| Internet Engineering Task Force (IETF) | C. Daboo |
| Request for Comments: 6578 | Apple Inc. |
| Category: Standards Track | A. Quillaud |
| ISSN: 2070-1721 | Oracle |
| March 2012 |
This specification defines an extension to Web Distributed Authoring and Versioning (WebDAV) that allows efficient synchronization of the contents of a WebDAV collection.¶
This is an Internet Standards Track document.¶
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.¶
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6578.¶
Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved.¶
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.¶
This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English.¶
WebDAV [RFC4918] defines the concept of 'collections', which are hierarchical groupings of WebDAV resources on an HTTP [RFC2616] server. Collections can be of arbitrary size and depth (i.e., collections within collections). WebDAV clients that cache resource content need a way to synchronize that data with the server (i.e., detect what has changed and update their cache). Currently, this can be done using a WebDAV PROPFIND request on a collection to list all members of a collection along with their DAV:getetag property values, which allows the client to determine which were changed, added, or deleted. However, this does not scale well to large collections, as the XML response to the PROPFIND request will grow with the collection size.¶
This specification defines a new WebDAV report that results in the server returning to the client only information about those member URLs that were added or deleted, or whose mapped resources were changed, since a previous execution of the report on the collection.¶
Additionally, a new property is added to collection resources that is used to convey a "synchronization token" that is guaranteed to change when the collection's member URLs or their mapped resources have changed.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].¶
This document uses XML DTD fragments ([W3C.REC-xml-20081126], Section 3.2) as a purely notational convention. WebDAV request and response bodies cannot be validated by a DTD due to the specific extensibility rules defined in Section 17 of [RFC4918] and due to the fact that all XML elements defined by this specification use the XML namespace name "DAV:". In particular: ¶
When an XML element type in the "DAV:" namespace is referenced in this document outside of the context of an XML fragment, the string "DAV:" will be prefixed to the element type.¶
One way to synchronize data between two entities is to use some form of synchronization token. The token defines the state of the data being synchronized at a particular point in time. It can then be used to determine what has changed between one point in time and another.¶
This specification defines a new WebDAV report that is used to enable client-server collection synchronization based on such a token.¶
In order to synchronize the contents of a collection between a server and client, the server provides the client with a synchronization token each time the synchronization report is executed. That token represents the state of the data being synchronized at that point in time. The client can then present that same token back to the server at some later time, and the server will return only those items that are new, have changed, or were deleted since that token was generated. The server also returns a new token representing the new state at the time the report was run.¶
Typically, the first time a client connects to the server it will need to be informed of the entire state of the collection (i.e., a full list of all member URLs that are currently in the collection). That is done by the client sending an empty token value to the server. This indicates to the server that a full listing is required.¶
As an alternative, the client might choose to do its first synchronization using some other mechanism on the collection (e.g., some other form of batch resource information retrieval such as PROPFIND, SEARCH [RFC5323], or specialized REPORTs such as those defined in CalDAV [RFC4791] and CardDAV [RFC6352]) and ask for the DAV:sync-token property to be returned. This property (defined in Section 4) contains the same token that can be used later to issue a DAV:sync-collection report.¶
In some cases, a server might only wish to maintain a limited amount of history about changes to a collection. In that situation, it will return an error to the client when the client presents a token that is "out of date". At that point, the client has to fall back to synchronizing the entire collection by re-running the report request using an empty token value.¶
Typically, a client will use the synchronization report to retrieve the list of changes and will follow that with requests to retrieve the content of changed resources. It is possible that additional changes to the collection could occur between the time of the synchronization report and resource content retrieval, which could result in an inconsistent view of the collection. When clients use this method of synchronization, they need to be aware that such additional changes could occur and track them, e.g., by differences between the ETag values returned in the synchronization report and those returned when actually fetching resource content, by using conditional requests as described in Section 5, or by repeating the synchronization process until no changes are returned.¶
If the DAV:sync-collection report is implemented by a WebDAV server, then the server MUST list the report in the "DAV:supported-report-set" property on any collection that supports synchronization.¶
To implement the behavior for this report, a server needs to keep track of changes to any member URLs and their mapped resources in a collection (as defined in Section 3 of [RFC4918]). This includes noting the addition of new member URLs, the changes to the mapped resources of existing member URLs, and the removal of member URLs. The server will track each change and provide a synchronization "token" to the client that describes the state of the server at a specific point in time. This "token" is returned as part of the response to the "sync-collection" report. Clients include the last token they got from the server in the next "sync-collection" report that they execute, and the server provides the changes from the previous state (represented by the token) to the current state (represented by the new token returned).¶
The synchronization token itself MUST be treated as an "opaque" string by the client, i.e., the actual string data has no specific meaning or syntax. However, the token MUST be a valid URI to allow its use in an If precondition request header (see Section 5). For example, a simple implementation of such a token could be a numeric counter that counts each change as it occurs and relates that change to the specific object that changed. The numeric value could be appended to a "base" URI to form the valid sync-token.¶
Marshalling: ¶
Preconditions: ¶
Postconditions: ¶
Servers MUST support only Depth:0 behavior with the DAV:sync-collection report, i.e., the report targets only the collection being synchronized in a single request. However, clients do need to "scope" the synchronization to different levels within that collection -- specifically, immediate children (level "1") and all children at any depth (level "infinite"). To specify which level to use, clients MUST include a DAV:sync-level XML element in the request. ¶
Note that when a server supports a DAV:sync-level XML element with a value of "infinite", it might not be possible to synchronize some child collections within the collection targeted by the report. When this occurs, the server MUST include a DAV:response element for the child collection with status 403 (Forbidden). The 403 response MUST be sent once, when the collection is first reported to the client. In addition, the server MUST include a DAV:error element in the DAV:response element, indicating one of two possible causes for this: ¶
When the DAV:sync-collection request contains an empty DAV:sync-token element, the server MUST return all member URLs of the collection (taking account of the DAV:sync-level XML element value as per Section 3.3, and optional truncation of the result set as per Section 3.6) and it MUST NOT return any removed member URLs. All types of member (collection or non-collection) MUST be reported.¶
When the DAV:sync-collection request contains a valid value for the DAV:sync-token element, two types of member URL state changes can be returned (changed or removed). This section defines what triggers each of these to be returned. It also clarifies the case where a member URL might have undergone multiple changes between two synchronization report requests. In all cases, the DAV:sync-level XML element value (as per Section 3.3) and optional truncation of the result set (as per Section 3.6) are taken into account by the server.¶
A member URL MUST be reported as changed if it has been newly mapped as a member of the target collection since the request sync-token was generated (e.g., when a new resource has been created as a child of the collection). For example, this includes member URLs that have been newly mapped as the result of a COPY, MOVE, BIND [RFC5842], or REBIND [RFC5842] request. All types of member URL (collection or non-collection) MUST be reported.¶
In the case where a mapping between a member URL and the target collection was removed, then a new mapping with the same URI was created, the member URL MUST be reported as changed and MUST NOT be reported as removed.¶
A member URL MUST be reported as changed if its mapped resource's entity tag value (defined in Section 3.11 of [RFC2616]) has changed since the request sync-token was generated.¶
A member URL MAY be reported as changed if the user issuing the request was granted access to this member URL, due to access control changes.¶
Collection member URLs MUST be returned as changed if they are mapped to an underlying resource (i.e., entity body) and if the entity tag associated with that resource changes. There is no guarantee that changes to members of a collection will result in a change in any entity tag of that collection, so clients cannot rely on a series of reports using the DAV:sync-level XML element value set to "1" at multiple levels to track all changes within a collection. Instead, a DAV:sync-level XML element with a value of "infinite" has to be used.¶
A member MUST be reported as removed if its mapping under the target collection has been removed since the request sync-token was generated, and it has not been remapped since it was removed. For example, this includes members that have been unmapped as the result of a MOVE, UNBIND [RFC5842], or REBIND [RFC5842] operation. This also includes collection members that have been removed, including ones that themselves do not support the DAV:sync-collection report.¶
If a member was added (and its mapped resource possibly modified), then removed between two synchronization report requests, it MUST be reported as removed. This ensures that a client that adds a member is informed of the removal of the member, if the removal occurs before the client has had a chance to execute a synchronization report.¶
A member MAY be reported as removed if the user issuing the request no longer has access to this member, due to access control changes.¶
For a report with the DAV:sync-level XML element value set to "infinite", where a collection is removed, the server MUST NOT report the removal of any members of the removed collection. Clients MUST assume that if a collection is reported as being removed, then all members of that collection have also been removed.¶
A server MAY limit the number of member URLs in a response, for example, to limit the amount of work expended in processing a request, or as the result of an explicit limit set by the client. If the result set is truncated, the response MUST use status code 207 (Multi-Status), return a DAV:multistatus response body, and indicate a status of 507 (Insufficient Storage) for the request-URI. That DAV:response element SHOULD include a DAV:error element with the DAV:number-of-matches-within-limits precondition, as defined in [RFC3744] (Section 9.2). DAV:response elements for all the changes being reported are also included.¶
When truncation occurs, the DAV:sync-token value returned in the response MUST represent the correct state for the partial set of changes returned. That allows the client to use the returned DAV:sync-token to fetch the next set of changes. In this way, the client can effectively "page" through the entire set of changes in a consistent manner.¶
Clients MUST handle the 507 status on the request-URI in the response to the report.¶
For example, consider a server that records changes using a strictly increasing integer to represent a "revision number" and uses that quantity as the DAV:sync-token value (appropriately encoded as a URI). Assume the last DAV:sync-token used by the client was "http://example.com/sync/10", and since then 15 additional changes to different resources have occurred. If the client executes a DAV:sync-collection request with a DAV:sync-token of "http://example.com/sync/10", without a limit, the server would return 15 DAV:response elements and a DAV:sync-token with value "http://example.com/sync/25". But if the server chooses to limit responses to at most 10 changes, then it would return only 10 DAV:response elements and a DAV:sync-token with value "http://example.com/sync/20", together with an additional DAV:response element for the request-URI with a status code of 507. Subsequently, the client can reissue the request with the DAV:sync-token value returned from the server and fetch the remaining 5 changes.¶
A client can limit the number of results returned by the server through use of the DAV:limit element ([RFC5323], Section 5.17) in the request body. This is useful when clients have limited space or bandwidth for the results. If a server is unable to truncate the result at or below the requested number, then it MUST fail the request with a DAV:number-of-matches-within-limits postcondition error. When the results can be correctly limited by the server, the server MUST follow the rules above for indicating a result set truncation to the client.¶
In this example, the client is making its first synchronization request to the server, so the DAV:sync-token element in the request is empty. It also asks for the DAV:getetag property and for a proprietary property. The server responds with the items currently in the targeted collection. The current synchronization token is also returned.¶
>> Request <<
REPORT /home/cyrusdaboo/ HTTP/1.1
Host: webdav.example.com
Depth: 0
Content-Type: text/xml; charset="utf-8"
Content-Length: xxxx
<?xml version="1.0" encoding="utf-8" ?>
<D:sync-collection xmlns:D="DAV:">
<D:sync-token/>
<D:sync-level>1</D:sync-level>
<D:prop xmlns:R="urn:ns.example.com:boxschema">
<D:getetag/>
<R:bigbox/>
</D:prop>
</D:sync-collection>
>> Response <<
HTTP/1.1 207 Multi-Status
Content-Type: text/xml; charset="utf-8"
Content-Length: xxxx
<?xml version="1.0" encoding="utf-8" ?>
<D:multistatus xmlns:D="DAV:">
<D:response>
<D:href
>http://webdav.example.com/home/cyrusdaboo/test.doc</D:href>
<D:propstat>
<D:prop>
<D:getetag>"00001-abcd1"</D:getetag>
<R:bigbox xmlns:R="urn:ns.example.com:boxschema">
<R:BoxType>Box type A</R:BoxType>
</R:bigbox>
</D:prop>
<D:status>HTTP/1.1 200 OK</D:status>
</D:propstat>
</D:response>
<D:response>
<D:href
>http://webdav.example.com/home/cyrusdaboo/vcard.vcf</D:href>
<D:propstat>
<D:prop>
<D:getetag>"00002-abcd1"</D:getetag>
</D:prop>
<D:status>HTTP/1.1 200 OK</D:status>
</D:propstat>
<D:propstat>
<D:prop>
<R:bigbox xmlns:R="urn:ns.example.com:boxschema"/>
</D:prop>
<D:status>HTTP/1.1 404 Not Found</D:status>
</D:propstat>
</D:response>
<D:response>
<D:href
>http://webdav.example.com/home/cyrusdaboo/calendar.ics</D:href>
<D:propstat>
<D:prop>
<D:getetag>"00003-abcd1"</D:getetag>
</D:prop>
<D:status>HTTP/1.1 200 OK</D:status>
</D:propstat>
<D:propstat>
<D:prop>
<R:bigbox xmlns:R="urn:ns.example.com:boxschema"/>
</D:prop>
<D:status>HTTP/1.1 404 Not Found</D:status>
</D:propstat>
</D:response>
<D:sync-token>http://example.com/ns/sync/1234</D:sync-token>
</D:multistatus>
In this example, the client is making a synchronization request to the server and is using the DAV:sync-token element returned from the last report it ran on this collection. The server responds, listing the items that have been added, changed, or removed. The (new) current synchronization token is also returned.¶
>> Request <<
REPORT /home/cyrusdaboo/ HTTP/1.1
Host: webdav.example.com
Content-Type: text/xml; charset="utf-8"
Content-Length: xxxx
<?xml version="1.0" encoding="utf-8" ?>
<D:sync-collection xmlns:D="DAV:">
<D:sync-token>http://example.com/ns/sync/1234</D:sync-token>
<D:sync-level>1</D:sync-level>
<D:prop xmlns:R="urn:ns.example.com:boxschema">
<D:getetag/>
<R:bigbox/>
</D:prop>
</D:sync-collection>
>> Response <<
HTTP/1.1 207 Multi-Status
Content-Type: text/xml; charset="utf-8"
Content-Length: xxxx
<?xml version="1.0" encoding="utf-8" ?>
<D:multistatus xmlns:D="DAV:">
<D:response>
<D:href
>http://webdav.example.com/home/cyrusdaboo/file.xml</D:href>
<D:propstat>
<D:prop>
<D:getetag>"00004-abcd1"</D:getetag>
</D:prop>
<D:status>HTTP/1.1 200 OK</D:status>
</D:propstat>
<D:propstat>
<D:prop>
<R:bigbox xmlns:R="urn:ns.example.com:boxschema"/>
</D:prop>
<D:status>HTTP/1.1 404 Not Found</D:status>
</D:propstat>
</D:response>
<D:response>
<D:href
>http://webdav.example.com/home/cyrusdaboo/vcard.vcf</D:href>
<D:propstat>
<D:prop>
<D:getetag>"00002-abcd2"</D:getetag>
</D:prop>
<D:status>HTTP/1.1 200 OK</D:status>
</D:propstat>
<D:propstat>
<D:prop>
<R:bigbox xmlns:R="urn:ns.example.com:boxschema"/>
</D:prop>
<D:status>HTTP/1.1 404 Not Found</D:status>
</D:propstat>
</D:response>
<D:response>
<D:href
>http://webdav.example.com/home/cyrusdaboo/test.doc</D:href>
<D:status>HTTP/1.1 404 Not Found</D:status>
</D:response>
<D:sync-token>http://example.com/ns/sync/1238</D:sync-token>
</D:multistatus>
In this example, the client is making its first synchronization request to the server, so the DAV:sync-token element in the request is empty. It also asks for the DAV:getetag property. The server responds with the items currently in the targeted collection but truncated at two items. The synchronization token for the truncated result set is returned.¶
>> Request <<
REPORT /home/cyrusdaboo/ HTTP/1.1
Host: webdav.example.com
Depth: 0
Content-Type: text/xml; charset="utf-8"
Content-Length: xxxx
<?xml version="1.0" encoding="utf-8" ?>
<D:sync-collection xmlns:D="DAV:">
<D:sync-token/>
<D:sync-level>1</D:sync-level>
<D:prop>
<D:getetag/>
</D:prop>
</D:sync-collection>
>> Response <<
HTTP/1.1 207 Multi-Status
Content-Type: text/xml; charset="utf-8"
Content-Length: xxxx
<?xml version="1.0" encoding="utf-8" ?>
<D:multistatus xmlns:D="DAV:">
<D:response>
<D:href
>http://webdav.example.com/home/cyrusdaboo/test.doc</D:href>
<D:propstat>
<D:prop>
<D:getetag>"00001-abcd1"</D:getetag>
</D:prop>
<D:status>HTTP/1.1 200 OK</D:status>
</D:propstat>
</D:response>
<D:response>
<D:href
>http://webdav.example.com/home/cyrusdaboo/vcard.vcf</D:href>
<D:propstat>
<D:prop>
<D:getetag>"00002-abcd1"</D:getetag>
</D:prop>
<D:status>HTTP/1.1 200 OK</D:status>
</D:propstat>
</D:response>
<D:response>
<D:href
>http://webdav.example.com/home/cyrusdaboo/</D:href>
<D:status>HTTP/1.1 507 Insufficient Storage</D:status>
<D:error><D:number-of-matches-within-limits/></D:error>
</D:response>
<D:sync-token>http://example.com/ns/sync/1233</D:sync-token>
</D:multistatus>
In this example, the client is making its first synchronization request to the server, so the DAV:sync-token element in the request is empty. It requests a limit of 1 for the responses returned by the server. It also asks for the DAV:getetag property. The server responds with the items currently in the targeted collection, but truncated at one item. The synchronization token for the truncated result set is returned.¶
>> Request <<
REPORT /home/cyrusdaboo/ HTTP/1.1
Host: webdav.example.com
Depth: 0
Content-Type: text/xml; charset="utf-8"
Content-Length: xxxx
<?xml version="1.0" encoding="utf-8" ?>
<D:sync-collection xmlns:D="DAV:">
<D:sync-token/>
<D:sync-level>1</D:sync-level>
<D:limit>
<D:nresults>1</D:nresults>
</D:limit>
<D:prop>
<D:getetag/>
</D:prop>
</D:sync-collection>
>> Response <<
HTTP/1.1 207 Multi-Status
Content-Type: text/xml; charset="utf-8"
Content-Length: xxxx
<?xml version="1.0" encoding="utf-8" ?>
<D:multistatus xmlns:D="DAV:">
<D:response>
<D:href
>http://webdav.example.com/home/cyrusdaboo/test.doc</D:href>
<D:propstat>
<D:prop>
<D:getetag>"00001-abcd1"</D:getetag>
</D:prop>
<D:status>HTTP/1.1 200 OK</D:status>
</D:propstat>
</D:response>
<D:response>
<D:href
>http://webdav.example.com/home/cyrusdaboo/</D:href>
<D:status>HTTP/1.1 507 Insufficient Storage</D:status>
<D:error><D:number-of-matches-within-limits/></D:error>
</D:response>
<D:sync-token>http://example.com/ns/sync/1232</D:sync-token>
</D:multistatus>
In this example, the client is making a synchronization request to the server with a valid DAV:sync-token element value. It requests a limit of 100 for the responses returned by the server. It also asks for the DAV:getetag property. The server is unable to limit the results to the maximum specified by the client, so it responds with a 507 status code and appropriate postcondition error code.¶
>> Request <<
REPORT /home/cyrusdaboo/ HTTP/1.1
Host: webdav.example.com
Depth: 0
Content-Type: text/xml; charset="utf-8"
Content-Length: xxxx
<?xml version="1.0" encoding="utf-8" ?>
<D:sync-collection xmlns:D="DAV:">
<D:sync-token>http://example.com/ns/sync/1232</D:sync-token>
<D:sync-level>1</D:sync-level>
<D:limit>
<D:nresults>100</D:nresults>
</D:limit>
<D:prop>
<D:getetag/>
</D:prop>
</D:sync-collection>
>> Response <<
HTTP/1.1 507 Insufficient Storage
Content-Type: text/xml; charset="utf-8"
Content-Length: xxxx
<?xml version="1.0" encoding="utf-8" ?>
<D:error xmlns:D="DAV:">
<D:number-of-matches-within-limits/>
</D:error>
In this example, the client is making its first synchronization request to the server, so the DAV:sync-token element in the request is empty, and it is using DAV:sync-level set to "infinite". It also asks for the DAV:getetag property and for a proprietary property. The server responds with the items currently in the targeted collection. The current synchronization token is also returned.¶
The collection /home/cyrusdaboo/collection1/ exists and has one child resource that is also reported. The collection /home/cyrusdaboo/collection2/ exists but has no child resources. The collection /home/cyrusdaboo/shared/ is returned with a 403 status indicating that a collection exists, but it is unable to report on changes within it in the scope of the current DAV:sync-level "infinite" report. Instead, the client can try a DAV:sync-collection report directly on the collection URI.¶
>> Request <<
REPORT /home/cyrusdaboo/ HTTP/1.1
Host: webdav.example.com
Depth: 0
Content-Type: text/xml; charset="utf-8"
Content-Length: xxxx
<?xml version="1.0" encoding="utf-8" ?>
<D:sync-collection xmlns:D="DAV:">
<D:sync-token/>
<D:sync-level>infinite</D:sync-level>
<D:prop xmlns:R="urn:ns.example.com:boxschema">
<D:getetag/>
<R:bigbox/>
</D:prop>
</D:sync-collection>
>> Response <<
HTTP/1.1 207 Multi-Status
Content-Type: text/xml; charset="utf-8"
Content-Length: xxxx
<?xml version="1.0" encoding="utf-8" ?>
<D:multistatus xmlns:D="DAV:">
<D:response>
<D:href>/home/cyrusdaboo/collection1/</D:href>
<D:propstat>
<D:prop>
<D:getetag>"00001-abcd1"</D:getetag>
<R:bigbox xmlns:R="urn:ns.example.com:boxschema">
<R:BoxType>Box type A</R:BoxType>
</R:bigbox>
</D:prop>
<D:status>HTTP/1.1 200 OK</D:status>
</D:propstat>
</D:response>
<D:response>
<D:href>/home/cyrusdaboo/collection1/test.doc</D:href>
<D:propstat>
<D:prop>
<D:getetag>"00001-abcd1"</D:getetag>
<R:bigbox xmlns:R="urn:ns.example.com:boxschema">
<R:BoxType>Box type A</R:BoxType>
</R:bigbox>
</D:prop>
<D:status>HTTP/1.1 200 OK</D:status>
</D:propstat>
</D:response>
<D:response>
<D:href>/home/cyrusdaboo/collection2/</D:href>
<D:propstat>
<D:prop>
<D:getetag/>
</D:prop>
<D:status>HTTP/1.1 404 Not Found</D:status>
</D:propstat>
<D:propstat>
<D:prop>
<R:bigbox xmlns:R="urn:ns.example.com:boxschema"/>
</D:prop>
<D:status>HTTP/1.1 404 Not Found</D:status>
</D:propstat>
</D:response>
<D:response>
<D:href>/home/cyrusdaboo/calendar.ics</D:href>
<D:propstat>
<D:prop>
<D:getetag>"00003-abcd1"</D:getetag>
</D:prop>
<D:status>HTTP/1.1 200 OK</D:status>
</D:propstat>
<D:propstat>
<D:prop>
<R:bigbox xmlns:R="urn:ns.example.com:boxschema"/>
</D:prop>
<D:status>HTTP/1.1 404 Not Found</D:status>
</D:propstat>
</D:response>
<D:response>
<D:href>/home/cyrusdaboo/shared/</D:href>
<D:status>HTTP/1.1 403 Forbidden</D:status>
<D:error><D:sync-traversal-supported/></D:error>
</D:response>
<D:sync-token>http://example.com/ns/sync/1234</D:sync-token>
</D:multistatus>
<!ELEMENT sync-token #PCDATA>
<!-- Text MUST be a valid URI -->
WebDAV provides an If precondition header that allows for "state tokens" to be used as preconditions on HTTP requests (as defined in Section 10.4 of [RFC4918]). This specification allows the DAV:sync-token value to be used as one such token in an If header. By using this, clients can ensure requests only complete when there have been no changes to the content of a collection, by virtue of an unchanged DAV:sync-token value. Servers MUST support use of DAV:sync-token values in If request headers.¶
In this example, the client has already used the DAV:sync-collection report to synchronize the collection /home/cyrusdaboo/collection/. Now it wants to add a new resource to the collection, but only if there have been no other changes since the last synchronization. Note that because the DAV:sync-token is defined on the collection and not on the resource targeted by the request, the If header value needs to use the "Resource_Tag" construct for the header syntax to correctly identify that the supplied state token refers to the collection resource.¶
>> Request <<
PUT /home/cyrusdaboo/collection/newresource.txt HTTP/1.1
Host: webdav.example.com
If: </home/cyrusdaboo/collection/>
(<http://example.com/ns/sync/12345>)
Content-Type: text/plain; charset="utf-8"
Content-Length: xxxx
Some content here...
>> Response <<
HTTP/1.1 201 Created
In this example, the client has already used the DAV:sync-collection report to synchronize the collection /home/cyrusdaboo/collection/. Now, it wants to add a new collection to the collection, but only if there have been no other changes since the last synchronization. Note that because the DAV:sync-token is defined on the collection and not on the resource targeted by the request, the If header value needs to use the "Resource_Tag" construct for the header syntax to correctly identify that the supplied state token refers to the collection resource. In this case, the request fails as another change has occurred to the collection corresponding to the supplied DAV:sync-token.¶
>> Request <<
MKCOL /home/cyrusdaboo/collection/child/ HTTP/1.1
Host: webdav.example.com
If: </home/cyrusdaboo/collection/>
(<http://example.com/ns/sync/12346>)
>> Response <<
HTTP/1.1 412 Precondition Failed
<!ELEMENT sync-collection (sync-token, sync-level, limit?, prop)>
<!-- DAV:limit defined in RFC 5323, Section 5.17 -->
<!-- DAV:prop defined in RFC 4918, Section 14.18 -->
<!ELEMENT sync-token CDATA>
<!-- Text MUST be a URI -->
<!ELEMENT sync-level CDATA>
<!-- Text MUST be either "1" or "infinite" -->
<!ELEMENT multistatus (response*, responsedescription?,
sync-token?) >
<!-- DAV:multistatus originally defined in RFC 4918, Section 14.16
but overridden here to add the DAV:sync-token element -->
<!-- DAV:response defined in RFC 4918, Section 14.24 -->
<!-- DAV:responsedescription defined in RFC 4918, Section 14.25 -->
This extension does not introduce any new security concerns beyond those already described in HTTP and WebDAV.¶
The following individuals contributed their ideas and support for writing this specification: Bernard Desruisseaux, Werner Donne, Mike Douglass, Ciny Joy, Andrew McMillan, Julian Reschke, and Wilfredo Sanchez. We would like to thank the Calendaring and Scheduling Consortium for facilitating interoperability testing for early implementations of this specification.¶
In prior draft versions of this specification, the Depth request header was used instead of the DAV:sync-level element to indicate the "scope" of the synchronization request. Servers that wish to be backwards compatible with clients conforming to the older specification should do the following: if a DAV:sync-level element is not present in the request body, use the Depth header value as the equivalent value for the missing DAV:sync-level element.¶
This appendix gives an example of how a client might accomplish collection synchronization using the WebDAV sync report defined in this specification. Note that this is provided purely as an example, and is not meant to be treated as a normative "algorithm" for client synchronization.¶
This example assumes a WebDAV client interacting with a WebDAV server supporting the sync report. The client keeps a local cache of resources in a targeted collection, "/collection/". Local changes are assumed to not occur. The client is only tracking changes to the immediate children of the collection resource.¶