|Network Working Group||J. Reschke|
|Updates: 2518 (if approved)||June 2004|
|Intended status: Informational|
|Expires: December 2004|
This document is an Internet-Draft and is subject to all provisions of section 3 of RFC 3667. By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she become aware will be disclosed, in accordance with RFC 3668.
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Copyright © The Internet Society (2004). All Rights Reserved.
This document specifies a set of methods and headers ancillary to HTTP/1.1 (RFC2616) and Distributed Authoring and Versioning (WebDAV, RFC2518) for the management of resource locking (collision avoidance). It updates those sections from RFC2518 that specify WebDAV's locking features.
[rfc.comment.1: Note that this document is not a product of the WebDAV working group. It is just an experiment to study the feasability of extracing the locking feature into a separate specification. --reschke]
Distribution of this document is unlimited. Please send comments to the WebDAV working group at email@example.com, which may be joined by sending a message with subject "subscribe" to firstname.lastname@example.org.
Discussions of the WEBDAV working group are archived at URL: http://lists.w3.org/Archives/Public/w3c-dist-auth/.
|I import-rfc3253-stuff (type: change, status: closed)|
|email@example.com||2004-04-25||Import error marshalling and terminology from RFC3253.|
|Associated changes in this document: A.2.|
|I import-gulp (type: change, status: open)|
|firstname.lastname@example.org||2004-05-25||Make specification text compatible with GULP where it isn't. Integrate GULP as normative specification of the locking behaviour.|
|I edit (type: edit, status: open)|
|email@example.com||2004-05-25||Umbrella issue for editorial fixes/enhancements.|
|Associated changes in this document: 2, 3.2, 4.1, 4.2, del-, 5, 7.2, 8, A.1.1, A.1.2, A.2.1, B, del-2, del-4, B.2.1.1, B.2.1.1, B.2.1.4, B.2.1.4, B.3, B.3.2, B.3.2, B.3.2, B.|
|I 008_URI_URL (type: change, status: closed)|
|firstname.lastname@example.org||1998-11-09||Perform a thorough review of the specification to ensure that URI and URL are used correctly, and consistently throughout.|
|email@example.com||2004-06-22||Use "request URI" when talking particularily about HTTP messages because this is the term used by RFC2616. Use "URL" when talking about HTTP/WebDAV resources. Use "(internal) member URI" when talking about collection membership (term is defined IN RFC2518). Otherwise use URI, making sure that if a relativeURI (as defined by RFC2396) is allowed, we say so.|
Seems to have been deferred: http://lists.w3.org/Archives/Public/w3c-dist-auth/2002AprJun/0216.html,
but there is some follow on discussion on what exactly needs to be clarified:
but no specific action was concluded besides the fact that we don't need to wait for RFC2396 to be updated or request any changes/clarifications to that.
Closed for now with the changes proposes by Julian R.
|Associated changes in this document: del-, del-, C.2, C.2, C.2.|
|I 040_LOCK_ISSUES_06 (type: change, status: open)|
Upon cursory reading of the rfc 2518 sec 8.10.4 through 8.11 I was confused by
the plethoria of error codes. Nothing seems to unify them.
8.10.4 speaks of a return code of 409 Conflict if a lock can't be granted.
- Firstly, I can't tell if it is saying that the 409 is within the multistatus body... or in the response header.
- Secondly, later text seems to use a different status codes and never mentions this one again.
8.10.7 lists status codes
- 200 OK, 412 Precondition Failed, and 423 Locked are listed, but 409 Conflict (mentioned above) is not.
- In the case of 412 Precondition Failed, the description the follows doesn't seem to describe a "precondition failed". And it sounds like it's talking about an access request that includes a "locktoken", not a LOCK request that generates one.
- The 423 Locked condition also sort of sounds like it's talking about an access request rather than a LOCK request.
8.10.10 lists LOCK status codes
- 207 Multistatus which was not mentioned above
- 403 Forbidden which was not mentioned above.
- 424 Failed dependency which was not mentioned above.
- we don't mention what the failure response should look like.
- comment: 200 OK seems like a better response than 204 No Content. The brief explanation isn't persuasive and seems to say that the response code should serve the purpose of the Content-Length. header.
- we should probably explicitly say if an UNLOCK can only be done on the original resource... and will fail even if the resource specified is locked by virtue of being a child of the original resource. Or is this too obvious? I know it's something easy to goof up in an implementation.
(1) 8.10.4 is wrong. The return code is 207. See issue 037_DEEP_LOCK_ERROR_STATUS,
resolved in draft 01.
(2) General agreement that descriptions of error marshalling needs to be redone. This applies both th LOCK and UNLOCK.
(3) Agreement that the argument given for 204 is lame; clients should handle all 2xx status codes with the notable exception of 207 as "success". We may want to explain that in RFC2518bis' section about 207.
|I 056_DEPTH_LOCK_AND_IF (type: change, status: closed)|
|firstname.lastname@example.org||2000-03-04||The specification is currently silent on how to use the If header for submitting a locktoken when performing a DELETE in a Depth infinity locked collection. Should the If header have both the collection URL and the Request-URI, or just the Request-URI? An example of this is needed.|
|email@example.com||2004-06-19||See http://lists.w3.org/Archives/Public/w3c-dist-auth/2004AprJun/0194.html: existing servers accept both. This is consistent with the concept of "if it appears in the If header, it's submitted". We still need to say something about lock state token evaluation in the If header, though.|
|2004-06-19||Resolution: Closed. See issue 066_MUST_AN_IF_HEADER_CHECK_THE_ROOT_OF_URL for the question of how lock tokens are evaluated in the If header.|
|I 063_LOCKS_SHOULD_THEY_USE_AN_IF_HEADER_TO_VERIFY (type: change, status: open)|
|firstname.lastname@example.org||Is the complexity of the IF header appropriate for the simple task o verifying that a client knowingly owns a lock? The IF header seems to serve a different purpose. One of those purposes is for the server to verify that you have the lock token (and that you know the root of it?). Another is for the client to check some preconditions before doing an action. Another seems to be to specify what lock to refresh in a lock refresh request. This seems to create ambiguity in our definition of the semantics of the IF: header.|
|email@example.com||It is felt by the group that it's important that the client not just own and hold the lock token, but that it also know where the lock is rooted before it does tasks related to that lock. This still leaves the lock referesh issue unresolved.|
|firstname.lastname@example.org||2004-06-02||Re.: using Lock-Token to identify the lock to be refreshed (http://lists.w3.org/Archives/Public/w3c-dist-auth/2004AprJun/0127.html): problems with current rfc2518bis-05 wording; also no support in popular implementations; suggestion to roll-back changes in -bis and keep "If" header based syntax.|
|I 088_DAVOWNER_FIELD_IS_CLIENT_CONTROLED (type: change, status: open)|
|The DAV:owner field of a lock is controlled by the locking client and should not be manipulated by the server. This is the only place the client can store info. The roundtrip details should match what we resolve for the PROP_ROUNDTRIP issue. Examples should also be checked.|
|Resolution: Resolved by repeated statement and no disagreement.|
|I 099_COPYMOVE_LOCKED_STATUS_CODE_CLARIFICATION (type: change, status: open)|
|email@example.com||What resource should be flagged in the multistatus response to locking issues in COPY/MOVE requests?|
|Resolution: Resolved to flag the locking errors at the source resource that was affected by the problem. The details of how to describe the error was deferred to a subsequent version of WebDAV. - 6/15/02 - 2518bis does not reflect this.|
|I 100_COPYMOVE_LOCKED_STATUS_DESCRIPTION (type: change, status: open)|
|The method of describing the details of (beyond what resolved by COPYMOVE_LOCKED_STATUS_CODE_CLARIFICATION) of the underlying cause of various locking and ACL COPY/MOVE problems is deferred. Two proposals were outlined in the discussion, but interest was not great and we clearly don't have interoperability to take these proposals forward.|
This document describes the "locking" extension to the Web Distributed Authoring and Versioning (WebDAV) protocol that allows to keep more than one person from working on a document at the same time. This helps preventing the "lost update problem," in which modifications are lost as first one author then another writes changes without merging the other author's changes.¶
This document uses XML DTD fragments ([XML]) as a purely notational convention. WebDAV request and response bodies cannot be validated due to the specific extensibility rules defined in section 23 of [RFC2518] and due to the fact that all XML elements defined by this specification use the XML namespace name "DAV:". In particular: ¶
A "precondition" of a method describes the state on the server that must be true for that method to be performed. A "postcondition" of a method describes the state on the server that must be true after that method has completed. If a method precondition or postcondition for a request is not satisfied and unless a more specific HTTP status code applies, the response status of the request MUST be either 403 (Forbidden) if the request should not be repeated because it will always fail, or 409 (Conflict) if it is expected that the user might be able to resolve the conflict and resubmit the request.¶
In order to allow better client handling of error responses, a distinct XML element type is associated with each method precondition and postcondition of a request. When a particular precondition is not satisfied or a particular postcondition cannot be achieved, the appropriate XML element MUST be returned as the child of a top-level DAV:error element in the response body, unless otherwise negotiated by the request. In a 207 Multi-Status response, the DAV:error element would appear in the appropriate DAV:responsedescription element.¶
The locking feature introduces the following properties for a resource.¶
<!ELEMENT lockscope (exclusive | shared) > <!ELEMENT exclusive EMPTY > <!ELEMENT shared EMPTY >
The DAV:lockdiscovery property returns a listing of who has a lock, what type of lock he has, the timeout type, the time remaining on the timeout, the associated lock token and the root of the lock. The server is free to withhold any or all of this information if the requesting principal does not have sufficient access rights to see the requested data.¶
<!ELEMENT lockdiscovery (activelock)* >
<!ELEMENT activelock (lockscope, locktype, depth, owner?, timeout?, locktoken?, lockroot) >
|I 3.2_lockdiscovery_depth (type: edit, status: closed)|
|firstname.lastname@example.org||2004-06-19||"depth" can never have the value "1".|
|Associated changes in this document: 3.2.|
<!ELEMENT depth (#PCDATA) >
owner: provides information about the principal taking out a lock; should be sufficient for either directly contacting a principal (such as a telephone number or email URI), or for discovering the principal (such as the URL of a homepage).
<!ELEMENT owner ANY>
timeout: the timeout associated with a lock (defined in Section 8.2).
<!ELEMENT timeout (#PCDATA) >
locktoken: the lock token associated with a lock; the href element contains the lock token.
<!ELEMENT locktoken (href) >
lockroot: the URL of the resource that was addressed in the LOCK request; the href element contains the URL of the resource to which the LOCK request has been applied.
<!ELEMENT lockroot (href) >
href: defined in [RFC2518], section 12.3.
<!ELEMENT lockroot (href) >
DAV:lockdiscovery property for a resource that has two shared write locks on it, with infinite timeouts:
<D:lockdiscovery xmlns:D="DAV:"> <D:activelock> <D:locktype><D:write/></D:locktype> <D:lockscope><D:shared/></D:lockscope> <D:depth>0</D:depth> <D:owner>Jane Smith</D:owner> <D:timeout>Infinite</D:timeout> <D:locktoken> <D:href >opaquelocktoken:f81de2ad-7f3d-a1b2-4f3c-00a0c91a9d76</D:href> </D:locktoken> <D:lockroot> <D:href >http://example.com/container/</D:href> </D:lockroot> </D:activelock> </D:lockdiscovery> <D:lockdiscovery> <D:activelock> <D:locktype><D:write/></D:locktype> <D:lockscope><D:shared/></D:lockscope> <D:depth>0</D:depth> <D:owner>John Doe</D:owner> <D:timeout>Infinite</D:timeout> <D:locktoken> <D:href >opaquelocktoken:f81de2ad-7f3d-a1b2-4f3c-00a0c91a9d77</D:href> </D:locktoken> <D:lockroot> <D:href >http://example.com/container/</D:href> </D:lockroot> </D:activelock> </D:lockdiscovery>
DAV:lockdiscovery property for a resource with no locks on it:
The DAV:supportedlock property of a resource returns a listing of the combinations of scope and access types which may be specified in a lock request on the resource. Note that the actual contents are themselves controlled by access controls so a server is not required to provide information the client is not authorized to see.¶
<!ELEMENT supportedlock (lockentry)* > <!ELEMENT lockentry (lockscope, locktype) >
DAV:supportedlock property for a resource that supports both exclusive and shares write locks:
<D:supportedlock xmlns:D="DAV:"> <D:lockentry> <D:lockscope><D:exclusive/></D:lockscope> <D:locktype><D:write/></D:locktype> </D:lockentry> <D:lockentry> <D:lockscope><D:shared/></D:lockscope> <D:locktype><D:write/></D:locktype> </D:lockentry> </D:supportedlock>
DAV:supportedlock property for a resource that doesn't support any locks at all:
The following sections describe the LOCK method, which is used to take out a lock of any access type or to refresh an existing lock.¶
|I 072_LOCK_URL_WITH_NO_PARENT_COLLECTION (type: change, status: closed)|
|dbrotsky@Adobe.COM||If a LOCK request is submitted to a URL that doesn't have a parent collection, what should be the correct response? Other methods, PUT, MKCOL, COPY, MOVE all require a 409 response in this case. Seems like LOCK should have this requirement as well.|
|2004-06-21||Resolution: Resolved that since LNRs no longer exist (see NULL_RESOURCE_CLARIFY) the server should return 409. We should insure that the new text we add to replace LNRs does not create an ambiguity: http://lists.w3.org/Archives/Public/w3c-dist-auth/2002JanMar/0164.html. See DAV:parent-resource-must-be-non-null precondition.|
LOCK /workspace/webdav/proposal.doc HTTP/1.1 Host: example.com Timeout: Infinite, Second-4100000000 Content-Type: text/xml; charset="utf-8" Content-Length: xxxx Authorization: Digest username="ejw", realm="email@example.com", nonce="...", uri="/workspace/webdav/proposal.doc", response="...", opaque="..." <?xml version="1.0" encoding="utf-8" ?> <D:lockinfo xmlns:D='DAV:'> <D:lockscope><D:exclusive/></D:lockscope> <D:locktype><D:write/></D:locktype> <D:owner> <D:href>http://example.org/~ejw/contact.html</D:href> </D:owner> </D:lockinfo>
HTTP/1.1 200 OK Lock-Token: <opaquelocktoken:e71d4fae-5dec-22d6-fea5-00a0c91e6be4> Content-Type: text/xml; charset="utf-8" Content-Length: xxxx <?xml version="1.0" encoding="utf-8" ?> <D:prop xmlns:D="DAV:"> <D:lockdiscovery> <D:activelock> <D:locktype><D:write/></D:locktype> <D:lockscope><D:exclusive/></D:lockscope> <D:depth>Infinity</D:depth> <D:owner> <D:href >http://example.org/~ejw/contact.html</D:href> </D:owner> <D:timeout>Second-604800</D:timeout> <D:locktoken> <D:href >opaquelocktoken:e71d4fae-5dec-22d6-fea5-00a0c91e6be4</D:href> </D:locktoken> <D:lockroot> <D:href >http://example.com/workspace/webdav/proposal.doc</D:href> </D:lockroot> </D:activelock> </D:lockdiscovery> </D:prop>
This example shows the successful creation of an exclusive write lock on resource http://example.com/workspace/webdav/proposal.doc. The resource http:/example.org/~ejw/contact.html contains contact information for the owner of the lock. The server has an activity-based timeout policy in place on this resource, which causes the lock to automatically be removed after 1 week (604800 seconds).¶
LOCK /webdav/ HTTP/1.1 Host: example.com Timeout: Infinite, Second-4100000000 Depth: infinity Content-Type: text/xml; charset="utf-8" Content-Length: xxxx Authorization: Digest username="ejw", realm="firstname.lastname@example.org", nonce="...", uri="/workspace/webdav/proposal.doc", response="...", opaque="..." <?xml version="1.0" encoding="utf-8" ?> <D:lockinfo xmlns:D="DAV:"> <D:locktype><D:write/></D:locktype> <D:lockscope><D:exclusive/></D:lockscope> <D:owner> <D:href>http://example.org/~ejw/contact.html</D:href> </D:owner> </D:lockinfo>
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>/webdav/secret</D:href> <D:status>HTTP/1.1 403 Forbidden</D:status> </D:response> <D:response> <D:href>/webdav/</D:href> <D:status>HTTP/1.1 424 Failed Dependency</D:status> </D:response> </D:multistatus>
This example shows a request for an exclusive write lock on a collection and all its children. In this request, the client has specified that it desires an infinite length lock, if available, otherwise a timeout of 4.1 billion seconds, if available. The request entity body contains the contact information for the principal taking out the lock, in this case a web page URL.¶
The error is a 403 (Forbidden) response on the resource http://example.com/webdav/secret. Because this resource could not be locked, none of the resources were locked.¶
A LOCK request with no request body is a "LOCK refresh" request. It's purpose is to restart all timers associated with a lock.¶
LOCK /workspace/webdav/proposal.doc HTTP/1.1 Host: example.com Timeout: Infinite, Second-4100000000 If: (<opaquelocktoken:e71d4fae-5dec-22d6-fea5-00a0c91e6be4>) Authorization: Digest username="ejw", realm="email@example.com", nonce="...", uri="/workspace/webdav/proposal.doc", response="...", opaque="..."
HTTP/1.1 200 OK Content-Type: text/xml; charset="utf-8" Content-Length: xxxx <?xml version="1.0" encoding="utf-8" ?> <D:prop xmlns:D="DAV:"> <D:lockdiscovery> <D:activelock> <D:locktype><D:write/></D:locktype> <D:lockscope><D:exclusive/></D:lockscope> <D:depth>Infinity</D:depth> <D:owner> <D:href >http://example.org/~ejw/contact.html</D:href> </D:owner> <D:timeout>Second-604800</D:timeout> <D:locktoken> <D:href >opaquelocktoken:e71d4fae-5dec-22d6-fea5-00a0c91e6be4</D:href> </D:locktoken> <D:lockroot> <D:href >http://example.com/workspace/webdav/proposal.doc</D:href> </D:lockroot> </D:activelock> </D:lockdiscovery> </D:prop>
This request would refresh the lock, resetting any time outs. Notice that the client asked for an infinite time out but the server choose to ignore the request.¶
The UNLOCK method removes the lock identified by the lock token in the Lock-Token request header from the resource identified by the Request-URI, and all other resources included in the lock. Note that the UNLOCK request may be submitted to any resource locked by that lock (even those that are locked indirectly).¶
If all resources which have been locked under the submitted lock token can not be unlocked then the UNLOCK request MUST fail.¶
Any DAV compliant resource which supports the LOCK method MUST support the UNLOCK method.¶
A server MAY allow principals other than a lock owner to unlock a resource. In this case, this capability SHOULD be under access control (see [RFC3744], section 3.5). Note that there is a tradeoff in allowing non-owners of a lock to unlock a resource. It can be beneficial to allow non-lock owners to perform UNLOCK requests because it allows the adminstrator of the server to configure the server to grant longer lock timeouts because the administrator knows that there is a process in place to allow users to deal with forgotten locks left by other users. On the other hand, a disadvantage of unlocking someone else's lock is that can create a situation where two users are working on modifications to the same resource at the same time which can result in a client having to perform an merge that wasn't previously planned.¶
UNLOCK /workspace/webdav/info.doc HTTP/1.1 Host: example.com Lock-Token: <opaquelocktoken:a515cfa4-5da4-22e1-f5b5-00a0451e6bf7> Authorization: Digest username="ejw", realm="firstname.lastname@example.org", nonce="...", uri="/workspace/webdav/proposal.doc", response="...", opaque="..."
HTTP/1.1 204 No Content
In this example, the lock identified by the lock token "opaquelocktoken:a515cfa4-5da4-22e1-f5b5-00a0451e6bf7" is successfully removed from the resource http://example.com/workspace/webdav/info.doc. If this lock included more than just one resource, the lock is removed from all resources included in the lock. The 204 (No Content) status code is used instead of 200 (OK) because there is no response entity body.¶
|I 044_REPORT_OTHER_RESOURCE_LOCKED (type: change, status: closed)|
|email@example.com||1999-07-23||In some cases, such as when the parent collection of a resource is locked, a 423 (Locked) status code is returned even though the resource identified by the Request-URI is not locked. This can be confusing, since it is not possible for a client to easily discover which resource is causing the locked status code to be returned. An improved status report would indicate the resource causing the lock message.|
|firstname.lastname@example.org||2004-04-25||Proposal to define a specific precondition element plus specific child elements similar to RFC3744, section 7.1.1.|
|email@example.com||2004-06-18||See summary and proposal in http://lists.w3.org/Archives/Public/w3c-dist-auth/2004AprJun/0192.html.|
|Associated changes in this document: 7.2.|
Lock-Token = "Lock-Token" ":" Coded-URL
The Lock-Token request header is used with the UNLOCK method to identify the lock to be removed. The lock token in the Lock-Token request header MUST identify a lock that contains the resource identified by Request-URI as a member.¶
The Lock-Token response header is used with the LOCK method to indicate the lock token created as a result of a successful LOCK request to create a new lock.¶
If the server supports locking, it MUST return both the compliance class names "2" and "locking" as fields in the "DAV" response header (see [RFC2518], section 9.1) from an OPTIONS request on any resource implemented by that server. A value of "2" or "locking" in the "DAV" response header MUST indicate that the server meets all class "1" requirements defined in [RFC2518] and supports all MUST level requirements and REQUIRED features specified in this document, including: ¶
When submitting a lock request a user agent may also submit an owner XML field giving contact information for the person taking out the lock (for those cases where a person, rather than a robot, is taking out the lock). This contact information is stored in a DAV:lockdiscovery property on the resource, and can be used by other collaborators to begin negotiation over access to the resource. However, in many cases this contact information can be very private, and should not be widely disseminated. Servers SHOULD limit read access to the DAV:lockdiscovery property as appropriate. Furthermore, user agents SHOULD provide control over whether contact information is sent at all, and if contact information is sent, control over exactly what information is sent.¶
This document is the collaborative product of ¶
This document has also benefited from thoughtful discussion by Mark Anderson, Dan Brotksy, Geoff Clemm, Jim Davis, Stefan Eissing, Rickard Falk, Larry Masinter, Joe Orton, Juergen Pill, Elias Sinderson, Greg Stein, Kevin Wiggen, and other members of the WebDAV working group.
|[ISO-11578]||International Organization for Standardization, “ISO/IEC 11578:1996. Information technology - Open Systems Interconnection - Remote Procedure Call (RPC)”, 1996.|
|[RFC2119]||Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels”, BCP 14, RFC 2119, March 1997.|
|[RFC2396]||Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifiers (URI): Generic Syntax”, RFC 2396, August 1998.|
|[RFC2518]||Goland, Y., Whitehead, E., Faizi, A., Carter, S., and D. Jensen, “HTTP Extensions for Distributed Authoring -- WEBDAV”, RFC 2518, February 1999.|
|[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.|
|[XML]||Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E., and F. Yergeau, “Extensible Markup Language (XML) 1.0 (Third Edition)”, W3C REC-xml-20040204, February 2004, <http://www.w3.org/TR/2004/REC-xml-20040204>.|
|[RFC3744]||Clemm, G., Reschke, J., Sedlar, E., and J. Whitehead, “Web Distributed Authoring and Versioning (WebDAV) Access Control Protocol”, RFC 3744, May 2004.|
In section 9.8, [RFC2518] specifies that locks should be refreshed implicitly every time "...any time an owner of the lock sends a method to any member of the lock, including unsupported methods, or methods which are unsuccessful." This features has been removed (locks need to be refreshed explicitly using the LOCK method).¶
In section 7.4, [RFC2518] specifies a special resource type called "lock-null resource" that's being created when a LOCK method request is applied to a null resource. In practice, no real interoperability was achieved because many servers failed to implement this feature properly and few clients (if any) ever relied on that particular functionality.¶
Removing this feature also means that there is no atomic way to create a collection in locked state, but in practice, this doesn't seem to be a problem.¶
Clients can take advantage of the new DAV:lockroot element to discover the URL to which the LOCK request (that created the lock) was applied.¶
The ability to lock a resource provides a mechanism for serializing access to that resource. Using a lock, an authoring client can provide a reasonable guarantee that another principal will not modify a resource while it is being edited. In this way, a client can prevent the "lost update" problem.
This specification allows locks to vary over two client-specified parameters, the number of principals involved (exclusive vs. shared) and the type of access to be granted. This document defines locking for only one access type, write. However, the syntax is extensible, and permits the eventual specification of locking for other access types.
The most basic form of lock is an exclusive lock. This is a lock where the access right in question is only granted to a single principal. The need for this arbitration results from a desire to avoid having to merge results.
However, there are times when the goal of a lock is not to exclude others from exercising an access right but rather to provide a mechanism for principals to indicate that they intend to exercise their access rights. Shared locks are provided for this case. A shared lock allows multiple principals to receive a lock. Hence any principal with appropriate access can get the lock.
With shared locks there are two trust sets that affect a resource. The first trust set is created by access permissions. Principals who are trusted, for example, may have permission to write to the resource. Among those who have access permission to write to the resource, the set of principals who have taken out a shared lock also must trust each other, creating a (typically) smaller trust set within the access permission write set.
Starting with every possible principal on the Internet, in most situations the vast majority of these principals will not have write access to a given resource. Of the small number who do have write access, some principals may decide to guarantee their edits are free from overwrite conflicts by using exclusive write locks. Others may decide they trust their collaborators will not overwrite their work (the potential set of collaborators being the set of principals who have write permission) and use a shared lock, which informs their collaborators that a principal may be working on the resource.
The WebDAV extensions to HTTP do not need to provide all of the communications paths necessary for principals to coordinate their activities. When using shared locks, principals may use any out of band communication channel to coordinate their work (e.g., face-to-face interaction, written notes, post-it notes on the screen, telephone conversation, Email, etc.) The intent of a shared lock is to let collaborators know who else may be working on a resource.
Shared locks are included because experience from web distributed authoring systems has indicated that exclusive locks are often too rigid. An exclusive lock is used to enforce a particular editing process: take out an exclusive lock, read the resource, perform edits, write the resource, release the lock. This editing process has the problem that locks are not always properly released, for example when a program crashes, or when a lock owner leaves without unlocking a resource. While both timeouts and administrative action can be used to remove an offending lock, neither mechanism may be available when needed; the timeout may be long or the administrator may not be available. With a shared lock, another user can at least take out another shared lock and start modifying the resource.
A WebDAV compliant server is not required to support locking in any form. If the server does support locking it may choose to support any combination of exclusive and shared locks for any access types.
The reason for this flexibility is that locking policy strikes to the very heart of the resource management and versioning systems employed by various storage repositories. These repositories require control over what sort of locking will be made available. For example, some repositories only support shared write locks while others only provide support for exclusive write locks while yet others use no locking at all. As each system is sufficiently different to merit exclusion of certain locking features, this specification leaves locking as the sole axis of negotiation within WebDAV.
A lock token is a type of state token, represented as a URI, which identifies a particular lock. A lock token is returned by every successful LOCK operation in the DAV:lockdiscovery property in the response body, and can also be found through lock discovery on a resource.
Lock token URIs MUST be unique across all resources for all time. This uniqueness constraint allows lock tokens to be submitted across resources and servers without fear of confusion.
This specification provides a lock token URI scheme called "opaquelocktoken" that meets the uniqueness requirements. However ↑↓
resources are free to return any URI scheme so long as it meets the uniqueness requirements.
Submitting a lock token provides no special access rights. Anyone can find out anyone else's lock token by performing lock discovery. Locks MUST be enforced based upon whatever authentication mechanism is used by the server, not based on the secrecy of the token values.
Since server lock support is optional, a client trying to lock a resource on a server can either try the lock and hope for the best, or perform some form of discovery to determine what lock capabilities the server supports. This is known as lock capability discovery. Lock capability discovery differs from discovery of supported access control types, since there may be access control types without corresponding lock types. A client can determine what lock types the server supports by retrieving the DAV:supportedlock property.
Any DAV compliant resource that supports the LOCK method MUST support the DAV:supportedlock property.
If another principal locks a resource that a principal wishes to access, it is useful for the second principal to be able to find out who the first principal is. For this purpose the DAV:lockdiscovery property is provided. This property lists all outstanding locks, describes their type, and where available, provides their lock token.
Any DAV compliant resource that supports the LOCK method MUST support the DAV:lockdiscovery property.
Although the locking mechanisms specified here provide some help in preventing lost updates, they cannot guarantee that updates will never be lost. Consider the following scenario:
There are several reasons why the WebDAV protocol itself cannot prevent this situation. First, it cannot force all clients to use locking because it must be compatible with HTTP clients that do not comprehend locking. Second, it cannot require servers to support locking because of the variety of repository implementations, some of which rely on reservations and merging rather than on locking. Finally, being stateless, it cannot enforce a sequence of operations like LOCK / GET / PUT / UNLOCK.
WebDAV servers that support locking can reduce the likelihood that clients will accidentally overwrite each other's changes by requiring clients to lock resources before modifying them. Such servers would effectively prevent HTTP 1.0 and HTTP 1.1 clients from modifying resources.
WebDAV clients can be good citizens by using a lock / retrieve / write /unlock sequence of operations (at least by default) whenever they interact with a WebDAV server that supports locking.
HTTP 1.1 clients can be good citizens, avoiding overwriting other clients' changes, by using entity tags in If-Match headers with any requests that would modify resources.
Information managers may attempt to prevent overwrites by implementing client-side procedures requiring locking before modifying WebDAV resources.
This section describes the semantics specific to the write lock type. The write lock is a specific instance of a lock type, and is the only lock type described in this specification.¶
If a request would modify the content for a locked resource, a dead property of a locked resource, a live property that is defined to be lockable for a locked resource, or an internal member URI of a locked collection, the request MUST fail unless the lock-token for that lock is submitted in the request. An internal member URI of a collection is considered to be modified if it is added, removed, or identifies a different resource. [rfc.comment.12: Copy of GULP, "Locked State". --reschke] ¶
A successful request for an exclusive or shared write lock MUST result in the generation of a unique lock token associated with the requesting principal. Thus if five principals have a shared write lock on the same resource there will be five lock tokens, one for each principal.¶
While those without a write lock may not alter a property on a resource it is still possible for the values of live properties to change, even while locked, due to the requirements of their schemas. Only dead properties and live properties defined to respect locks are guaranteed not to change while write locked.¶
A write lock on a collection, whether created by a "Depth: 0" or "Depth: infinity" lock request, prevents the addition or removal of member URIs of the collection by non-lock owners. As a consequence, when a principal issues a PUT or POST request to create a new resource under a URI which needs to be an internal member of a write locked collection to maintain HTTP namespace consistency, or issues a DELETE to remove an internal member URI of a write locked collection, this request MUST fail if the principal does not have a write lock on the collection.¶
However, if a write lock request is issued to a collection containing member URIs identifying resources that are currently locked in a manner which conflicts with the write lock, the request MUST fail with a 423 (Locked) status code (Note that this can only occur for a request of a "Depth: infinity" write lock).¶
If a lock owner causes the URI of a resource to be added as an internal member URI of a "Depth: infinity" locked collection then the new resource MUST be automatically added to the lock. This is the only mechanism that allows a resource to be added to a write lock. Thus, for example, if the collection /a/b/ is write locked and the resource /c is moved to /a/b/c then resource /a/b/c will be added to the write lock.¶
If a user agent is not required to have knowledge about a lock when requesting an operation on a locked resource, the following scenario might occur. Program A, run by User A, takes out a write lock on a resource. Program B, also run by User A, has no knowledge of the lock taken out by Program A, yet performs a PUT to the locked resource. In this scenario, the PUT succeeds because locks are associated with a principal, not a program, and thus program B, because it is acting with principal A's credential, is allowed to perform the PUT. However, had program B known about the lock, it would not have overwritten the resource, preferring instead to present a dialog box describing the conflict to the user. Due to this scenario, a mechanism is needed to prevent different programs from accidentally ignoring locks taken out by other programs with the same authorization.¶
In order to prevent these collisions a lock token MUST be submitted in the If header for all locked resources that a method may interact with or the method MUST fail. For example, if a resource is to be moved and both the source and destination are locked then two lock tokens must be submitted, one for the source and the other for the destination.¶
Servers SHOULD restrict usage of the lock token to exactly the authenticated principal who created the lock.¶
COPY /~fielding/index.html HTTP/1.1 Host: example.com Destination: http://example.com/users/f/fielding/index.html If: <http://example.com/users/f/fielding/index.html> (<opaquelocktoken:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>)
HTTP/1.1 204 No Content
In this example, even though both the source and destination are locked, only one lock token must be submitted, for the lock on the destination. This is because the source resource is not modified by a COPY, and hence unaffected by the write lock. In this example, user agent authentication has previously occurred via a mechanism outside the scope of the HTTP protocol, in the underlying transport layer.¶
A COPY method invocation MUST NOT duplicate any write locks active on the source. However, as previously noted, if the COPY copies the resource into a collection that is locked with "Depth: infinity", then the resource will be added to the lock.¶
A successful MOVE request on a write locked resource MUST NOT move the write lock with the resource. However, the resource is subject to being added to an existing lock at the destination, as specified in Appendix B.1.4. For example, if the MOVE makes the resource a child of a collection that is locked with "Depth: infinity", then the resource will be added to that collection's lock. Additionally, if a resource locked with "Depth: infinity" is moved to a destination that is within the scope of the same lock (e.g., within the namespace tree covered by the lock), the moved resource will again be a added to the lock. In both these examples, as specified in Appendix B.1.5, an If header must be submitted containing a lock token for both the source and destination.¶
A LOCK method invocation creates the lock specified by the lockinfo XML element on the resource identified by the Request-URI. Lock method requests SHOULD have a XML request body which contains an owner XML element for this lock request, unless this is a refresh request. The LOCK request may have a Timeout header.
Clients MUST assume that locks may arbitrarily disappear at any time, regardless of the value given in the Timeout header. The Timeout header only indicates the behavior of the server if "extraordinary" circumstances do not occur. For example, an administrator may remove a lock at any time or the system may crash in such a way that it loses the record of the lock's existence. The response for a successful LOCK creation request MUST contain the value of the DAV:lockdiscovery property in a prop XML element.¶
In order to indicate the lock token associated with a newly created lock, a Lock-Token response header MUST be included in the response for every successful LOCK request for a new lock. Note that the Lock-Token header would not be returned in the response for a successful refresh LOCK request because a new lock was not created.
The scope of a lock is the entire state of the resource, including its body and associated properties. As a result, a lock on a resource MUST also lock the resource's properties.¶
For collections, a lock also affects the ability to add or remove members. The nature of the effect depends upon the type of access control involved.¶
A resource may be made available through more than one URI. However locks apply to resources, not URIs. Therefore a LOCK request on a resource MUST NOT succeed if can not be honored by all the URIs through which the resource is addressable.¶
The Depth header may be used with the LOCK method. Values other than 0 or infinity MUST NOT be used with the Depth header on a LOCK method. All resources that support the LOCK method MUST support the Depth header.
A Depth header of value 0 means to just lock the resource specified by the Request-URI.¶
If the Depth header is set to infinity then the resource specified in the Request-URI along with all its internal members, all the way down the hierarchy, are to be locked. A successful result MUST return a single lock token which represents all the resources that have been locked. If an UNLOCK is successfully executed on this token, all associated resources are unlocked. If the lock cannot be granted to all resources, a 207 (Multistatus) status code MUST be returned with a response entity body containing a multistatus XML element describing which resource(s) prevented the lock from being granted. Hence, partial success is not an option. Either the entire hierarchy is locked or no resources are locked.¶
If no Depth header is submitted on a LOCK request then the request MUST act as if a "Depth:infinity" had been submitted.
The interaction of a LOCK with various methods is dependent upon the lock type. However, independent of lock type, a successful DELETE of a resource MUST cause all of its locks to be removed.¶
The table below describes the behavior that occurs when a lock request is made on a resource.
|Current lock state / Lock request||Shared Lock||Exclusive Lock|
Legend: True = lock may be granted. False = lock MUST NOT be granted. *=It is illegal for a principal to request the same lock twice.
The current lock state of a resource is given in the leftmost column, and lock requests are listed in the first row. The intersection of a row and column gives the result of a lock request. For example, if a shared lock is held on a resource, and an exclusive lock is requested, the table entry is "false", indicating the lock must not be granted.¶
200 (OK) - The lock request succeeded and the value of the DAV:lockdiscovery property is included in the body.¶
412 (Precondition Failed) - The included lock token was not enforceable on this resource or the server could not satisfy the request in the lockinfo XML element.¶
423 (Locked) - The resource is locked, so the method has been rejected.¶
A LOCK request with a non-empty body creates a new lock, and the resource identified by the ↑↓
request-URL is directly locked by that lock. The "lock-root" of the new lock is the ↑↓ request-URL. If at the time of the request, the ↑ request-URL is not mapped to a resource, a new resource with empty content MUST be created by the request.¶
If a collection is directly locked by a depth:infinity lock, all members of that collection (other than the collection itself) are indirectly locked by that lock. In particular, if an internal member resource is added to a collection that is locked by a depth:infinity lock, and if the resource is not locked by that lock, then the resource becomes indirectly locked by that lock. Conversely, if a resource is indirectly locked with a depth:infinity lock, and if the result of deleting an internal member URI is that the resource is no longer a member of the collection that is directly locked by that lock, then the resource is no longer locked by that lock.¶
An UNLOCK request deletes the lock with the specified lock token. The request-URL of the request MUST identify a resource that is either directly or indirectly locked by that lock. After a lock is deleted, no resource is locked by that lock.¶
A lock token is "submitted" in a request when it appears in an "If" request header.¶
If a request would modify the content for a locked resource, a dead property of a locked resource, a live property that is defined to be lockable for a locked resource, or an internal member URI of a locked collection, the request MUST fail unless the lock-token for that lock is submitted in the request. An internal member URI of a collection is considered to be modified if it is added, removed, or identifies a different resource.¶
If a request causes a directly locked resource to no longer be mapped to the lock-root of that lock, then the request MUST fail unless the lock-token for that lock is submitted in the request. If the request succeeds, then that lock MUST have been deleted by that request.¶
|I 015_MOVE_SECTION_6.4.1_TO_APPX (type: change, status: closed)|
|firstname.lastname@example.org||1998-11-24||The discussion of generating UUID node fields without using the IEEE 802 address in section 6.4.1 can be moved to an appendix.|
|email@example.com||2004-04-31||Plan: get rid of the section altogether and refer to draft-mealling-uuid-urn. In the meantime, move the whole opaquelocktoken discussion into an appendix.|
|2004-06-27||Resolution: (1) Moved to appendix. (2) UUID URNs aren't ready; delay this to future revisions.|
The opaquelocktoken URI scheme is designed to be unique across all resources for all time. Due to this uniqueness quality, a client may submit an opaque lock token in an If header on a resource other than the one that returned it.¶
All resources MUST recognize the opaquelocktoken scheme and, at minimum, recognize that the lock token does not refer to an outstanding lock on the resource.¶
Opaquelocktoken generators, however, have a choice of how they create these tokens. They can either generate a new UUID for every lock token they create or they can create a single UUID and then add extension characters. If the second method is selected then the program generating the extensions MUST guarantee that the same extension will never be used twice with the associated UUID.¶
OpaqueLockToken-URI = "opaquelocktoken:" UUID [Extension] ; The UUID production is the string representation of a UUID, as defined in [ISO-11578]. Note that white space (LWS) is not allowed between elements of this production.¶
UUIDs, as defined in [ISO-11578], contain a "node" field that contains one of the IEEE 802 addresses for the server machine. As noted in Section 10.1, there are several security risks associated with exposing a machine's IEEE 802 address. This section provides an alternate mechanism for generating the "node" field of a UUID which does not employ an IEEE 802 address. WebDAV servers MAY use this algorithm for creating the node field when generating UUIDs. The text in this section is originally from an Internet-Draft by Paul Leach and Rich Salz, who are noted here to properly attribute their work.¶
The ideal solution is to obtain a 47 bit cryptographic quality random number, and use it as the low 47 bits of the node ID, with the most significant bit of the first octet of the node ID set to 1. This bit is the unicast/multicast bit, which will never be set in IEEE 802 addresses obtained from network cards; hence, there can never be a conflict between UUIDs generated by machines with and without network cards.¶
If a system does not have a primitive to generate cryptographic quality random numbers, then in most systems there are usually a fairly large number of sources of randomness available from which one can be generated. Such sources are system specific, but often include:¶
(Note that it is precisely the above kinds of sources of randomness that are used to seed cryptographic quality random number generators on systems without special hardware for their construction.)¶
In addition, items such as the computer's name and the name of the operating system, while not strictly speaking random, will help differentiate the results from those obtained by other systems.¶
The exact algorithm to generate a node ID using these data is system specific, because both the data available and the functions to obtain them are often very system specific. However, assuming that one can concatenate all the values from the randomness sources into a buffer, and that a cryptographic hash function such as MD5 is available, then any 6 bytes of the MD5 hash of the buffer, with the multicast bit (the high bit of the first byte) set will be an appropriately random node ID.¶
Other hash functions, such as SHA-1, can also be used. The only requirement is that the result be suitably random in the sense that the outputs from a set uniformly distributed inputs are themselves uniformly distributed, and that a single bit change in the input can be expected to cause half of the output bits to change.¶
Add and resolve issue "rfc2606-compliance". Resolve issues "extract-locking", "updated-rfc2068", "022_COPY_OVERWRITE_LOCK_NULL", "025_LOCK_REFRESH_BY_METHODS", "037_DEEP_LOCK_ERROR_STATUS", "039_MISSING_LOCK_TOKEN", "040_LOCK_ISSUES_01", "040_LOCK_ISSUES_02", "040_LOCK_ISSUES_05", "043_NULL_LOCK_SLASH_URL", "065_UNLOCK_WHAT_URL", "077_LOCK_NULL_STATUS_CREATION", "080_DEFER_LOCK_NULL_RESOURCES_IN_SPEC", "089_FINDING_THE_ROOT_OF_A_DEPTH_LOCK", "101_LOCKDISCOVERY_FORMAT_FOR_MULTIPLE_SHARED_LOCKS", "109_HOW_TO_FIND_THE_ROOT_OF_A_LOCK" and "111_MULTIPLE_TOKENS_PER_LOCK". Add issue "import-gulp". Start work on moving text from RFC2518 excerpts into new sections. Define new compliance class "locking" (similar to "bis" in RFC2518bis, but only relevant to locking). Reformatted "GULP" into separate subsections for easier reference.¶
Update "008_URI_URL", "040_LOCK_ISSUES_06", "063_LOCKS_SHOULD_THEY_USE_AN_IF_HEADER_TO_VERIFY", "067_UNLOCK_NEEDS_IF_HEADER", "068_UNLOCK_WITHOUT_GOOD_TOKEN". Re-opened "065_UNLOCK_WHAT_URL". Close "070_LOCK_RENEWAL_SHOULD_NOT_USE_IF_HEADER". Rewrite UNLOCK and LOCK refresh method descriptions. Fix page title (TXT version). Close "052_LOCK_BODY_SHOULD_BE_MUST", "054_IF_AND_AUTH", "060_LOCK_REFRESH_BODY" and "079_UNLOCK_BY_NON_LOCK_OWNER". Add and resolve "8.10.1_lockdiscovery_on_failure". Started attempt to clarify status code.¶
Resolve issues "040_LOCK_ISSUES_03", "040_LOCK_ISSUES_04", "040_LOCK_ISSUES_08" "053_LOCK_INHERITANCE", "057_LOCK_SEMANTICS", "067_UNLOCK_NEEDS_IF_HEADER" and "068_UNLOCK_WITHOUT_GOOD_TOKEN". Resolve issue "065_UNLOCK_WHAT_URL"; update to new GULP version (5.7). Add and resolve new issue "7.5_DELETE_vs_URIs". Start work on "additional marshalling" and "introduction". Update issues "044_REPORT_OTHER_RESOURCE_LOCKED" and "066_MUST_AN_IF_HEADER_CHECK_THE_ROOT_OF_URL".¶
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