Network Working Group C. Daboo
Internet-Draft Apple, Inc.
Intended status: Standards Track A. Quillaud
Expires: May 22, 2010 Sun Microsystems
November 18, 2009
Collection Synchronization for WebDAV
draft-daboo-webdav-sync-02
Abstract
This specification defines an extension to WebDAV that allows
efficient synchronization of the contents of a WebDAV collection.
Editorial Note (To be removed by RFC Editor before publication)
Please send comments to the Distributed Authoring and Versioning
(WebDAV) working group at <mailto:w3c-dist-auth@w3.org>, which may be
joined by sending a message with subject "subscribe" to
<mailto:w3c-dist-auth-request@w3.org>. Discussions of the WEBDAV
working group are archived at
<http://lists.w3.org/Archives/Public/w3c-dist-auth/>.
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Copyright Notice
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Copyright (c) 2009 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . 3
3. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. WebDAV Synchronization . . . . . . . . . . . . . . . . . . . . 4
4.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.2. DAV:sync-collection report . . . . . . . . . . . . . . . . 5
4.3. Types of changes reported . . . . . . . . . . . . . . . . 7
4.3.1. New resource . . . . . . . . . . . . . . . . . . . . . 7
4.3.2. Modified resource . . . . . . . . . . . . . . . . . . 7
4.3.3. Removed resource . . . . . . . . . . . . . . . . . . . 7
4.4. Example: Initial DAV:sync-collection REPORT . . . . . . . 8
4.5. Example: DAV:sync-collection Report with token . . . . . . 10
5. DAV:sync-token Property . . . . . . . . . . . . . . . . . . . 11
6. XML Element Definitions . . . . . . . . . . . . . . . . . . . 12
6.1. DAV:sync-collection XML Element . . . . . . . . . . . . . 12
6.2. DAV:sync-token XML Element . . . . . . . . . . . . . . . . 12
6.3. DAV:multistatus XML Element . . . . . . . . . . . . . . . 13
6.4. DAV:sync-response XML Element . . . . . . . . . . . . . . 13
7. Security Considerations . . . . . . . . . . . . . . . . . . . 14
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 14
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
10.1. Normative References . . . . . . . . . . . . . . . . . . . 14
10.2. Informative References . . . . . . . . . . . . . . . . . . 14
Appendix A. Change History (to be removed prior to
publication as an RFC) . . . . . . . . . . . . . . . 15
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1. Introduction
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). This can currently
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 resources 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 resources
which have changed, are new or were deleted 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 resources within the collection have changed.
2. Conventions Used in This Document
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:
1. element names use the "DAV:" namespace,
2. element ordering is irrelevant unless explicitly stated,
3. extension elements (elements not already defined as valid child
elements) may be added anywhere, except when explicitly stated
otherwise,
4. extension attributes (attributes not already defined as valid for
this element) may be added anywhere, except when explicitly
stated otherwise.
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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.
This document inherits, and sometimes extends, DTD productions from
Section 14 of [RFC4918].
3. Open Issues
1. We are extending the multistatus response element in a
significant way. The DAV:sync-response differs from a regular
DAV:response because, in the case of a created or modified
resource, it contains both a DAV:status (201 for created, 200 for
modified) and a DAV:propstat. The DAV:response on the other hand
contains either a DAV:status or a DAV:propstat but not both.
Shall we define a DAV:multisyncstatus response element instead of
overloading DAV:multistatus ?
2. Do clients really need to be notified that a resource was created
versus modified ? They should be able to figure that out by
looking at the state of their current cache. If this information
is not necessary, the response would not need to contain a DAV:
status along with the DAV:propstat. This would allow the use of
a regular multistatus (simply extended with a sync-token
element).
4. WebDAV Synchronization
4.1. Overview
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 since 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.
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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 resources that are currently contained 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 may 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 [I-D.ietf-vcarddav-carddav])
and ask for the DAV:sync-token property to be returned. This
property (defined in Section 5) contains the same token that can be
used later on to issue a DAV:sync-collection report.
In some cases a server may 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. ACL changes may also cause a token to
become invalid.
4.2. DAV:sync-collection report
This specification defines the DAV:sync-collection report.
If this report is implemented by a WebDAV server, then the server
MUST list the report in the "DAV:supported-report-set" property on
any collection supporting synchronization.
To implement the behavior for this report a server needs to keep
track of changes to any resources in a collection. This includes
noting the addition of new resources, changes to existing resources
and removal of resources (where "removal" could be the result of a
DELETE or MOVE WebDAV request). Only internal members of the
collection (as defined in [RFC4918]) are to be considered. 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 is an "opaque" string - i.e., the
actual string data has no specific meaning or syntax. A simple
implementation of such a token would be a numeric counter that counts
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each change as it occurs and relates that change to the specific
object that changed.
Marshalling:
The request URI MUST be a collection. The request body MUST be a
DAV:sync-collection XML element (see Section 6.1), which MUST
contain one DAV:sync-token XML element, and optionally a DAV:
propstat XML element.
The request MUST include a Depth header with a value of "1".
The response body for a successful request MUST be a DAV:
multistatus XML element, which MUST contain one DAV:sync-token
element in addition to any DAV:sync-response elements.
The response body for a successful DAV:sync-collection report
request MUST contain a DAV:sync-response element for each resource
that was created, has changed or been deleted since the last
synchronization operation as specified by the DAV:sync-token
provided in the request. A given resource MUST appear only once
in the response.
The DAV:status element in each DAV:sync-response element is used
to indicate how the resource may have changed:
A status code of '201 Created' is used to indicate resources
that are new.
A status code of '200 OK' is used to indicate resources that
have changed.
A status code of '404 Not Found' is used to indicate resources
that have been removed.
The conditions under which each type of change may occur is
further described in Section 4.3.
Preconditions:
(DAV:valid-sync-token): The DAV:sync-token element value MUST map
to a valid token previously returned by the server. A token may
become invalid as the result of being "out of date" (out of the
range of change history maintained by the server), or for other
reasons (e.g. collection deleted, then recreated, ACL changes,
etc...).
Postconditions:
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None.
4.3. Types of changes reported
Three types of resource state changes can be returned by the DAV:
sync-collection report (new, modified, removed). This section
further defines under which condition each of them shall be used. It
also clarifies the case where a resource may have undergone multiple
changes in between two synchronizations.
4.3.1. New resource
A resource MUST be reported as new if it has been mapped directly
under the target collection since the request sync token was
generated. This includes resources that have been mapped as the
result of a COPY, MOVE or BIND ([I-D.ietf-webdav-bind]) operation.
This also includes collection resources that have been created.
In the case where a mapping between a resource and the target
collection was removed, then a new mapping with the same URI created,
the new resource MUST be reported as new, while the old resource MUST
NOT be reported as removed. For example, if a resource was deleted,
then recreated using the same URI, it should be reported as a new
resource only.
A resource MAY be reported as new if the user issuing the request was
granted access to this resource, due to access control changes.
4.3.2. Modified resource
A resource MUST be reported as modified if it is not reported as new
and if its entity tag value (defined in [RFC2616]) has changed since
the request sync token was generated. In other words, the new
resource change indicator takes precedence over the modified resource
change indicator.
Collection resources MUST NOT be returned as modified. Instead
clients are expected to synchronize changes in child collection
resources on an individual basis.
4.3.3. Removed resource
A resource 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 re-mapped since it was removed. This
includes resources that have been unmapped as the result of a MOVE or
UNBIND ([I-D.ietf-webdav-bind]) operation. This also includes
collection resources that have been removed.
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If a resource was created (and possibly modified), then removed in
between two synchronizations, it MUST NOT be reported as new,
modified or removed.
A resource MAY be reported as removed if the user issuing the request
has no longer access to this resource, due to access control changes.
4.4. Example: Initial DAV:sync-collection REPORT
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
(indicating that they are 'new' via the '201 Created' status code).
The current synchronization token is also returned.
>> Request <<
REPORT /home/cyrusdaboo/ HTTP/1.1
Host: webdav.example.com
Depth: 1
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:prop>
<D:getetag/>
</D:prop>
</D:sync-collection>
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>> 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:sync-response>
<D:href
>http://webdav.example.com/home/cyrusdaboo/test.doc</D:href>
<D:status>HTTP/1.1 201 Created</D:status>
<D:propstat>
<D:prop>
<D:getetag>"00001-abcd1"</D:getetag>
</D:prop>
<D:status>HTTP/1.1 200 OK</D:status>
</D:propstat>
</D:sync-response>
<D:sync-response>
<D:href
>http://webdav.example.com/home/cyrusdaboo/vcard.vcf</D:href>
<D:status>HTTP/1.1 201 Created</D:status>
<D:propstat>
<D:prop>
<D:getetag>"00002-abcd1"</D:getetag>
</D:prop>
<D:status>HTTP/1.1 200 OK</D:status>
</D:propstat>
</D:sync-response>
<D:sync-response>
<D:href
>http://webdav.example.com/home/cyrusdaboo/calendar.ics</D:href>
<D:status>HTTP/1.1 201 Created</D:status>
<D:propstat>
<D:prop>
<D:getetag>"00003-abcd1"</D:getetag>
</D:prop>
<D:status>HTTP/1.1 200 OK</D:status>
</D:propstat>
</D:sync-response>
<D:sync-token>1234</D:sync-token>
</D:multistatus>
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4.5. Example: DAV:sync-collection Report with token
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
Depth: 1
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>1234</D:sync-token>
<D:prop>
<D:getetag/>
</D:prop>
</D:sync-collection>
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>> 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:sync-response>
<D:href
>http://webdav.example.com/home/cyrusdaboo/file.xml</D:href>
<D:status>HTTP/1.1 201 Created</D:status>
<D:propstat>
<D:prop>
<D:getetag>"00004-abcd1"</D:getetag>
</D:prop>
<D:status>HTTP/1.1 200 OK</D:status>
</D:propstat>
</D:sync-response>
<D:sync-response>
<D:href
>http://webdav.example.com/home/cyrusdaboo/vcard.vcf</D:href>
<D:status>HTTP/1.1 200 OK</D:status>
<D:propstat>
<D:prop>
<D:getetag>"00002-abcd2"</D:getetag>
</D:prop>
<D:status>HTTP/1.1 200 OK</D:status>
</D:propstat>
</D:sync-response>
<D:sync-response>
<D:href
>http://webdav.example.com/home/cyrusdaboo/test.doc</D:href>
<D:status>HTTP/1.1 404 Not Found</D:status>
</D:sync-response>
<D:sync-token>1238</D:sync-token>
</D:multistatus>
5. DAV:sync-token Property
Name: sync-token
Namespace: DAV:
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Purpose: Contains the value of the synchronization token as it would
be returned by a DAV:sync-collection report.
Value: Any text.
Protected: MUST be protected because this value is created and
controlled by the server.
COPY/MOVE behavior: This property value is dependent on the final
state of the destination resource, not the value of the property
on the source resource.
Description: The DAV:sync-token property MUST be defined on all
resources that support the DAV:sync-collection report. It
contains the value of the synchronization token as it would be
returned by a DAV:sync-collection report on that resource at the
same point in time. It SHOULD NOT be returned by a PROPFIND DAV:
allprop request (as defined in [RFC4918]).
Definition:
<!ELEMENT sync-token #PCDATA>
6. XML Element Definitions
6.1. DAV:sync-collection XML Element
Name: sync-collection
Namespace: DAV:
Purpose: WebDAV report used to synchronize data between client and
server.
Description: See Section 4.
<!ELEMENT sync-collection (sync-token, DAV:prop?)>
6.2. DAV:sync-token XML Element
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Name: sync-token
Namespace: DAV:
Purpose: The synchronization token provided by the server and
returned by the client.
Description: See Section 4.
<!ELEMENT sync-token CDATA>
6.3. DAV:multistatus XML Element
Name: multistatus
Namespace: DAV:
Purpose: Extends the DAV:multistatus element to include
synchronization details.
Description: See Section 4.
<!ELEMENT multistatus ((DAV:response*, DAV:responsedescription?) |
(DAV:sync-response*, DAV:sync-token,
DAV:responsedescription?))>
6.4. DAV:sync-response XML Element
Name: sync-response
Namespace: DAV:
Purpose: Contains the synchronization results returned by the
server.
Description: See Section 4.
<!ELEMENT sync-response (DAV:href, DAV:status, DAV:propstat?)>
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7. Security Considerations
This extension does not introduce any new security concerns than
those already described in HTTP and WebDAV.
8. IANA Considerations
This document does not require any actions on the part of IANA.
9. Acknowledgments
The following individuals contributed their ideas and support for
writing this specification: Bernard Desruisseaux, Mike Douglass, Ciny
Joy and Julian Reschke.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs
to Indicate Requirement Levels", BCP 14,
RFC 2119, March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J.,
Frystyk, H., Masinter, L., Leach, P.,
and T. Berners-Lee, "Hypertext Transfer
Protocol -- HTTP/1.1", RFC 2616,
June 1999.
[RFC4918] Dusseault, L., "HTTP Extensions for Web
Distributed Authoring and Versioning
(WebDAV)", RFC 4918, June 2007.
[W3C.REC-xml-20081126] Sperberg-McQueen, C., Yergeau, F., Bray,
T., Paoli, J., and E. Maler, "Extensible
Markup Language (XML) 1.0 (Fifth
Edition)", World Wide Web Consortium
Recommendation REC-xml-20081126,
November 2008, <http://www.w3.org/TR/
2008/REC-xml-20081126>.
10.2. Informative References
[I-D.ietf-vcarddav-carddav] Daboo, C., "vCard Extensions to WebDAV
(CardDAV)",
draft-ietf-vcarddav-carddav-10 (work in
progress), November 2009.
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[I-D.ietf-webdav-bind] Clemm, G., Crawford, J., Reschke, J.,
and J. Whitehead, "Binding Extensions to
Web Distributed Authoring and Versioning
(WebDAV)", draft-ietf-webdav-bind-26
(work in progress), September 2009.
[RFC4791] Daboo, C., Desruisseaux, B., and L.
Dusseault, "Calendaring Extensions to
WebDAV (CalDAV)", RFC 4791, March 2007.
[RFC5323] Reschke, J., Reddy, S., Davis, J., and
A. Babich, "Web Distributed Authoring
and Versioning (WebDAV) SEARCH",
RFC 5323, November 2008.
Appendix A. Change History (to be removed prior to publication as an
RFC)
Changes in -02:
1. Added definition of sync-token WebDAV property.
2. Added references to SEARCH, CalDAV, CardDAV as alternative ways
to first synchronize a collection.
3. Added section defining under which condition each state change
(new, modified, removed) should be reported. Added reference to
BIND.
4. Incorporated feedback from Julian Reschke and Ciny Joy.
5. More details on the use of the DAV:valid-sync-token precondition.
Changes in -01:
1. Updated to 4918 reference.
2. Fixed examples to properly include DAV:status in DAV:propstat
3. Switch to using XML conventions text from RFC5323.
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Authors' Addresses
Cyrus Daboo
Apple Inc.
1 Infinite Loop
Cupertino, CA 95014
USA
EMail: cyrus@daboo.name
URI: http://www.apple.com/
Arnaud Quillaud
Sun Microsystems
180, Avenue de l'Europe
Saint Ismier cedex, 38334
France
EMail: arnaud.quillaud@sun.com
URI: http://www.sun.com/
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