1. Introduction

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 can advertise the capabilities it supports. Remote systems can 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 can 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.

This document is structured as follows:

• The remainder of this section provides example use cases and introduces some notational conventions.
• Section 2. ResourceSync Basics provides a brief overview of the various ResourceSync capabilities. Reading this section is advised before continuing on to further technical details.
• Section 3. Sitemap Document Formats describes the way in which the document formats introduced by the Sitemap protocol are used to convey synchronization related information in all ResourceSync capabilities.
• Section 4. Describing Resources, Section 5. Packaging Resources, Section 6. Describing Changes, Section 7. Packaging Changes, and Section 8. Linking to Related Resources each provide details about capabilities that a server may implement in order to enable remote systems to remain aligned with its evolving data.
• Section 9. Advertising Capabilities describes how a server can advertise the ResourceSync capabilities it supports.

1.1. 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 web pages. The maintainer can create a Resource List of these web pages 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 can 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.

For many years now, the arXiv.org collection of scientific articles has used a custom mirroring solution to propagate resource changes to a set of mirror sites and interacting services on a daily basis. The collection contains about 2.4 million files and there are about 1,600 changes (creates, updates) per day. The mirroring system currently in place 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 interested third party systems 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. Leveraging ResourceSync capabilities including Resource Lists, Resource Dumps, Change Lists, and Change Dumps, both existing mirrors, such as lanl.arXiv.org, and new parties could remain accurately in sync with the arXiv.org collection. This would extend the openly available metadata sharing capability provided by arXiv.org, currently implemented via OAI-PMH, to full-text sharing in a web-friendly fashion.

1.2. Notational Conventions

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

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

Table 1.1: Namespace prefix bindings used in this document

2. ResourceSync Basics

This section provides an overview of the various ResourceSync capabilities that a server may support in order to enable remote systems to become and remain synchronized with its evolving resources. The following terms are introduced:

• A Source is a server that hosts resources subject to synchronization.
• A Destination is a system that synchronizes itself with the Source's resources.

2.1. 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 2.1, 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. It expresses that the Sitemap implements ResourceSync's Resource List capability and conveys the datetime when the Resource List was valid, allowing 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"
         from="2013-01-03T09:00:00Z"/>
  <url>
      <loc>http://example.com/res1</loc>
  </url>
  <url>
      <loc>http://example.com/res2</loc>
  </url>
</urlset>

Example 2.1: 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 2.2 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. Example 2.2 shows this information conveyed using the hash attribute on 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:md capability="resourcelist"
         from="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"/>
  </url>
</urlset>

Example 2.2: 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. 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 2.3 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 being 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="2013-01-01T11:00:00Z"
         until="2013-01-03T11:00:00Z"/>
  <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 2.3: 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 2.4 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 2.5. 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> attribute conveys the URI associated with the bitstream at the Source.

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

Example 2.4: 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"
         from="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 2.5: 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 2.6 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 a <rs:ln> child element of <urlset> that links 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 href="http://example.com/info_about_set1_of_resources.xml"
         rel="describedby"/>
  <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 2.6: A Capability List with the description of the ResourceSync capabilities of a Source for a set of resources

There are two ways for a Destination to discover whether and how a Source supports ResourceSync: a Source-wide and a resource-specific discovery approach. The Source-wide approach leverages the well-known URI specification [RFC5785] and consists of the Source making a ResourceSync Description, like the one shown in Example 2.7 available at /.well-known/resourcesync. The ResourceSync 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 ResourceSync Description contains only one pointer. The resource-specific discovery approach consists of a Source providing a link in an (X)HTML document or in an HTTP Link header that points at a Capability List that covers the resource that provides the link.

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

Example 2.7: A ResourceSync 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 group the documents described so far. For example, the Sitemap protocol currently prescribes a maximum of 50,000 resources per Sitemap and a Source may easily 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 2.8 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"
         from="2013-01-03T09:00:00Z"/>
  <sitemap>
      <loc>http://example.com/resourcelist-part2.xml</loc>
      <lastmod>2013-01-03T09:00:00Z</lastmod>
  </sitemap>
  <sitemap>
      <loc>http://example.com/resourcelist-part1.xml</loc>
      <lastmod>2013-01-03T09:00:00Z</lastmod>
  </sitemap>
</sitemapindex>

Example 2.8: A Resource List Index expressed using the <sitemapindex> document format

2.2. Overview of Synchronization Processes

The previous section provides a concrete walkthrough of some capabilities that a Source can implement and describes how a Destination can leverage 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 in a Destination's processes aimed at remaining in step with changes.

2.2.1. Source Perspective

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

Describing Content - In order to describe its data, a Source can maintain an up-to-date Resource List. A basic Resource List minimally provides the URIs of resources that the Source makes available for synchronization. However, additional information can be added to the Resource List to optimize the Destination's process of obtaining the Source's resources, including the most recent modification time of resources and fixity information such as content-based checksum or hash and length. Figure 1 shows a Source publishing up-to-date Resource Lists at times t2 and t4. At t4, too many resources need to be listed to fit in a single Resource List and hence multiple Resource Lists are published and grouped in a Resource List Index.

Packaging Content - In order to make its data available for download, a Source can recurrently make an up-to-date Resource Dump of its content available. A Resource Dump points at one or more packages, each of which contains bitstreams associated with resources hosted by the Source. Each package also contains a Resource Dump Manifest that provides metadata about the bitstreams contained in the package, minimally including their associated URI and their file path in the ZIP file. Figure 1 shows a Source publishing up-to-date Resource Dumps at times t1 and t3. At time t3, multiple Resource Dumps are published and grouped in a Resource Dump Index.

Describing Changes - In order to achieve lower synchronization latency and/or to improve transfer efficiency, a Source may publish a Change List that provides information about changes to its resources. It is up to the Source to decide what the temporal interval is that is covered by a Change List, for example, expressing all the changes that occurred during the previous hour, the current day, or since the most recent publication of a Resource List. Per resource change, a Change List minimally conveys the URI of the changed resource as well as the datetime and nature of the change (create, update, delete). Since a Change List is organized on the basis of changes, it may list the same resource multiple times, once per change. Figure 1 shows three Change Lists. The first Change List covers resource changes that occurred between t1 and t3, the second between t3 and t5, and the third between t5 and t7. Since too many changes occurred between t5 and t7 to fit in a single Change List, multiple Change Lists are published and grouped in a Change List Index.

Packaging Changes - In order to make content changes available for download, a Source can publish a Change Dump. A Change Dump points at one or more packages, each of which contains bitstreams that correspond to changes that occurred to a Source's resources. Each package also contains a Change Dump Manifest that provides metadata about the bitstreams provided in the Change Dump. Per bitstream, the Change Dump Manifest minimally includes the associated URI, the datetime when the change that resulted in the bitstream occurred, the nature of the change (create, update, delete) and the file path of the bitstream in the ZIP file. It is up to a Source to decide the temporal interval covered by a Change Dump, for example, covering all the resource changes that occurred during the previous hour, the current day, or since the most recent publication of a Dump. Since a Change Dump is organized on the basis of changes, the package(s) it points at may contain multiple bitstreams associated with any given resource, one per change. Figure 1 shows three Change Dumps. The first Change Dump covers resource changes that occurred between t2 and t4, the second between t4 and t6, and the third between t6 and t8. During the time period between t6 and t8, multiple Change Dumps are published and grouped in a Change Dump Index.

Linking to Related Resources - There are several reasons to provide additional links from a resource subject to synchronization to related resources:

• Alternate Content Transfer - The default mechanisms by which a Destination obtains content for a resource are to issue an HTTP GET against its URI found in a Resource List or Change List, or to unpack packages obtained via a Resource Dump or Change Dump. However, additional approaches may also be supported. For example, a Source may prefer, for synchronization purposes, that content be obtained from a mirror server and hence from a different URI. Also, a Source can allow obtaining only the changes that a resource underwent, instead of the entire changed resource. This may be desirable when the resource size is considerable and/or the frequency of changes high. Such an Alternate Content Transfer approach is expressed by means of a link from the resource to another resource that makes the content available in an alternate way. It is possible that certain Destinations do not understand a specific Alternate Content Transfer approach, in which case ignoring the link and dereferencing the resource's URI remains the fallback approach.
• Resources and Metadata about Resources - Cases exist where both resources and metadata about resources must be synchronized, for example, a collection of scientific publications and metadata describing each. From the ResourceSync perspective, both the resource and the metadata about it are regarded as resources with distinct URIs that are subject to synchronization. Their inter-relationship is expressed by means of links with appropriate relation types.
• Prior Versions of Resources - In some cases a Destination requires a copy of each version of a resource, not just the most recent one. A Source may support discovery and access to prior resource versions through links. Two approaches are provided, one based on linking to resource versions, the other based on linking to a TimeGate that supports datetime negotiation to obtain an appropriate temporal version of a resource [Memento Internet Draft].

Figure 1: ResourceSync Source perspective

2.2.2. Destination Perspective

From the perspective of a Destination, three key processes are enabled by the ResourceSync capabilities; Figure 2 provides an overview:

Baseline Synchronization - In order to become synchronized with a Source, the Destination must make an initial copy of the Source's data. A Destination can obtain the Resource List that conveys the URIs of the Source's resources, and subsequently dereference those URIs one by one. A Destination can also obtain a Resource Dump that conveys the URIs of one or more content packages each of which contains bitstreams associated with the Source's resources. A Destination can dereference those URIs and subsequently unpack the retrieved content packages, guided by the contained Resource Dump Manifest.

Incremental Synchronization - A Destination can remain in sync with a Source by repeatedly performing a Baseline Synchronization. To increase efficiency and decrease latency, a Source may communicate information about changes to its resources via Change Lists. This allows a Destination to obtain up-to-date content by dereferencing the URIs of newly created and updated resources listed in the Change List. It also allows a Destination to remove its copies of deleted resources, if needed. A Source can also make a Change Dump available that points at one or more packages, each of which contains bitstreams that correspond to changes that occurred to a Source's resources. In this case the Destination first obtains the Change Dump, then obtains the package(s) by dereferencing the URI(s) listed in the Change Dump, and subsequently unpacks those, guided by the contained Change Dump Manifest.

Audit - In order to verify whether it is in sync with the Source, a Destination must be able to check that the content it obtained matches the current resources hosted by the Source both regarding coverage and accuracy. This requires an up-to-date list of resources hosted by the Source, which can be compiled on the basis of a Resource List and Change Lists. It also requires these Lists to contain metadata per resource that characterizes its most recent state, such as last modification time, length, and content-based hash.

Figure 2: ResourceSync Destination perspective

2.2.3. Structural Perspective

All capabilities in the ResourceSync framework are implemented on the basis of the <urlset> and <sitemapindex> Sitemap document formats. Figure 3 depicts the overall structure of the set of documents that is used:

• On top of the picture is the mandatory ResourceSync Description. It is a Destination's typical entry point to discover a Source's ResourceSync implementation. The ResourceSync Description enumerates all Capability Lists offered by a Source, one Capability List per set of resources. If a Source only offers one set of resources, the ResourceSync Description contains a single pointer. A ResourceSync Description is expressed as a <urlset> document and, per Capability List, a <url> element is introduced. The <loc> child element of <url> contains the URI of the Capability List, and the capabilitylist value for the capability attribute of the <rs:md> child element of <url> makes clear that the URI is that of a Capability List.
• A Capability List enumerates all capabilities supported for a set of the Source's resources. The capabilities available in the pull-based ResourceSync framework are Resource List, Change List, Resource Dump, and Change Dump. A Capability List is expressed as a <urlset> document and, for each supported capability, a <url> element is introduced. The <loc> child element of <url> contains the URI of the document that implements a capability, and the type of capability is expressed by means of the value of the capability attribute of the <rs:md> child element of <url>, e.g. <resourcelist> for a Resource List.
• A Resource List and a Change List point at resources. A representation of a resource can be obtained by dereferencing its URI, listed as the value of the <loc> child element of <url>.
• A Resource Dump and a Change Dump point at packages, each of which contains bitstream representations of resources, as well as a Manifest that describes the bitstreams provided in the package.
• The Manifest contained in a package of bitstreams is expressed as a <urlset> document. For each bitstream contained in the package, that document contains a <url> element; the <loc> child element of <url> provides the URI that corresponds to the bitstream, whereas the path attribute of the <rs:md> child element of <url> provides the path of the bitstream in the package.
• If a single document suffices to express a ResourceSync Description, a Resource List, a Change List etc., then the <urlset> document format is used. If multiple documents are required, each is expressed using the <urlset> document format, and a <sitemapindex> document is introduced as an Index to point at all individual <urlset> documents. As a result, the URI of, for example, a Resource List provided in a Capability List can be either that of a <urlset> or a <sitemapindex> document. The <urlset> or <sitemapindex> documents used for a specific capability (e.g. Resource List) have the same value for the capability attribute (e.g. resourcelist).

The Resource List branch of Figure 3 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.

Figure 3: ResourceSync structural perspective

2.2.4. Discovery Perspective

ResourceSync has provisions that allow a Destination to determine whether and how a Source implements ResourceSync capabilities. This section provides an overview that is summarized by Figure 4, while details are provided in Section 9.

The Source must publish a ResourceSync Description at the well-known URI /.well-known/resourcesync. The ResourceSync Description enumerates Capability Lists, one per distinct set of resources. Each Capability List entry in the ResourceSync Description has a link with a describedby relation type that points at a document that describes the set of resources covered by the respective Capability List.

In order to advertise the capabilities it supports for a set of resources, a Source publishes a Capability List. Such a list has one entry per supported capability, and the URI where the capability can be accessed as well as the capability type is conveyed for each. For example, Figure 4 depicts a Capability List that indicates support for Resource List, Change List, and Resource Dump. A Capability List also contains a link with a describedby relation type that points at a document that describes the set of resources covered by the Capability List. In addition, a Capability List provides a link with a resourcesync relation type to support discovery of the ResourceSync Description.

A Capability List can also be discovered via a link with a resourcesync relation type provided in an (X)HTML page or HTTP Link header. In this case, the linked Capability List must pertain to the resource that provides the link; this means that the resource must be covered by all capabilities listed in the linked Capability List.

A document that conveys a capability, such as a Resource List, can also include a link with a resourcesync relation type to point at the Capability List it resorts under.

Figure 4: ResourceSync discovery perspective

2.2.5. Overview Summary

Table 2.1 provides a summary of this Overview section. The table lists Destination processes as columns and Source capabilities as rows, with cells indicating the applicability of a capability for a given process.

Table 2.1: Source capabilities versus Destination processes

3. 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 is it 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 50MB.

The document formats, as well as their ResourceSync extension elements, are shown in Table 3.1. The <rs:md> and <rs:ln> elements are introduced to express metadata and links, respectively. Both are in the ResourceSync XML Namespace and can have attributes. The attributes defined in this namespace are listed in Table 3.2 and detailed below. To improve readability of this document, the attributes in all examples are shown without the default ResourceSync 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 3.2 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.

<?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="http://www.sitemaps.org/schemas/sitemap/0.9"
              xmlns:rs="http://www.openarchives.org/rs/terms/">
  <rs:md />
  <rs:ln />
  <sitemap>
      <loc />
      <lastmod />
      <rs:md />
      <rs:ln />
  </sitemap>
  <sitemap>
      ...
  </sitemap>
</sitemapindex>
  

Table 3.1: 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; they have one mandatory and one optional child element:
  • <rs:md> - In this context, the element conveys information about the document itself. Its use is mandatory and it can have the following attributes:
    • capability - This attribute is mandatory in all ResourceSync documents. The value of the attribute conveys the nature of the document, e.g. whether the document is a Resource List, a Change List, a Manifest, etc. Defined values are resourcelist, changelist, resourcedump, changedump, resourcedump-manifest, changedump-manifest, capabilitylist, and resourcesync.
    • from - When used without the until attribute, the value of the from attribute conveys the datetime at which the document was valid. It represents the time of a snapshot such as t1, t2, t3, and t4 of Figure 1. This attribute is mandatory for Resource Lists, Resource List Indexes, Resource Dumps, Resource Dump Indexes, and Resource Dump Manifests. The attribute value is expressed as a W3C Datetime and the use of a complete date and time expressed in UTC using the format YYYY-MM-DDThh:mm:ss[.s]Z is recommended.
    • from and until - When used together, the values of these attributes convey the start and end of a temporal interval. The value of the until attribute must be equal to or more recent than the value of the from attribute. Both attributes are mandatory in Change Lists, Change List Indexes, Change Dumps, Change Dump Indexes, and Change Dump Manifests. The attribute values guarantee that all resource changes that occurred within the expressed temporal interval are listed in the document. For example, the first Change List in Figure 1 would have a from value of t1 and an until value of t3 and the second Change List would have a from value of t3 and an until value of t5. The values are expressed as a W3C Datetime and the use of complete dates and times expressed in UTC using the format YYYY-MM-DDThh:mm:ss[.s]Z is recommended.
  • <rs:ln> - An optional and repeatable element used to support discovery of other documents by means of a link. It can have several attributes and the ones defined in the ResourceSync XML Namespace are as follows:
    • href - A mandatory attribute to convey the URI of the document.
    • rel - A mandatory attribute to express a relationship. The following values are explicitly used in this specification:
      • describedby - for linking from a Capability List to a document that describes the set of resources covered by it, and from a ResourceSync Description to a document that describes the Source.
      • resourcesync - for linking from a Capability List to the ResourceSync Description and from a document that conveys a capability, such as a Resource List, to the Capability List it resorts under.
      • up - for linking from a document that conveys a capability (e.g. a Resource List) to a parent Index document (e.g. a Resource List Index).
  • <url> or <sitemap> - The <urlset> element should have zero or more <url> child elements, and the <sitemapindex> element has zero or more <sitemap> child elements. Each such child element is used to convey information about a resource that plays a role in the ResourceSync framework. They can have the following child elements:
    • <loc> - A mandatory element that conveys the URI of the resource that plays a role in the ResourceSync framework.
    • <lastmod> - An element that conveys the last modification time of the resource with the URI provided in <loc>, expressed as a W3C Datetime as described above. Its use is optional in some, and mandatory in other documents.
    • <changefreq> - An optional element that provides a hint about the change frequency of the resource with the URI provided in <loc>. Defined values are always, hourly, daily, weekly, monthly, yearly, and never. The value always should be used for resources that change each time they are accessed. The value never should be used for archived resources.
    • <rs:md> - In this context, the element conveys metadata pertaining to the resource with the URI provided in <loc>. The element is not repeatable, and is mandatory for some documents and optional for others. It can have several attributes and the ones defined in the ResourceSync XML Namespace are as follows:
      • capability - This attribute is mandatory in ResourceSync Descriptions and Capability Lists. Its value indicates the nature of the resource identified by the URI in the <loc> element e.g., a Resource List, a Change List, a Change Dump, etc. Defined values are resourcelist, changelist, resourcedump, changedump, and capabilitylist.
      • change - The value of the attribute conveys the type of change that a resource underwent. Defined values are created, updated, and deleted to convey the creation, update, and deletion of a resource, respectively. This attribute is used in Change Lists and Change Dump Manifests.
      • encoding - The value of the attribute conveys what content codings have been applied to the resource. The value of the encoding attribute should be equal to the value of the content-encoding header in the HTTP response as defined in RFC 2616, Sec. 14.11.
      • hash - The value of the attribute conveys fixity information for a resource representation returned when the URI in <loc> is dereferenced. The attribute value is expressed in the form of a whitespace-delimited list of hash values. Each hash value is represented by a hex-encoded digest and is preceded by a token that identifies the utilized hash algorithm, e.g. md5:, sha-256:.
      • length - The value of the attribute conveys the content length of a resource representation returned when the URI in <loc> is dereferenced. The value of the length attribute should be equal to the value of the Content-Length header in the HTTP response and must be computed as defined in RFC 2616, Sec. 4.4.
      • path - The attribute is only used in Resource Dump Manifests and Change Dump Manifests. Its value conveys the file path of the bitstream associated with the URI in <loc> in the ZIP file. That is the relative file system path where the bitstream would reside if the ZIP were unpacked.
      • type - The value of the attribute conveys the Media Type of a resource representation returned when the URI in <loc> is dereferenced. Registered values are listed in the IANA Media Type registry.
    • <rs:ln> - In this context, an optional and repeatable element used to link to resources related to the one with the URI provided in <loc>, such as a copy on a mirror site, a prior version of the resource, etc. (see Linking to Related Resources in Section 2.2.1). It can have several attributes and the ones defined in the ResourceSync XML Namespace are as follows:
      • href - A mandatory attribute to convey the URI of the related resource.
      • rel - A mandatory attribute to convey the relationship between the resource with the URI in <loc> and the one with the URI in href. Values for the rel attribute featured in this specification are:
        • duplicate - for a link to a resource's mirror location.
        • alternate and canonical - for links to alternate representations of a resource.
        • http://www.openarchives.org/rs/terms/patch - for a link detailing the difference between changed resources.
        • describedby and describes - for links providing additional information about a resource.
        • memento and timegate - for links to access prior versions of a resource.
        • collection - for links that express collection membership.
        • via - for links providing provenance information.
        The relation type values used in the ResourceSync framework are summarized in Table 3.3.
      • encoding, hash, length, modified, path, type - Optional attributes with meanings as described above and pertaining to the related resource.
      • pri - An optional attribute used to express a priority among links with the same relation type. The attribute value is an integer between 1 and 999,999, with a lower integer indicating a higher priority and the absence of the attribute indicating a value of 999,999.

Table 3.2 lists the elements used in ResourceSync documents and for each shows the attributes in the ResourceSync XML Namespace that can 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 can 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.

Table 3.3 summarizes the relation types used in the ResourceSync framework. Other relation types registered in the IANA Link Relation Type Registry or expressed as URIs as specified in RFC 5988, Sec. 4.2 may be used.

Table 3.2: Elements and associated attributes defined for the ResourceSync documents

Table 3.3: Relation types for the <rs:ln> elements

4. 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.

4.1. 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:

• The mandatory <rs:md> child element of <urlset> must have a capability attribute that has a value of resourcelist and it must have a from attribute that conveys the time the Resource List was valid.
• A recommended <rs:ln> child element of <urlset> points to the Capability List with the relation type resourcesync (see Section 10).
• One <url> child element of <urlset> per resource. This element does not have attributes, but uses child elements to convey information about the resource. The <url> element has the following child elements:
  • A mandatory <loc> child element provides the URI of the resource.
  • A recommended <lastmod> child element and an optional <changefreq> element with semantics as described in Section 3.
  • An optional <rs:md> child element provides further metadata about the resource. It can have the attributes hash, length, and type, as described in Section 3.
  • Optional <rs:ln> child elements link to related resources as detailed in Section 8.

Example 4.1 shows a Resource List with two resources.

<?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="resourcesync"
         href="http://example.com/dataset1/capabilitylist.xml"/>
  <rs:md capability="resourcelist"
         from="2013-01-03T09:00:00Z"/>
  <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 4.1: A Resource List

4.2. 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 particular point in time - the creation time of the Resource List(s).

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 mandatory <rs:md> child element of <sitemapindex> must have a capability attribute that has a value of resourcelist and it must have a from attribute that conveys the time the Resource List Index was valid.
• A recommended <rs:ln> child element of <sitemapindex> points to the Capability List with the relation type resourcesync (see Section 10).
• One <sitemap> child element of <sitemapindex> per Resource List. This element does not have attributes, but uses child elements to convey information about the Resource List. The <sitemap> element has the following child elements:
  • A mandatory <loc> child element provides the URI of the Resource List.
  • A recommended <lastmod> child element with semantics as described in Section 3.

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 is shown in Example 4.2.

<?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="resourcesync"
         href="http://example.com/dataset1/capabilitylist.xml"/>
  <rs:md capability="resourcelist"
         from="2013-01-03T09:00:00Z"/>
  <sitemap>
      <loc>http://example.com/resourcelist3.xml</loc>
      <lastmod>2013-01-03T09:00:00Z</lastmod>
  </sitemap>
  <sitemap>
      <loc>http://example.com/resourcelist2.xml</loc>
      <lastmod>2013-01-03T09:00:00Z</lastmod>
  </sitemap>
  <sitemap>
      <loc>http://example.com/resourcelist1.xml</loc>
      <lastmod>2013-01-03T09:00:00Z</lastmod>
  </sitemap>
</sitemapindex>

Example 4.2: A Resource List Index

Example 4.2 refers to three Resource Lists identified by:

• http://example.com/resourcelist3.xml, and
• http://example.com/resourcelist2.xml, and
• http://example.com/resourcelist1.xml.

Example 4.3 shows the content of the Resource List identified by the URI http://example.com/resourcelist3.xml. Structurally, it is identical to the Resource List shown in Example 4.1 but it contains an additional <rs:ln> child element of <urlset> that provides a navigational link with the relation type up to the parent Resource List Index shown in Example 4.2. This link is meant to ease navigation for Destinations and their adoption is therefore strongly 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="resourcesync"
         href="http://example.com/dataset1/capabilitylist.xml"/>
  <rs:ln rel="up"
         href="http://example.com/dataset1/resourcelist-index.xml"/>
  <rs:md capability="resourcelist"
         from="2013-01-03T09:00:00Z"/>
  <url>
      <loc>http://example.com/res3</loc>
      <lastmod>2013-01-03T09:00:00Z</lastmod>
      <rs:md hash="md5:1584abdf8ebdc9802ac0c6a7402c8753"
             length="4385"
             type="application/pdf"/>
  </url>
  <url>
      <loc>http://example.com/res4</loc>
      <lastmod>2013-01-03T09:00:00Z</lastmod>
      <rs:md hash="md5:4556abdf8ebdc9802ac0c6a7402c9881"
             length="883"
             type="image/png"/>
  </url>
</urlset>

Example 4.3: A Resource List with a navigational link to its parent Resource List Index

5. Packaging Resources

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

5.1. Resource Dump

A Source can publish a Resource Dump, which provides links to packages of the resources' bitstreams. The Resource Dump represents the Source's state at a particular 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. A typical scenario in which a Destination would obtain a Resource Dump is the Baseline Synchronization process.

The ResourceSync framework specifies the use of the ZIP file format as the packaging format for content packages made discoverable by a Resource Dump. Other communities can define their own packaging format but one format should be consistently used throughout a Resource Dump.

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:

• The mandatory <rs:md> child element of <urlset> must have a capability attribute that has a value of resourcedump and it must have a from attribute that conveys the time the Resource Dumps was valid.
• A recommended <rs:ln> child element of <urlset> points to the Capability List with the relation type resourcesync (see Section 10).
• One <url> child element of <urlset> per ZIP package. This element does not have attributes, but uses child elements to convey information about the package. The <url> element has the following child elements:
  • A mandatory <loc> child element provides the URI of the package.
  • A recommended <lastmod> child element with semantics as described in Section 3.
  • A recommended <rs:md> child element with the type attribute to convey the Media Type of the package and the length attribute to convey the length of the package. The child element may further have attributes such as hash and type, as described in Section 3.

Example 5.1 shows a Resource Dump document that points to three ZIP files.

<?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="resourcesync"
         href="http://example.com/dataset1/capabilitylist.xml"/>
  <rs:md capability="resourcedump"
         from="2013-01-03T09:00:00Z"/>
  <url>
      <loc>http://example.com/resourcedump-part3.zip</loc>
      <lastmod>2013-01-03T09:00:00Z</lastmod>
      <rs:md type="application/zip"
             length="4765"/>
  </url>
  <url>
      <loc>http://example.com/resourcedump-part2.zip</loc>
      <lastmod>2013-01-03T09:00:00Z</lastmod>
      <rs:md type="application/zip"
             length="9875"/>
  </url>
  <url>
      <loc>http://example.com/resourcedump-part1.zip</loc>
      <lastmod>2013-01-03T09:00:00Z</lastmod>
      <rs:md type="application/zip"
             length="2298"/>
  </url>
</urlset>

Example 5.1: A Resource Dump document

If a Source needs to or chooses to publish and group multiple Resource Dumps, it needs to establish a Resource Dump Index, in a manner that is similar to what was described in Section 4.2.

5.1.1. Resource Dump Manifest

Each content 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:

• The mandatory <rs:md> child element of <urlset> must have a capability attribute with a value of resourcedump-manifest and it must have a from attribute that conveys the time the Resource Dump Manifest was valid.
• A recommended <rs:ln> child element of <urlset> points to the Capability List with the relation type resourcesync (see Section 10).
• One <url> child element of <urlset> per bitstream. This element does not have attributes, but uses child elements to convey information about the bitstream. The <url> element has the following child elements:
  • A mandatory <loc> child element provides the URI which the Source associates with the bitstream.
  • A recommended <lastmod> child element and an optional <changefreq> element with semantics as described in Section 3.
  • A mandatory <rs:md> child element must have a path attribute to convey the location of the bitstream within the package. The value of the attribute is relative to root of the package and it is expressed with a leading slash (/). The use of the type attribute in the <rs:md> element is strongly recommended to help Destinations determine the Media Type of the bitstream. The <rs:md> element can further have the attributes hash and length, as described in Section 3.
  • Optional <rs:ln> child elements link to related resources as detailed in Section 8.

<?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="resourcesync"
         href="http://example.com/dataset1/capabilitylist.xml"/>
  <rs:md capability="resourcedump-manifest"
         from="2013-01-03T09:00:00Z"/>
  <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 5.2: A Resource Dump Manifest

6. Describing Content Changes

A Source may publish a record of the changes to its content over a period of time. This enables Destinations to efficiently follow the changes and hence to synchronize incrementally.

6.1. Change List

A Change List is a document that contains a description of changes to the resources at a Source. If a resource underwent multiple changes, it will be listed once per change. It is up to the Source to determine the frequency with which it publishes or updates Change Lists and also the time period that the Change List covers. A Source may choose to publish some number of recent changes, or only the changes from the a particular period, such as the last day or week.

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 mandatory <rs:md> child element of <urlset> must have a capability attribute that has a value of changelist. It also must have both the from and the until attributes to convey the temporal interval covered by the Change List: all changes that occurred in the expressed interval must be listed.
• A recommended <rs:ln> child element of <urlset> points to the Capability List with the relation type resourcesync (see Section 10).
• One <url> child element of <urlset> per changed resource. This element does not have attributes, but uses child elements to convey information about the changed resource. The <url> element has the following child elements:
  • A mandatory <loc> child element provides the URI of the changed resource.
  • A mandatory <lastmod> child element with semantics as described in Section 3.
  • A mandatory <rs:md> child element must have the attribute change to convey the nature of the change. It may take values created, updated, and deleted. It can further have attributes hash, length, and type, as described in Section 3.
  • Optional <rs:ln> child elements link to related resources as detailed in Section 8.

The datetime of the resource change can be used by Destinations to determine if it has already been processed. A Destination can walk through the Change List until it reaches a datetime before it last requested the Change List, and then start processing the new changes in order of their occurrence. In the same manner as processing a Resource List, the Destination can retrieve a representation of the resource by dereferencing its URI.

All entries in a Change List must be in forward chronological order. In particular, 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. This ordering supports Destinations in processing the changes, however, sophisticated Destinations may reorder the Change List to avoid unnecessary processing, for example, only processing the most recent change to a resource.

Example 6.1 shows the content of a Change List with changes to three resources. The example shows one creation, one update, and one deletion and the changes are in forward chronological order. One resource underwent two changes and hence is listed twice.

<?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="resourcesync"
         href="http://example.com/dataset1/capabilitylist.xml"/>
  <rs:md capability="changelist"
         from="2013-01-01T11:00:00Z"
         until="2013-01-03T11:00:00Z"/>
  <url>
      <loc>http://example.com/res1.html</loc>
      <lastmod>2013-01-02T11:00:00Z</lastmod>
      <rs:md change="created"/>
  </url>
  <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>
  <url>
      <loc>http://example.com/res2.pdf</loc>
      <lastmod>2013-01-02T21:00:00Z</lastmod>
      <rs:md change="updated"/>
  </url>
</urlset>

Example 6.1: A Change List describing four resource changes

A unique identifier for each change might be useful in some situations. No explicit identity is defined in this specification, but the combination of the content in the <loc> and of the <lastmod> elements of the <url> element is recommended for this purpose. The Source is responsible for providing a sufficiently granular time for the content of the <lastmod> element to ensure that this combination results in a truly unique identifier.

If a Source needs to publish and group multiple Change Lists, it needs to establish a Change List Index, in a manner similar to what was described in Section 4.2.

7. Packaging Changes

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

7.1. Change Dump

To make content changes available for download, a Source can publish Change Dumps that refer to packages of the changed bitstreams. Similar to Change Lists, it is up to the Source to determine the time period a Change Dump covers or how many bitstreams are contained in each package. Each package is a ZIP file that contains bitstreams of the resources after each change.

The ResourceSync framework specifies the use of the ZIP file format as the packaging format for content packages made discoverable by a Change Dump. Other communities can define their own packaging format but one format should be consistently used throughout a Change Dump.

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:

• The mandatory <rs:md> child element of <urlset> must have a capability attribute that has a value of changedump. It also must have both the from and the until attributes to convey the temporal interval covered by the Change Dump: all changes that occurred during the specified interval must be included in the package(s) of bitstreams pointed at by the Change Dump.
• A recommended <rs:ln> child element of <urlset> points to the Capability List with the relation type resourcesync (see Section 10).
• One <url> child element of <urlset> per ZIP package of changed content. This element does not have attributes, but uses child elements to convey information about the package of changed content. The <url> element has the following child elements:
  • A mandatory <loc> child element provides the URI of the package.
  • A mandatory <lastmod> child element with semantics as described in Section 3.
  • A recommended <rs:md> child element with a type attribute to convey the Media Type of the package and a length attribute to convey the length of the package. It may further have the attributes hash and type, as described in Section 3.

Example 7.1 shows a Change Dump document with three pointers to packages of changed content.

<?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="resourcesync"
         href="http://example.com/dataset1/capabilitylist.xml"/>
  <rs:md capability="changedump"
         from="2013-01-01T11:00:00Z"
         until="2013-01-08T11:00:00Z"/>
  <url>
      <loc>http://example.com/changedump-part3.zip</loc>
      <lastmod>2013-01-03T09:00:00Z</lastmod>
      <rs:md type="application/zip"
             length="3109"/>
  </url>
  <url>
      <loc>http://example.com/changedump-part2.zip</loc>
      <lastmod>2013-01-03T09:00:00Z</lastmod>
      <rs:md type="application/zip"
             length="6629"/>
  </url>
  <url>
      <loc>http://example.com/changedump-part1.zip</loc>
      <lastmod>2013-01-03T09:00:00Z</lastmod>
      <rs:md type="application/zip"
             length="8124"/>
  </url>
</urlset>

Example 7.1: A Change Dump

If a Source needs to or chooses to publish and group multiple Change Dumps, it needs to establish a Change Dump Index, in a manner similar to what was described in Section 4.2.

7.1.1. Change Dump Manifest

Each package of changed content referred to from a Change Dump must contain a Change Dump Manifest file that describes the file's constituent bitstreams. The file has to be named manifest.xml and has to be packaged at the top level of the ZIP package. Like in a Change List, all entries in a Change Dump Manifest must be in forward chronological order, meaning the document starts with a reference to the least recently changed bitstream and ends with a reference to the most recently changed bitstream.

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

• The mandatory <rs:md> child element of <urlset> must have a capability attribute that has a value of changedump-manifest. It also must have both the from and the until attributes to convey the temporal interval covered by the Change Dump Manifest: all changes that occurred during the specified interval must be included in the Change Dump Manifest.
• A recommended <rs:ln> child element of <urlset> points to the Capability List with the relation type resourcesync (see Section 10).
• One <url> child element of <urlset> per changed bitstream. This element does not have attributes, but uses child elements to convey information about the bitstream. The <url> element has the following child elements:
  • A mandatory <loc> child element provides the URI which the Source associates with the changed bitstream.
  • A mandatory <lastmod> child element with semantics as described in Section 3.
  • A mandatory <rs:md> child element must have a change attribute to convey the type of change to the resource. It may take values created, updated, and deleted. It also must have a path attribute to convey the location of the bitstream within the ZIP package. The path is relative to root of the package and it is expressed with a leading slash (/). The use of the type attribute in the <rs:md> element is strongly recommended to help Destinations anticipate the Media Type of the bitstream. The <rs:md> element can further have the attributes hash and length, as described in Section 3.
  • Optional <rs:ln> child elements link to related resources as detailed in Section 8.

A Change Dump Manifest with a total of four changes to three resources is shown in Example 7.2. The resource identified by the URI http://example.com/res1.html is included twice. It was first created and later updated, which accounts for the two changes. While the URI in <loc> child element is the same, the path attribute of the <rs:md> child element refers to a different bitstream for each.

<?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="resourcesync"
         href="http://example.com/dataset1/capabilitylist.xml"/>
  <rs:md capability="changedump-manifest"
         from="2013-01-01T11:00:00Z"
         until="2013-01-03T11:00:00Z"/>
  <url>
      <loc>http://example.com/res1.html</loc>
      <lastmod>2013-01-01T15:00:00Z</lastmod>
      <rs:md change="created"
             hash="md5:1c1b0e264fa9b7e1e9aa6f9db8d6362b"
             length="4339"
             type="text/html"
             path="/changes/res1.html"/>
  </url>
  <url>
      <loc>http://example.com/res2.pdf</loc>
      <lastmod>2013-01-01T19:00:00Z</lastmod>
      <rs:md change="updated"
             hash="md5:f906610c3d4aa745cb2b986f25b37c5a
                   sha-256:f138185cddef488264a0323aee56e7647e89cd7a4d6e45ba28b3be26234a6d09"
             length="38297"
             type="application/pdf"
             path="/changes/res2.pdf"/>
  </url>
  <url>
      <loc>http://example.com/res3.tiff</loc>
      <lastmod>2013-01-02T11:00:00Z</lastmod>
      <rs:md change="deleted"/>
  </url>
  <url>
      <loc>http://example.com/res1.html</loc>
      <lastmod>2013-01-02T14:00:00Z</lastmod>
      <rs:md change="updated"
             hash="md5:0988647082c8bc51778894a48ec3b576"
             length="5426"
             type="text/html"
             path="/changes/res1-v2.html"/>
  </url>
</urlset>

Example 7.2: A Change Dump Manifest

8. Linking to Related Resources

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 can 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:

• The Source makes the resource content available at a mirror location;
• The Source provides alternate representations for the resource;
• The Source details the difference between the current and the previous version of the resource;
• The Source makes a resource as well as metadata about the resource available for synchronization;
• The Source provides access to prior versions of the resource;
• The Source provides collection membership information about the resource;
• A Destination republishes a resource obtained from a Source, makes that republished resource available for synchronization (i.e. acts as a Source itself), and links to the original resource that it republished.

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 3. 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 a <rs:ln> element, it should refrain from accessing the related resource and rather use the URI provided in <loc> to retrieve the resource.

8.1. Mirrored Content

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

• A mandatory rel attribute with a value of duplicate.
• A mandatory href attribute that conveys the URI of the mirrored resource.
• An optional pri attribute to express a prioritization among multiple mirror locations, each expressed by means of an individual <rs:ln> element. The use of pri is detailed in Section 3.
• Other attributes are as described for the <rs:ln> child element of <url> in Section 3.

Example 8.1 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="resourcesync"
         href="http://example.com/dataset1/capabilitylist.xml"/>
  <rs:md capability="changelist"
         from="2013-01-01T11:00:00Z"
         until="2013-01-03T11:00:00Z"/>
  <url>
      <loc>http://example.com/res1</loc>
      <lastmod>2013-01-02T17:00:00Z</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="2013-01-02T18:00:00Z"/>
      <rs:ln rel="duplicate"
             pri="2"
             href="http://mirror2.example.com/res1"
             modified="2013-01-02T18:30:00Z"/>
      <rs:ln rel="duplicate"
             pri="3"
             href="gsiftp://gridftp.example.com/res1"
             modified="2013-01-02T18:30:00Z"/>
  </url>
</urlset>

Example 8.1: Mirrored content

8.2. 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 can 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, a <rs:ln> element is introduced. This element has the following attributes:

• A mandatory rel attribute with a value of alternate.
• A mandatory href attribute that conveys the specific URI of the alternate representation of the resource.
• A recommended type attribute that conveys the Media Type of the alternate representation.
• Other attributes are as described for the <rs:ln> child element of <url> in Section 3.

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 8.2 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="resourcesync"
         href="http://example.com/dataset1/capabilitylist.xml"/>
  <rs:md capability="changelist"
         from="2013-01-01T11:00:00Z"
         until="2013-01-03T11:00:00Z"/>
  <url>
      <loc>http://example.com/res1</loc>
      <lastmod>2013-01-02T18:00:00Z</lastmod>
      <rs:md change="updated"/>
      <rs:ln rel="alternate"
             href="http://example.com/res1.html"
             modified="2013-01-02T18:00:00Z"
             type="text/html"/>
      <rs:ln rel="alternate"
             href="http://example.com/res1.pdf"
             modified="2013-01-02T18:00:00Z"
             type="application/pdf"/>
  </url>
</urlset>

Example 8.2: 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, can be provided using a <rs:ln> element. This element has the following attributes:

• A mandatory rel attribute with a value of canonical.
• A mandatory href attribute that conveys the generic URI associated with the specific URI provided in <loc>.
• Other attributes are as described for the <rs:ln> child element of <url> in Section 3.

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

Example 8.3 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="resourcesync"
         href="http://example.com/dataset1/capabilitylist.xml"/>
  <rs:md capability="changelist"
         from="2013-01-01T11:00:00Z"
         until="2013-01-03T11:00:00Z"/>
  <url>
      <loc>http://example.com/res1.html</loc>
      <lastmod>2013-01-02T18:00:00Z</lastmod>
      <rs:md change="updated"
             hash="md5:1584abdf8ebdc9802ac0c6a7402c03b6"
             length="8876"/>
      <rs:ln rel="canonical"
             href="http://example.com/res1"
             modified="2013-01-02T18:00:00Z"/>
  </url>
</urlset>

Example 8.3: Specific URI and alternate with generic URI

8.3. Patching Content

In order to increase the efficiency of updating a resource, a Source may make a description of the changes that the resource underwent available, 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 can express that it makes a description of resource changes available by providing the URI of the resource in <loc>, as usual, and by introducing a <rs:ln> element with the following attributes:

• A mandatory rel attribute with a value of http://www.openarchives.org/rs/terms/patch.
• A mandatory href attribute that conveys the URI of the description of the resource changes.
• A mandatory type attribute that conveys the Media Type of the change description. That Media Type must be such that it allows to unambiguously apply the described changes to the previous version of the resource to construct the current one.
• Other attributes are as described for the <rs:ln> child element of <url> in Section 3.

Example 8.4 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 8.4) 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="resourcesync"
         href="http://example.com/dataset1/capabilitylist.xml"/>
  <rs:md capability="changelist"
         from="2013-01-01T11:00:00Z"
         until="2013-01-03T11:00:00Z"/>
  <url>
      <loc>http://example.com/res4</loc>
      <lastmod>2013-01-02T17:00:00Z</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="2013-01-02T17:00:00Z"
             hash="sha-256:y66dER_t_HWEIKpesdkeb7rtSc-ippjf9823742opld"
             length="73"
             type="application/json-patch"/>
  </url>
  <url>
      <loc>http://example.com/res5-full.tiff</loc>
      <lastmod>2013-01-02T18:00:00Z</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="2013-01-02T18:00:00Z"
             hash="sha-256:h986gT_t_87HTkjHYE76G558hY-jdfgy76t55sadJUYT"
             length="4533"
             type="application/x-tiff-diff"/>
  </url>
</urlset>

Example 8.4: A Change List with links to document that detail how to patch resources

8.4. 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 a <rs:ln> element with appropriate relation types added to each block. The <rs:ln> element has the following attributes:

• A mandatory rel attribute. When pointing from a resource to metadata that describes it, its value is describedby; when pointing from metadata to the resource described by the metadata, its value is describes.
• A mandatory href attribute. When pointing from a resource to metadata that describes it, its value is the URI of the metadata resource; when pointing from metadata to the resource described by it, the value is the URI of the described resource.
• Other attributes are as described for the <rs:ln> child element of <url> in Section 3.

Example 8.5 shows how a Source can express this inter-relationship between the two resources.

<?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="resourcesync"
         href="http://example.com/dataset1/capabilitylist.xml"/>
  <rs:md capability="changelist"
         from="2013-01-01T11:00:00Z"
         until="2013-01-03T11:00:00Z"/>
  <url>
      <loc>http://example.com/res2.pdf</loc>
      <lastmod>2013-01-02T18:00:00Z</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="2013-01-02T18:00:00Z"
             type="application/xml"/>
  </url>
  <url>
      <loc>http://example.com/res2_dublin-core_metadata.xml</loc>
      <lastmod>2013-01-02T19:00:00Z</lastmod>
      <rs:md change="updated"
             type="application/xml"/>
      <rs:ln rel="describes"
             href="http://example.com/res2.pdf"
             modified="2013-01-02T19:00:00Z"
             hash="md5:1584abdf8ebdc9802ac0c6a7402c03b6"
             length="8876"
             type="application/pdf"/>
  </url>
</urlset>

Example 8.5: Linking between a resource and metadata about a resource in a Change List

8.5. 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:

• The existence of a time-generic URI from which, at any moment in time, the current representation of the resource is available.
• The existence of a time-specific URI for each resource version, from which a specific temporal version of the resource is available.

When communicating about the resource, its time-generic URI is provided in <loc>.

A first approach consists of conveying the time-specific URI of the resource for the moment the communication about it takes place. This is achieved by introducing a <rs:ln> element with the following attributes:

• A mandatory rel attribute with a value of memento.
• A mandatory href attribute that conveys the time-specific URI of the resource at the moment of communication. This URI allows a Destination to obtain that specific version during a catch-up operation, for example because it had been offline, even if the resource has meanwhile changed again.
• Other attributes are as described for the <rs:ln> child element of <url> in Section 3. It is recommended to include the last modification and fixity information for both the time-generic and the time-specific URI as doing so unambiguously conveys the tight temporal relationship between both.

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 to obtain the version as it existed at the moment of communication about the resource by using the <lastmod> value for datetime negotiation, but it also allows obtaining other versions by using different datetime values. A pointer to a TimeGate is introduced by using a <rs:ln> element with the following attributes:

• A mandatory rel attribute with a value of timegate.
• A mandatory href attribute that conveys the URI of TimeGate associated with the time-generic resource.
• The other attributes described for the <rs:ln> child element of <url> in Section 3 should not be used as they are meaningless for TimeGates.

Example 8.6 shows a Change List with a link to a prior version of a resource as well as a link to a Timegate. 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="resourcesync"
         href="http://example.com/dataset1/capabilitylist.xml"/>
  <rs:md capability="changelist"
         from="2013-01-01T11:00:00Z"
         until="2013-01-03T11:00:00Z"/>
  <url>
      <loc>http://example.com/res1</loc>
      <lastmod>2013-01-02T18:00:00Z</lastmod>
      <rs:md hash="md5:1584abdf8ebdc9802ac0c6a7402c03b6"
             length="8876"
             type="text/html"
             change="updated"/>
      <rs:ln rel="memento"
             href="http://example.com/20130103070000/res1"
             modified="2013-01-02T19:00:00Z"
             hash="md5:1584abdf8ebdc9802ac0c6a7402c03b6"
             length="8876"
             type="text/html"/>
      <rs:ln rel="timegate"
             href="http://example.com/timegate/http://example.com/res1"/>
  </url>
</urlset>

Example 8.6: Links to a resource version and a Memento TimeGate

8.6. Collection Membership

A Source can express that a resource is a member of a collection such as an OAI-ORE Aggregation or an OAI-PMH Set. A Source can 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 a <rs:ln> element with the following attributes:

• A mandatory rel attribute with a value of collection.
• A mandatory href attribute that conveys the URI that identifies the collection.
• Other attributes are as described for the <rs:ln> child element of <url> in Section 3.

Example 8.7 shows a Change List with one resources 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="resourcesync"
         href="http://example.com/dataset1/capabilitylist.xml"/>
  <rs:md capability="changelist"
         from="2013-01-01T11:00:00Z"
         until="2013-01-03T11:00:00Z"/>
  <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 8.7: A resource as a member of a collections

8.7. 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 from which the Aggregator obtained content. When communicating about a republished resource, the Aggregator can provide such provenance information by introducing a <rs:ln> element with the following attributes:

• A mandatory rel attribute with a value of via.
• A mandatory href attribute that conveys the URI of the resource at the Source from which the Aggregator obtained the content.
• Other attributes are as described for the <rs:ln> child element of <url> in Section 3.

If a chain of such aggregations takes place, existing via links should be maintained and additional ones should be added in order to allow tracing the entire provenance chain. This is shown in examples 8.8, 8.9, and 8.10 that illustrate a process that starts with an original Source that publishes a Change List. A first Aggregator consumes that Change List, obtains the changed resource and integrates it into its own collection. It then publishes its own Change List that includes the description of the change but also gives credit to where the change originated from. The cycle repeats as a second Aggregator consumes the Change List from the first. Note the datetimes in all three examples.

<?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="resourcesync"
         href="http://example.com/dataset1/capabilitylist.xml"/>
  <rs:md capability="changelist"
         from="2013-01-01T11:00:00Z"
         until="2013-01-03T11:00:00Z"/>
  <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 8.8: An original Source publishes

The example below shows a primary Aggregator's Change List that refers to the original Source's 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="resourcesync"
         href="http://aggregator1.example.com/dataset1/capabilitylist.xml"/>
  <rs:md capability="changelist"
         from="2013-01-01T11:00:00Z"
         until="2013-01-03T21:00:00Z"/>
  <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 8.9: A primary aggregator republishes

A second Aggregator obtains the changed resource as it consumes the Change List of the primary Aggregator and republishes its Change List where it adds yet another <rs:ln> child element to convey the original Source from its perspective. The values of the from and until attributes are always from the perspective of the Source that publishes the capability document. Hence it is possible, as seen in Example 8.10, that the from datetime of the Change List is more recent than the <lastmod> datetime of the original Source's resource, which is conveyed using an <rs:ln> link with the via relation type.

<?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="resourcesync"
         href="http://aggregator2.example.com/dataset1/capabilitylist.xml"/>
  <rs:md capability="changelist"
         from="2013-01-03T11:00:00Z"
         until="2013-01-04T11:00:00Z"/>
  <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"/>
      <rs:ln rel="via"
             href="http://aggregator1.example.com/res1.html"
             modified="2013-01-03T20:00:00Z"
             hash="md5:1584abdf8ebdc9802ac0c6a7402c03b6"
             length="8876"
             type="text/html"/>
  </url>
</urlset>

Example 8.10: A second aggregator republishes

9. Advertising Capabilities

In order to make use of the capabilities that a Source provides, it is first necessary to determine which capabilities are supported and what the URIs of the respective capability documents are. To that end, the ResourceSync framework introduces a ResourceSync Description and Capability Lists and various ways to discover them.

9.1. ResourceSync Description

A ResourceSync 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 ResourceSync Description will enumerate as many Capability Lists as the Source has distinct sets of resources.

The ResourceSync Description can only be based on the <urlset> format. It has the <urlset> root element and the following structure:

• The mandatory <rs:md> child element of <urlset> must have a capability attribute that has a value of resourcesync.
• A recommended <rs:ln> child element of <urlset> with the relation type describedby points to a document that provides information about the Source.
• One <url> child element of <urlset> per Capability List offered by the Source. This element does not have attributes, but uses child elements to convey information about the Capability Lists. The <url> element has the following child elements:
  • A mandatory <loc> child element provides the URI of the respective Capability List document.
  • A mandatory <rs:md> child element must have a capability attribute to convey that the URI points to a Capability List.
  • A recommended <rs:ln> child element with a describedby relation type points to a document that describes the set of resources described by the Capability List.

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

Example 9.1 shows a ResourceSync 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"
         type="application/xml"/>
  <rs:md capability="resourcesync"/>
  <url>
      <loc>http://example.com/capabilitylist1.xml</loc>
      <rs:md capability="capabilitylist"/>
      <rs:ln href="http://example.com/info_about_set1_of_resources.xml"
             rel="describedby"/>
  </url>
  <url>
      <loc>http://example.com/capabilitylist2.xml</loc>
      <rs:md capability="capabilitylist"/>
      <rs:ln href="http://example.com/info_about_set2_of_resources.xml"
             rel="describedby"/>
  </url>
  <url>
      <loc>http://example.com/capabilitylist3.xml</loc>
      <rs:md capability="capabilitylist"/>
      <rs:ln href="http://example.com/info_about_set3_of_resources.xml"
             rel="describedby"/>
  </url>
</urlset>

Example 9.1: A ResourceSync Description

If a Source needs to or chooses to publish and group multiple ResourceSync Descriptions, it needs to establish a ResourceSync Description Index, in a manner that is similar to what was described in Section 4.2.

9.2. Capability List

A Capability List is a document that enumerates all capabilities supported by a Source for one set of resources. The Source defines which resources pertain to which set of resources. In the simplest case, all resources subject to synchronization belong to one and the same set of resources. However, the Source can distinguish sets, for example, between Media Types, collections, resources subject to different change frequencies, or other criteria. If a Source offers multiple sets of resources, it must publish one Capability List for each set.

A Capability List points at the capability documents for the corresponding set of resources: Resource List, Resource Dump, Change List, and Change Dump as introduced in Section 4, Section 5.1, Section 6, and Section 7.1, respectively. A Capability List can only contain one entry per capability.

A resource that is covered by one capability listed in a Capability List must also be covered by all other capabilities that are enumerated in that Capability List. Take, for example, a resource that is included in a Resource List that is pointed at by a Capability List. If that resource changes, and if the Capability List also points at a Change List, then the change to the resource must also be listed in the Change List.

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

• The mandatory <rs:md> child element of <urlset> must have a capability attribute that has a value of capabilitylist.
• A recommended <rs:ln> child element of <urlset> with the relation type resourcesync points to the ResourceSync Description document that enumerates all Capability Lists offered by the Source.
• A recommended <rs:ln> child element of <urlset> with the relation type describedby points to a document that provides information about resources that are subject to synchronization.
• One <url> child element of <urlset> per capability offered by the Source. This element does not have attributes, but uses child elements to convey information about the capabilities. The <url> element has the following child elements:
  • A mandatory <loc> child element provides the URI of the respective capability document.
  • A mandatory <rs:md> child element must have a capability attribute to convey the type of the respective listed capability.

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 9.2 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. Only the capability document must be downloaded and parsed to make this determination.

<?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"
         type="application/xml"/>
  <rs:ln rel="resourcesync"
         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 9.2: A Capability List

If a Source needs to or chooses to publish and group multiple Capability Lists, one for each set of resources, it needs to establish a Capability List Index, in a manner similar to what was described in Section 4.2.

9.3. Discovery

ResourceSync provides two ways for a Destination to discover whether and how a Source supports ResourceSync: a Source-wide approach detailed in Section 9.3.1 and a resource-specific discovery approach detailed in Section 9.3.2 and Section 9.3.3.

9.3.1. ResourceSync Well-Known URI

A Source must publish a ResourceSync Description, such as the one shown in Example 9.1, and it should be published at the well-known URI [RFC 5785] /.well-known/resourcesync defined in this specification. The ResourceSync 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.

9.3.2. X/HTML Link Element

A Capability List can be made discoverable by means of a link in an (X)HTML document. In order to do so, 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 9.4 shows the structure of a web page that contains a link to a Capability List.

As shown in Example 9.2 the ResourceSync Description can be discovered from the Capability List by following the link provided in the <rs:ln> element with the relation type resourcesync.

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

Example 9.4: X/HTML link discovery syntax

9.3.3. HTTP Link Header

A Capability List can be made discoverable by means of an HTTP Link header that can 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 IRI 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 9.5 contains part of an HTTP response header. It includes an HTTP Link header with the relation type resourcesync to make a Capability List that pertains to the resource that provides the Link discoverable.

As shown in Example 9.2 the ResourceSync Description can be discovered from the Capability List by following the link provided in the <rs:ln> element with the relation type resourcesync.

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 9.5: HTTP link discovery syntax

10. References

[Atom Link Extensions]

Atom Link Extensions, J. Snell, 08 June 2012.

[JSON-Patch]

JSON Patch, P.Bryan, M. Nottingham, Draft, January 2013.

[Web Architecture]

Architecture of the World Wide Web, Volume One, I. Jacobs and N. Walsh, Editors, World Wide Web Consortium, 15 January 2004.

[RFC 2616]

IETF RFC 2616 Hypertext Transfer Protocol -- HTTP/1.1, R. Fielding, et al., June 1999.

[RFC 4287]

IETF RFC 4287: The Atom Syndication Format, M. Nottingham, R. Sayre, December 2005.

[RFC 5261]

IETF RFC 5261: An Extensible Markup Language (XML) Patch Operations Framework Utilizing XML Path Language (XPath) Selectors, J. Urpalainen, September 2008.

[RFC 5785]

IETF RFC 5785: Defining Well-Known Uniform Resource Identifiers (URIs), M. Nottingham, E. Hammer-Lahav, April 2010.

[RFC 5988]

IETF RFC 5988: Web Linking, M. Nottingham, October 2010.

[RFC 6249]

IETF RFC 6249: Metalink/HTTP: Mirrors and Hashes, A. Bryan, N. McNab, T. Tsujikawa, P. Poeml, H. Nordstrom, June 2011.

[RFC 6573]

IETF RFC 6573: The Item and Collection Link Relations, M. Amundsen, April 2012

[RFC 6596]

IETF RFC 6596: The Canonical Link Relation, M. Ohye, J. Kupke, April 2012.

[Sitemaps]

Sitemaps XML format and protocol, sitemaps.org, 27 February 2008.

[W3C Datetime]

Date and Time Formats, Misha Wolf, Charles Wicksteed, 15 September 1997.

[The Open Archives Initiative Protocol for Metadata Harvesting]

The Open Archives Initiative Protocol for Metadata Harvesting, C. Lagoze, H. Van de Sompel, Michael Nelson, Simeon Warner, December 2008

[ORE Specification - Abstract Data Model]

ORE Specification - Abstract Data Model, C. Lagoze, H. Van de Sompel, Pete Johnston, Michael Nelson, Robert Sanderson, Simeon Warner, October 2008

[Memento Internet Draft]

Memento Internet Draft, H. Van de Sompel, M. L. Nelson, R. D. Sanderson, May 2012

[Protocol for Web Description Resources (POWDER): Description Resources]

Protocol for Web Description Resources (POWDER): Description Resources, P. Archer, K. Smith, A. Perego, September 2009

[The 'describes' Link Relation Type]

The 'describes' Link Relation Type, E. Wilde, October 2012

[HTML 5]

HTML 5, R. Berjon, T. Leithead, E.D. Navara, E. O'Connor, S. Pfeiffer, December 2012

[.ZIP File Format Specification]

.ZIP File Format Specification, PKWARE Inc., September 2012