the URI of a changed resource help the Destination to determine the first unprocessed change in a not fully processed Change List. The Destination should start processing there; it can retrieve a representation of a changed resource by dereferencing its URI provided in the <loc> child element of the <url> element that conveys the change. In order for the determination of the first unprocessed change to be accurate, the combination of the URI of a changed resource and the datetime of its change should be unique. Hence, a Source should provide change datetime values at a sufficiently fine granularity.

Example 19 shows a Change List that indicates that four resource changes occurred since 2013-01-03T00:00:00Z: one creation, two updates, and one deletion. One resource underwent two of these changes and hence is listed twice. The Change List has no until attribute, which indicates that it will report further changes; a Destination should keep polling this Change 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="changelist"
         from="2013-01-03T00:00:00Z"/>
  <url>
      <loc>http://example.com/res1.html</loc>
      <lastmod>2013-01-03T11:00:00Z</lastmod>
      <rs:md change="created"/>
  </url>
  <url>
      <loc>http://example.com/res2.pdf</loc>
      <lastmod>2013-01-03T13:00:00Z</lastmod>
      <rs:md change="updated"/>
  </url>
  <url>
      <loc>http://example.com/res3.tiff</loc>
      <lastmod>2013-01-03T18:00:00Z</lastmod>
      <rs:md change="deleted"/>
  </url>
  <url>
      <loc>http://example.com/res2.pdf</loc>
      <lastmod>2013-01-03T21:00:00Z</lastmod>
      <rs:md change="updated"/>
  </url>
</urlset>

Example 19: An open Change List describing four resource changes

12.2 Change List Index

If a Source needs to publish multiple Change Lists, it must group them in a Change List Index. A Change List Index must enumerate Change Lists in forward chronological order.

A Change List Index is based on the <sitemapindex> document format introduced by the Sitemap protocol. It has the <sitemapindex> root element and the following structure:

The Destination should determine whether it has reached a Change List or a Change List Index based on whether the root element is <urlset> or <sitemapindex> respectively.

A Change List Index that points to three Change Lists is shown in Example 20. Two of those Change Lists are closed, as indicated by the provision of <lastmod>, and one is open, as indicated by its absence. The closed Change List http://example.com/20130102-changelist.xml is shown in Example 21. Note that the value for <lastmod> for this Change List in the Change List Index is the same as the value of the until attribute in the Change List: 2013-01-02T23:59:59Z. The open Change List could be the one shown in Example 19, in which case that list would have an additional link with an index relation type pointing to the Change List Index.

<?xml version="1.0" encoding="UTF-8"?>
<sitemapindex 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="changelist"
         from="2013-01-01T00:00:00Z"/>
  <sitemap>
      <loc>http://example.com/20130101-changelist.xml</loc>
      <rs:md from="2013-01-01T00:00:00Z" 
             until="2013-01-02T00:00:00Z"/>
  </sitemap>
  <sitemap>
      <loc>http://example.com/20130102-changelist.xml</loc>
      <rs:md from="2013-01-02T00:00:00Z" 
             until="2013-01-03T00:00:00Z"/>
  </sitemap>
  <sitemap>
      <loc>http://example.com/20130103-changelist.xml</loc>
      <rs:md from="2013-01-03T00:00:00Z"/>
  </sitemap>
</sitemapindex>

Example 20: A Change List Index

<?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:ln rel="index"
         href="http://example.com/dataset1/changelist.xml"/>
  <rs:md capability="changelist"
         from="2013-01-02T00:00:00Z"
         until="2013-01-03T00:00:00Z"/>
  <url>
      <loc>http://example.com/res7.html</loc>
      <lastmod>2013-01-02T12:00:00Z</lastmod>
      <rs:md change="created"/>
  </url>
  <url>
      <loc>http://example.com/res9.pdf</loc>
      <lastmod>2013-01-02T13:00:00Z</lastmod>
      <rs:md change="updated"/>
  </url>
  <url>
      <loc>http://example.com/res5.tiff</loc>
      <lastmod>2013-01-02T19:00:00Z</lastmod>
      <rs:md change="deleted"/>
  </url>
  <url>
      <loc>http://example.com/res7.html</loc>
      <lastmod>2013-01-02T20:00:00Z</lastmod>
      <rs:md change="updated"/>
  </url>
</urlset>

Example 21: A closed Change List pointing back to its Index

13. Packaging Changes

In order to reduce the number of requests required to obtain resource changes, a Source may provide packaged bitstreams for changed resources.

13.1 Change Dump

To make content changes available for download, a Source may publish Change Dumps. A Change Dump is a document that points to packages containing bitstreams for the Source's changed resources. The ResourceSync framework specifies the use of the ZIP file format as the packaging format. Communities may define their own packaging format. A Change Dump should only point to packages of the same format.

It is up to the Source to determine the publication frequency of these packages, as well as the temporal interval they cover. For example, a Source may choose to publish a fixed number of changes per package, or all the changes in a period of fixed length, such as an hour, a day, or a week. If a resource underwent multiple changes in the period covered by a package, then the package will contain multiple bitstreams for the resource, one per change. As new packages are published, new entries are added to the Change Dump that points at them. All entries in a Change Dump should be provided in forward chronological order: the least recently published package listed at the beginning of the Change Dump, the most recent package listed at the end of the document.

A Change Dump is based on the <urlset> document format introduced by the Sitemap protocol. It has the <urlset> root element and the following structure:

Example 22 shows a Change Dump with pointers to three bitstream packages associated with changed resources. The absence of the until attribute indicates that further packages will be added. The example also includes within each <url> element a pointer to a copy of the Change Dump Manifest associated with the package. This pointer is optional and intended to allow a Destination to determine whether the package should be downloaded. If such pointers are provided, the Source must ensure that the Manifest referred to matches the Manifest included in the bitstream package.

<?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="changedump"
         from="2013-01-01T00:00:00Z"/>
  <url>
      <loc>http://example.com/20130101-changedump.zip</loc>
      <lastmod>2013-01-01T23:59:59Z</lastmod>
      <rs:md type="application/zip" 
             length="3109"
             from="2013-01-01T00:00:00Z"
             until="2013-01-02T00:00:00Z"/>           
      <rs:ln rel="contents"
             href="http://example.com/20130101-changedump-manifest.xml"
             type="application/xml"/>
  </url>
  <url>
      <loc>http://example.com/20130102-changedump.zip</loc>
      <lastmod>2013-01-02T23:59:59Z</lastmod>
      <rs:md type="application/zip"
             length="6629"
             from="2013-01-02T00:00:00Z"
             until="2013-01-03T00:00:00Z"/>
      <rs:ln rel="contents"
             href="http://example.com/20130102-changedump-manifest.xml"
             type="application/xml"/>
  </url>
  <url>
      <loc>http://example.com/20130103-changedump.zip</loc>
      <lastmod>2013-01-03T23:59:59Z</lastmod>
      <rs:md type="application/zip"
             length="8124"
             from="2013-01-03T00:00:00Z"
             until="2013-01-04T00:00:00Z"/>
      <rs:ln rel="contents"
             href="http://example.com/20130103-changedump-manifest.xml"
             type="application/xml"/>
  </url>
</urlset>

Example 22: A Change Dump

If a Source needs to publish multiple Change Dumps, it must group them in a Change Dump Index, in a manner similar to what was described in Section 12.2.

13.2 Change Dump Manifest

Each ZIP package referred to from a Change Dump must contain a Change Dump Manifest file that describes the constituent bitstreams of the package. The file must be named manifest.xml and must be located at the top level of the ZIP package. All entries in a Change Dump Manifest must be provided in forward chronological order: the bitstream associated with the least recent resource change is listed first, and the bitstream associated with the most recent change is listed last.

The Change Dump Manifest is based on the <urlset> format. It has the <urlset> root element and the following structure:

Example 23 shows the Change Dump Manifest associated with the second entry in the Resource Dump from Example 22. The Manifest must be named manifest.xml at the top level of the ZIP package. A copy of the Manifest may also be provided at a location indicated by an optional <rs:ln> element with the relation type contents in the Change Dump, http://example.com/20130102-changedump-manifest.xml in Example 22. The Manifest covers the same changes as conveyed in the closed Change List of Example 21. The resource http://example.com/res7.html is listed twice, once because it was created, and once because it was updated. Both entries have the same URI. The ZIP package in which this Manifest is contained has two bitstreams for this resource, available at different paths in the package.

<?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="changedump-manifest"
         from="2013-01-02T00:00:00Z"
         until="2013-01-03T00:00:00Z"/>
  <url>
      <loc>http://example.com/res7.html</loc>
      <lastmod>2013-01-02T12:00:00Z</lastmod>
      <rs:md change="created"
             hash="md5:1c1b0e264fa9b7e1e9aa6f9db8d6362b"
             length="4339"
             type="text/html"
             path="/changes/res7.html"/>
  </url>
  <url>
      <loc>http://example.com/res9.pdf</loc>
      <lastmod>2013-01-02T13:00:00Z</lastmod>
      <rs:md change="updated"
             hash="md5:f906610c3d4aa745cb2b986f25b37c5a"
             length="38297"
             type="application/pdf"
             path="/changes/res9.pdf"/>
  </url>
  <url>
      <loc>http://example.com/res5.tiff</loc>
      <lastmod>2013-01-02T19:00:00Z</lastmod>
      <rs:md change="deleted"/>
  </url>
  <url>
      <loc>http://example.com/res7.html</loc>
      <lastmod>2013-01-02T20:00:00Z</lastmod>
            <rs:md change="updated"
             hash="md5:0988647082c8bc51778894a48ec3b576"
             length="5426"
             type="text/html"
             path="/changes/res7-v2.html"/>
  </url>
</urlset>

Example 23: A Change Dump Manifest

14. Linking to Related Resources

14.1 Overview

In order to facilitate alternative approaches to obtain content for a resource that is subject to synchronization or to provide additional information about it, a Source may provide links from that resource to related resources. Such links may occur in Resource Lists, Resource Dump Manifests, Change Lists, and Change Dump Manifests. The following cases are considered and detailed (in examples of Change Lists) in the remainder of this section:

As always, the <loc> child element of <url> conveys the URI of the resource that is subject to synchronization. The related resource is provided by means of the <rs:ln> child element of <url>. The possible attributes for <rs:ln> as well as the link relation types used to address the aforementioned use cases are detailed in Section 7. Links to meet needs other than the ones listed may be provided, and appropriate relation types may be selected from the IANA Link Relation Type Registry or expressed as URIs as specified in RFC 5988, Sec. 4.2.

In case a Destination is not able to adequately interpret the information conveyed in an <rs:ln> element, it should refrain from accessing the related resource and rather use the URI provided in <loc> to retrieve the resource.

14.2 Mirrored Content

In order to reduce the load on its primary access mechanism, a Source may convey one or mirror locations for a resource. An <rs:ln> element is introduced to express each mirror location for the resource. This element has the following attributes:

Example 24 shows how a Source conveys information about prioritized mirror locations for a resource. Since the three locations conveyed by <rs:ln> elements point to duplicates of the resource specified in <loc>, the values for each of the attributes of <rs:md> are expected to be identical for the resource and its mirrors. Hence, they should be omitted from the <rs:ln> elements. The last <rs:ln> element points to a mirror location where the resource is accessible via a protocol other than HTTP as can be seen from the URI scheme. Even though the resources are duplicates, their last modified datetimes may vary.

<?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="changelist"
         from="2013-01-03T00:00:00Z"/>
  <url>
      <loc>http://example.com/res1</loc>
      <lastmod>2013-01-03T18:00:00Z</lastmod>
      <rs:md change="updated"
             hash="md5:1584abdf8ebdc9802ac0c6a7402c03b6"
             length="8876"
             type="text/html"/>
      <rs:ln rel="duplicate"
             pri="1"
             href="http://mirror1.example.com/res1"
             modified="2013-01-03T18:00:00Z"/>
      <rs:ln rel="duplicate"
             pri="2"
             href="http://mirror2.example.com/res1"
             modified="2013-01-03T18:00:00Z"/>
      <rs:ln rel="duplicate"
             pri="3"
             href="gsiftp://gridftp.example.com/res1"
             modified="2013-01-03T18:00:00Z"/>
  </url>
</urlset>

Example 24: Mirrored content

14.3 Alternate Representations

A resource may have multiple representations available from different URIs. A resource may, for example, be identified by a generic URI such as http://example.com/res1. After performing content negotiation with the server, a client may, for example, obtain the resource's HTML representation available from the specific URI http://example.com/res1.html. Another client may ask for and retrieve the PDF representation of the resource from the specific URI http://example.com/res1.pdf. Which representation a client obtains, can, amongst others, depend on its preferences in terms of Media Type and language, its geographical location, and its device type.

A Source may express that a resource is subject to synchronization by conveying its generic URI in <loc>. In this case, per alternate representation that the Source wants to advertise, an <rs:ln> element is introduced. This element has the following attributes:

Cases exist in which there is no generic URI for a resource, only specific URIs. This may occur, for example, when a resource has different representations available for different devices. In this case the URI in <loc> will be a specific URI, and <rs:ln> elements with an alternate relation type are still used to refer to alternate representations available from other specific URIs.

Example 25 shows how to promote a generic URI in <loc> while also pointing to alternate representations available from specific URIs, for example, through content negotiation.

<?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="changelist"
         from="2013-01-03T11:00:00Z"/>
  <url>
      <loc>http://example.com/res1</loc>
      <lastmod>2013-01-03T18:00:00Z</lastmod>
      <rs:md change="updated"/>
      <rs:ln rel="alternate"
             href="http://example.com/res1.html"
             modified="2013-01-03T18:00:00Z"
             type="text/html"/>
      <rs:ln rel="alternate"
             href="http://example.com/res1.pdf"
             modified="2013-01-03T18:00:00Z"
             type="application/pdf"/>
  </url>
</urlset>

Example 25: Generic URI and alternates with specific URIs

In cases where a particular representation is considered the subject of synchronization, its specific URI is provided in <loc>. The associated generic URI, if one exists, may be provided using an <rs:ln> element. This element has the following attributes:

This approach might be most appropriate for Resource Dump Manifests and Change Dump Manifests that describe bitstreams contained in a ZIP file.

Example 26 shows a Source promoting a specific URI in <loc> while also pointing to the resource's generic URI by means of an <rs:ln> element. Metadata pertaining to the representation available from that specific URI is conveyed by means of attributes of the <rs:md> element.

<?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="changelist"
         from="2013-01-03T00:00:00Z"/>
  <url>
      <loc>http://example.com/res1.html</loc>
      <lastmod>2013-01-03T18:00:00Z</lastmod>
      <rs:md change="updated"
             hash="md5:1584abdf8ebdc9802ac0c6a7402c03b6"
             length="8876"/>
      <rs:ln rel="canonical"
             href="http://example.com/res1"
             modified="2013-01-03T18:00:00Z"/>
  </url>
</urlset>

Example 26: Specific URI and alternate with generic URI

14.4 Patching Content

In order to increase the efficiency of updating a resource, a Source may make a description available of the changes that the resource underwent, in addition to the entire changed resource. Especially when frequent minor changes and/or changes to large resources are concerned, such an approach may be attractive. It will, however, require an unambiguous way to describe the changes in such a way that a Destination can construct the most recent version of the resource by appropriately patching the previous version with the description of the changes.

A Source may express that it makes a description of resource changes available by providing the URI of the resource in <loc>, as usual, and by introducing an <rs:ln> element with the following attributes:

Example 27 shows a Source that expresses changes that a JSON resource underwent expressed using the application/json-patch Media Type introduced in JSON Patch. It also shows the Source conveying changes to a large TIFF file using an experimental Media Type that may, for example, be described in a community specification. A Destination that does not understand the Media Type should ignore the description of changes and use the URI in <loc> to obtain the most recent version of the resource. Another example of a well-specified Media Type for expressing changes to XML document is application/patch-ops-error+xml, as specified in RFC 5261.

Expressing resource changes in this manner is only applicable to Change Lists (as in Example 27) and Change Dumps. When doing so for a Change Dump, the entry in the Change Dump Manifest must have the path attribute for the <rs:ln> element that points to the change description that is included in the content package.

<?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="changelist"
         from="2013-01-03T00:00:00Z"/>
  <url>
      <loc>http://example.com/res4</loc>
      <lastmod>2013-01-03T17:00:00Z</lastmod>
      <rs:md change="updated"
             hash="sha-256:f4OxZX_x_DFGFDgghgdfb6rtSx-iosjf6735432nklj"
             length="56778"
             type="application/json"/>
      <rs:ln rel="http://www.openarchives.org/rs/terms/patch"
             href="http://example.com/res4-json-patch"
             modified="2013-01-03T17:00:00Z"
             hash="sha-256:y66dER_t_HWEIKpesdkeb7rtSc-ippjf9823742opld"
             length="73"
             type="application/json-patch"/>
  </url>
  <url>
      <loc>http://example.com/res5-full.tiff</loc>
      <lastmod>2013-01-03T18:00:00Z</lastmod>
      <rs:md change="updated"
             hash="sha-256:f4OxZX_x_FO5LcGBSKHWXfwtSx-j1ncoSt3SABJtkGk"
             length="9788456778"
             type="image/tiff"/>
      <rs:ln rel="http://www.openarchives.org/rs/terms/patch"
             href="http://example.com/res5-diff"
             modified="2013-01-03T18:00:00Z"
             hash="sha-256:h986gT_t_87HTkjHYE76G558hY-jdfgy76t55sadJUYT"
             length="4533"
             type="application/x-tiff-diff"/>
  </url>
</urlset>

Example 27: A Change List with links to document that detail how to patch resources

14.5 Resources and Metadata about Resources

Cases exist where both resources and metadata about those resources must be synchronized. From the ResourceSync perspective, both the resource and the metadata about it are regarded as resources with distinct URIs that are subject to synchronization. As usual, each gets its distinct <url> block and each URI is conveyed in a <loc> child element of the respective block. If required, the inter-relationship between both resources is expressed by means of an <rs:ln> element with appropriate relation types added to each block. The <rs:ln> element has the following attributes:

Example 28 shows how a Source may express this inter-relationship between the two resources. Note that a Destination can use the metadata that describes a resource as a filtering mechanism to only synchronize with those resources that meet its metadata-based selection criteria. Note also in the <url> element that conveys the metadata record update, the use of the link with a profile relation type [RFC 6906] to express the kind of metadata that is used to describe the resource, in this case, expressed by means of its XML Namespace. The link should provide a URI that supports the Destination in interpreting the metadata information. For example, it could refer to a namespace, an XML schema, or a description of MARC.

<?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="changelist"
         from="2013-01-03T00:00:00Z"/>
  <url>
      <loc>http://example.com/res2.pdf</loc>
      <lastmod>2013-01-03T18:00:00Z</lastmod>
      <rs:md change="updated"
             hash="md5:1584abdf8ebdc9802ac0c6a7402c03b6"
             length="8876"
             type="application/pdf"/>
      <rs:ln rel="describedby"
             href="http://example.com/res2_dublin-core_metadata.xml"
             modified="2013-01-01T12:00:00Z"
             type="application/xml"/>
  </url>
  <url>
      <loc>http://example.com/res2_dublin-core_metadata.xml</loc>
      <lastmod>2013-01-03T19:00:00Z</lastmod>
      <rs:md change="updated"
             type="application/xml"/>
      <rs:ln rel="describes"
             href="http://example.com/res2.pdf"
             modified="2013-01-03T18:00:00Z"
             hash="md5:1584abdf8ebdc9802ac0c6a7402c03b6"
             length="8876"
             type="application/pdf"/>
      <rs:ln rel="profile"
             href="http://purl.org/dc/elements/1.1/"/>
  </url>
</urlset>

Example 28: Linking between a resource and metadata about a resource in a Change List

14.6 Prior Versions of Resources

A Source may provide access to prior versions of a resource to allow Destinations to obtain a historical perspective, rather than just remaining synchronized with the most recent version. The approach to do so leverages a common resource versioning paradigm that consists of:

When communicating about the resource, its time-generic URI is provided in <loc> and <lastmod> must be used to provide the resource's last modification time.

A first approach consists of conveying the time-specific URI of the resource that corresponds with the time of last modification, as given in the <lastmod> element. This is achieved by introducing a single <rs:ln> element with the following attributes:

A second approach consists of pointing to a TimeGate associated with the time-generic resource. A TimeGate supports negotiation in the datetime dimension, as introduced in the Memento protocol [Memento Internet Draft], to obtain a version of the resource as it existed at a specified moment in time. This allows the Destination to obtain the version that existed at the time of last modification by using the <lastmod> value in the datetime negotiation process, but also allows the Destination to obtain other versions by using different datetime values. A pointer to a TimeGate is introduced by using an <rs:ln> element with the following attributes:

A third approach consists of pointing to a TimeMap associated with the time-generic resource. A TimeMap, as introduced in the Memento protocol [Memento Internet Draft], enables Destinations to retrieve a comprehensive list of all time-specific resources known to a server. This allows Destinations to choose a particular version of a resource from that list. A pointer to a TimeMap is introduced by using an <rs:ln> element with the following attributes:

Example 29 shows a Change List with a link to a prior version of a resource, a link to a TimeGate, as well as a link to a TimeMap. Note that the values of the hash, length, and type attributes are identical between the <rs:md> child element and the <rs:ln> child element that points to the prior version.

<?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="changelist"
         from="2013-01-03T00:00:00Z"/>
  <url>
      <loc>http://example.com/res1</loc>
      <lastmod>2013-01-03T18:00:00Z</lastmod>
      <rs:md hash="md5:1584abdf8ebdc9802ac0c6a7402c03b6"
             length="8876"
             type="text/html"
             change="updated"/>
      <rs:ln rel="memento"
             href="http://example.com/20130103070000/res1"
             modified="2013-01-02T18:00:00Z"
             hash="md5:1584abdf8ebdc9802ac0c6a7402c03b6"
             length="8876"
             type="text/html"/>
      <rs:ln rel="timegate"
             href="http://example.com/timegate/http://example.com/res1"/>
      <rs:ln rel="timemap"
             href="http://example.com/timemap/http://example.com/res1"
             type="application/link-format"/>
  </url>
</urlset>

Example 29: Links to a resource version, and a Memento TimeGate and TimeMap

14.7 Collection Membership

A Source may express that a resource is a member of a collection such as an OAI-ORE Aggregation or an OAI-PMH Set. A Source may express collection membership of a resource that is subject to synchronization by providing the URI of that resource in <loc>, as usual, and by introducing an <rs:ln> element with the following attributes:

Example 30 shows a Change List with one resource that is a member of an OAI-ORE Aggregation.

<?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="changelist"
         from="2013-01-03T00:00:00Z"/>
  <url>
      <loc>http://example.com/res1</loc>
      <lastmod>2013-01-03T07:00:00Z</lastmod>
      <rs:md hash="md5:1584abdf8ebdc9802ac0c6a7402c03b6"
             length="8876"
             type="text/html"
             change="updated"/>
      <rs:ln rel="collection"
             href="http://example.com/aggregation/0601007"/>
  </url>
</urlset>

Example 30: A resource as a member of a collection

14.8 Republishing Resources

A special kind of Destination, henceforth called an aggregator, may retrieve content from a Source, republish it, and in its turn act as a Source for the republished content. In such an aggregator scenario, it may be important for a Destination that synchronizes with the aggregator to understand the provenance of the content and to be able to verify its accuracy with the original Source of the content. When communicating about a republished resource, the aggregator may provide such provenance information by introducing an <rs:ln> element with the following attributes:

Example 31 shows a Change List in which a Source publishes information about a change to a single resource.

<?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="changelist"
         from="2013-01-03T00:00:00Z"/>
  <url>
      <loc>http://original.example.com/res1.html</loc>
      <lastmod>2013-01-03T07:00:00Z</lastmod>
      <rs:md hash="md5:1584abdf8ebdc9802ac0c6a7402c03b6"
             length="8876"
             type="text/html"
             change="updated"/>
  </url>
</urlset>

Example 31: An original Source publishes

Example 32 shows a primary aggregator's Change List that refers to the original Source's resource. It includes a link with the relation type via that has attributes such as href to convey information about the origin of the resource. This information corresponds with the data provided in the <url> block of the Change List shown in Example 31. For example, the value of the href attribute in Example 32 equals the value of the <loc> child element in the <url> block in Example 31.

<?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://aggregator1.example.com/dataset1/capabilitylist.xml"/>
  <rs:md capability="changelist"
         from="2013-01-03T11:00:00Z"/>
  <url>
      <loc>http://aggregator1.example.com/res1.html</loc>
      <lastmod>2013-01-03T20:00:00Z</lastmod>
      <rs:md hash="md5:1584abdf8ebdc9802ac0c6a7402c03b6"
             length="8876"
             type="text/html"
             change="updated"/>
      <rs:ln rel="via"
             href="http://original.example.com/res1.html"
             modified="2013-01-03T07:00:00Z"
             hash="md5:1584abdf8ebdc9802ac0c6a7402c03b6"
             length="8876"
             type="text/html"/>
  </url>
</urlset>

Example 32: A primary aggregator republishes

If a secondary aggregator obtains the changed resource by consuming the Change List of the primary aggregator and republishes its Change List, a chain of aggregations is created. In this case each aggregator should maintain only the existing via link in order to convey information about the origin of the resource.

Example 33 shows the Change List of a secondary aggregator with information about the changed resource and the via link equal to Example 32. The data conveyed with the link corresponds to the data provided in the <url> block in Example 31.

<?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://aggregator2.example.com/dataset1/capabilitylist.xml"/>
  <rs:md capability="changelist"
         from="2013-01-03T12:00:00Z"/>
  <url>
      <loc>http://aggregator2.example.com/res1.html</loc>
      <lastmod>2013-01-04T09:00:00Z</lastmod>
      <rs:md hash="md5:1584abdf8ebdc9802ac0c6a7402c03b6"
             length="8876"
             type="text/html"
             change="updated"/>
      <rs:ln rel="via"
             href="http://original.example.com/res1.html"
             modified="2013-01-03T07:00:00Z"
             hash="md5:1584abdf8ebdc9802ac0c6a7402c03b6"
             length="8876"
             type="text/html"/>
  </url>
</urlset>

Example 33: A second aggregator republishes

The values of the at, completed, from, and until attributes must always be expressed from the perspective of the Source that publishes the document that contains them. Hence it is possible that the from datetime of a Change List is more recent than the <lastmod> datetime of the original Source's resource described in the Change List, which is conveyed using an <rs:ln> link with the via relation type.

An aggregator should be cautious when inheriting links, other than the one with the via relation type, from a Source that precedes it in an aggregation chain. It should make sure that each such link remains appropriate from its own perspective and refrain from inheriting it when it is not. For example, a link with the relation type collection or canonical expressed by the original Source may not be appropriate in the context of the aggregator's copy, and hence should not be included in the description of the changed resource in the aggregator's capability document.

A. Time Attribute Requirements

(normative)

Table 4 provides an overview of the requirements for use of the at and from attributes in ResourceSync documents. The top label in the column headings represents the <sitemapindex> root element for index documents, and the <urlset> root element for all other documents. The child label in the column headings represents the <sitemap> child element for index documents, and the <url> child element for all other documents.

The optional attributes completed and until are not shown in the table, but they may be added wherever the corresponding at and from attributes are mandatory, recommended, or optional.

Table 4 shows that, for example, a Change List must contain the <rs:md> child element of the <urlset> root element with the attribute from to convey the temporal interval covered by the Change List. The table also shows that the <url> child element of the <urlset> root element in a Change List must have the <lastmod> child element to convey the last modification time of a resource. Both mandatory attributes are, for example, shown in Example 19 in Section 12.1.

Capability Document/top/rs:md/@at/top/rs:md/@from/top/child/rs:md/@at/top/child/rs:md/@from/top/child/lastmod
Resource List Mandatory X X X Optional
Resource List Index Mandatory X Optional X Optional
Resource Dump Mandatory X Optional X Optional
Resource Dump Index Mandatory X Optional X Optional
Resource Dump Manifest Mandatory X X X Optional
Change List X Mandatory X X Mandatory
Change List Index X Mandatory X Recommended Optional
Change Dump X Mandatory X Recommended Optional
Change Dump Index X Mandatory X Recommended Optional
Change Dump Manifest X Mandatory X X Mandatory

Table 4: Required and optional use of at and from attributes in ResourceSync documents

B. Bibliography

(This Bibliography is not part of the ResourceSync Framework Specification, ANSI/NISO Z39.99-2014. It is included for information only.)

[arXiv]
arXiv.org [website]. Available at: http://arxiv.org/
[HTML Links]
Raggett, Dave, Arnaud Le Hors, and Ian Jacobs, eds. "Links." Section 12 in: HTML 4.01 Specification. W3C Recommendation. World Wide Web Consortium, December 24, 1999. Available at: http://www.w3.org/TR/html401/struct/links.html
[JSON-Patch]
Bryan, P., and M. Nottingham, eds. JSON Patch. Internet Draft. Internet Engineering Task Force (IETF), January 20, 2013. Available at: http://tools.ietf.org/html/draft-ietf-appsawg-json-patch-10
[OAI-PMH]
Lagoze, Carl, Herbert Van de Sompel, Michael Nelson, and Simeon Warner, eds. The Open Archives Initiative Protocol for Metadata Harvesting. Version 2.0. Open Archives Initiative, December 7, 2008. Available at: http://www.openarchives.org/OAI/openarchivesprotocol.html
[ORE]
Lagoze, Carl, Herbert Van de Sompel, Pete Johnston, Michael Nelson, Robert Sanderson, and Simeon Warner, eds. ORE Specification - Abstract Data Model. Open Archives Initiative, October 17, 2008. Available at: http://www.openarchives.org/ore/1.0/datamodel
[Relation Types]
Klein, Martin, Robert Sanderson, Herbert Van de Sompel, Simeon Warner, Graham Klyne, Bernhard Haslhofer, Michael Nelson, and Carl Lagoze, eds. Relation Types Used in the ResourceSync Framework. Beta draft. Open Archives Initiative, September 27, 2013. Available at: http://www.openarchives.org/rs/relationtypes
[RFC 5261]
Urpalainen, J. An Extensible Markup Language (XML) Patch Operations Framework Utilizing XML Path Language (XPath) Selectors. RFC 5261. Internet Engineering Task Force (IETF), September 2008. Available at: http://www.ietf.org/rfc/rfc5261.txt
[RFC 5785]
Nottingham, M., and E. Hammer-Lahav. Defining Well-Known Uniform Resource Identifiers (URIs). RFC 5785. Internet Engineering Task Force (IETF), April 2010. Available at: http://www.ietf.org/rfc/rfc5785.txt
[RFC 6596]
Ohye, M., and J. Kupke. The Canonical Link Relation. RFC 6596. Internet Engineering Task Force (IETF), April 2012. Available at: http://www.ietf.org/rfc/rfc6596.txt
[rsync]
rsync. From Wikipedia, the free encyclopedia. Available at: http://en.wikipedia.org/wiki/Rsync
[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/
[XHTML Links]
McCarron, Shane, et al., eds. "Link Module." Section 5.19 in: XHTML Modularization 1.1. Second Edition. World Wide Web Consortium, July 29, 2010. Available at: http://www.w3.org/TR/xhtml-modularization/abstract_modules.html#s_linkmodule

C. Front Matter, Authorship, Acknowledgements

(This appendix is contains front matter, authorship and acknowledgements from the PDF version of this specification. They have been moved here to preserve the readability of this web document.)

About NISO Standards

NISO standards are developed by Working Groups of the National Information Standards Organization. The development process is a strenuous one that includes a rigorous peer review of proposed standards open to each NISO Voting Member and any other interested party. Final approval of the standard involves verification by the American National Standards Institute that its requirements for due process, consensus, and other approval criteria have been met by NISO. Once verified and approved, NISO Standards also become American National Standards.

These standards may be revised or withdrawn at any time. For current information on the status of this standard contact the NISO office or visit the NISO website at: www.niso.org.

Published by
NISO
3600 Clipper Mill Road
Suite 302
Baltimore, MD 21211
www.niso.org

ISSN: 1041-5653 (National Information standards series)
ISBN: 978-1-937522-20-9 (HTML)
ISBN: 978-1-937522-19-3 (PDF)

Foreword

(This foreword is not part of the ResourceSync Framework Specification, ANSI/NISO Z39.99-2014. It is included for information only.)

About This Standard

This document is structured as follows:

Trademarks, Services Marks

Wherever used in this standard, all terms that are trademarks or service marks are and remain the property of their respective owners.

NISO Voting Members

At the time this standard was balloted and approved, the following were members of the NISO Z39.99-201x ResourceSync voting pool:

American Institute of Physics (AIP)
Evan Owens (primary), Aravind Akella (alternate)
Los Alamos National Laboratory
Dianna Magnoni (primary), Herbert Van de Sompel (alternate)
American Library Association (ALA)
Nancy Kraft
Microsoft Corporation
Alex Wade
Minitex
Cecelia Boone (primary), Valerie Horton (alternate), Paul Swanson (alternate)
American Theological Library Association (ATLA)
Brenda Bailey-Hainer (primary), Maria Stanton (alternate)
Music Library Association
Nara Newcomer (primary), David Sommerfield (alternate)
Association for Information Science & Technology (ASIS&T)
Mark Needleman
National Archives and Records Administration (NARA)
Laura McCarthy (primary), Marilyn Redman (alternate)
Association of Research Libraries (ARL)
Julia Blixrud
National Federation of Advanced Information Services (NFAIS)
Marjorie Hlava
EBSCO Information Services
Oliver Pesch
National Library of Finland
Juha Hakala
EnvisionWare, Inc.
Robert Walsh (primary), Michael Monk (alternate)
National Library of Medicine (NLM)
Barbara Rapp (primary), Jacque-Lynne Schulman (alternate)
Ex Libris, Inc.
Mike Dicus
OCLC Online Computer Library Center
Tam Dalrymple (primary), Thomas Hickey (alternate)
Innovative Interfaces, Inc.
Brad Jung (primary), John McCullough (alternate)
SAGE Publications
Carol Richman
International DOI Foundation (IDF)
Norman Paskin
Scholarly iQ
Gary Van Overborg (primary), John Milligan (alternate)
ITHAKA/JSTOR/Portico
Amy Kirchhoff (primary), Bruce Heterick (alternate)
Statewide California Electronic Library Consortium (SCELC)
Rick Burke
John Wiley & Sons, Ltd.
Duncan Campbell (primary), Craig Van Dyck (alternate)
The Library Corporation (TLC)
Juli Marsh (primary), Wayne Hicks (alternate)
Library of Congress
Sally McCallum (primary), John Zagas (alternate)

NISO D2D Topic Committee

At the time NISO approved this standard, the following individuals served on the Discovery to Delivery (D2D) Topic Committee that had oversight for this project.

Kristin Antelman
North Carolina State University Libraries
Peter Murray
Lyrasis
Pascal Calarco, Co-chair
University of Waterloo Library
Tim Shearer
University of North Carolina Chapel Hill Libraries
Lucy Harrison, Co-chair
Florida Virtual Campus
Christine Stohn
Ex Libris, Inc.
Juli Marsh
The Library Corporation (TLC)
Chris Shillum
Reed Elsevier

NISO ResourceSync Working Group Members

The following individuals served on the NISO ResourceSync Working Group, which developed and approved this standard:

Todd Carpenter, Co-chair
NISO
Michael Nelson
Old Dominion University
Bernhard Haslhofer
Vienna University of Technology
Shlomo Sanders
Ex Libris, Inc.
Richard Jones
Cottage Labs
Robert Sanderson
Los Alamos National Laboratory
Martin Klein
Los Alamos National Laboratory
Herbert Van de Sompel, Co-chair
Los Alamos National Laboratory
Graham Klyne
University of Oxford
Paul Walk
EDINA
Carl Lagoze
University of Michigan
Simeon Warner
Cornell University
Stuart Lewis
JISC Collections
Zhiwu Xie
Virginia Tech University Libraries
Peter Murray
Lyrasis
Jeff Young
OCLC Online Computer Library Center

Acknowledgements

This specification is the collaborative work of NISO and the Open Archives Initiative. Funding for ResourceSync is provided by the Alfred P. Sloan Foundation. UK participation is supported by Jisc. We also thank numerous individual contributors including: Martin Haye (California Digital Library), David Rosenthal (Stanford University Libraries), Ed Summers (Library of Congress), and Vincent Wehren (Microsoft).

D. Change Log

(This appendix is not part of the ResourceSync Framework Specification, ANSI/NISO Z39.99-2014. It is included for information only.)

Date Editor Description
2014-04-21 martin, herbert, simeon verson 1.0, ANSI/NISO Z39.99-2014
2013-08-05 martin, herbert, rob, simeon version 0.9.1
2013-06-07 martin, herbert, rob, simeon version 0.9
2013-05-01 martin, herbert, rob, simeon version 0.6
2013-02-01 martin, herbert, rob, simeon beta spec draft
2012-08-13 martin, herbert, simeon, bernhard first alpha spec draft

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