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Clarification of URI vs. resource

From: Jim Whitehead <ejw@ics.uci.edu>
Date: Tue, 3 Nov 1998 23:29:33 -0800
To: WEBDAV WG <w3c-dist-auth@w3.org>
Message-ID: <007101be07c4$de92ce20$d115c380@galileo.ics.uci.edu>
On September 9, 1998, Larry Masinter started a thread titled, "aliasing and
other (primarily) editorial issues with -protocol-08", based on discussion
at the WebDAV WG meeting at the Chicago IETF.  He wrote:

> As you may recall, the issue I raised was one of whether the
> webdav draft required that a DAV-compliant resource had one, and
> only one, URI. I pointed out the various situations of aliasing,
> of which 'case sensitive URI' is only one instance, but other kinds
> of aliasing might also be possible.

Mark Anderson also touched on this issue in his October 30, 1998 post.

The core issues here are:

a) Can a resource have more than one URI, or does each URI imply a new
resource?

b) If a resource can have more than one URI, the language in the WebDAV
spec. needs to be more precise in distinguishing between URI and resource.
In particular, this would be expected to affect the definition of a
collection.

Larry also re-raised this issue in an October 7 email to the list titled
"DAV and "one URL per resource"?"  Despite this, the issue was not
substantively addressed in the -09 specification.  In retrospect, looking at
the list traffic, this issue should have been addressed in the -09 revision.
Larry speaks as chair of the HTTP Working Group and a co-author of the URI
syntax specification, after all.

This message consists of a set of proposed changes to the -09 draft to
clarify:

a) A resource can have more than one URI.
b) A collection is a set of URIs, not a set of resources.

The rest of the modifications are dependent on these clarifications.  These
changes do not introduce any new functional requirements -- they merely
clarify and strengthen the discussion of collections, and operations which
affect the namespace (MOVE, COPY, PUT, etc.)

The intent of this message is to give the working group the opportunity to
review these proposed changes -- and only these proposed changes! -- before
they are added to the draft.  I'd prefer to hear any comments back by
Tuesday, November 10, midnight Pacific time.

--------------------

3 Terminology

URI/URL - As defined in [RFC2396].

Collection - A resource that contains a set of URIs, termed member URIs,
which identify member resources and meets the requirements in section 5 of
this specification.

Member URI - A URI which is a member of the set of URIs contained by a
collection.

Internal Member URI - A Member URI that is immediately relative to the URI
of the collection (the definition of immediately relative is given in
section 5.2).

{the remainder of section 3 is unchanged}

---------------------

5.1 HTTP URL Namespace Model

The HTTP URL namespace is a hierarchical namespace where the hierarchy is
delimited with the "/" character.

An HTTP URL namespace is said to be consistent if it meets the following
conditions: for every URL in the HTTP hierarchy there exists a collection
that contains that URL as an internal member. The root collection of the
namespace is exempt from the previous rule.

Neither HTTP/1.1 nor WebDAV require that the entire HTTP URL namespace be
consistent.  However, certain WebDAV methods are prohibited from producing
results that cause namespace inconsistencies.

Although implicit in [RFC2068] and [RFC2396], a resource MAY be identified
by more than one URI. For example, a resource could be identified by
multiple HTTP URLs.

5.2 Collection Resources

A collection is a resource whose state consists of at least a list of
internal member URIs and a set of properties, but which may have additional
state such as entity bodies returned by GET.  An internal member URI MUST be
immediately relative to the base URI of the collection.  That is, the
internal member URI is equal to the containing collection's URI plus an
additional segment for non-collection resources, or additional segment plus
trailing slash "/" for collection resources, where segment is defined in
section 3.3 of [RFC2396].

Any given internal member URI MUST only belong to the collection once, i.e.,
it is illegal to have multiple instances of the same URI in a collection.
Properties defined on collections behave exactly as do properties on
non-collection resources.

For all WebDAV compliant resources A and B, identified by URLs U and V, for
which U is immediately relative to V, B MUST be a collection that has U as
an internal member URI. So, if the resource with URL http://foo.com/bar/blah
is WebDAV compliant and if the resource with URL http://foo.com/bar/ is
WebDAV compliant then the resource with URL http://foo.com/bar/ must be a
collection and must contain URL http://foo.com/bar/blah as an internal
member.

Collection resources MAY list the URLs of non-WebDAV compliant children in
the HTTP URL namespace hierarchy as internal members but are not required to
do so. For example, if the resource with URL http://foo.com/bar/blah is not
WebDAV compliant and the URL http://foo.com/bar/ identifies a collection
then URL http://foo.com/bar/blah may or may not be an internal member of the
collection with URL http://foo.com/bar/.

{The final three paragraphs of this section are unchanged.}

If a WebDAV compliant resource has no WebDAV compliant children in the HTTP
URL namespace hierarchy then the WebDAV compliant resource is not required
to be a collection.

There is a standing convention that when a collection is referred to by its
name without a trailing slash, the trailing slash is automatically appended.
Due to this, a resource may accept a URI without a trailing "/" to point to
a collection. In this case it SHOULD return a content-location header in the
response pointing to the URL ending with the "/".  For example, if a client
invokes a method on http://foo.bar/blah (no trailing slash), the resource
http://foo.bar/blah/ (trailing slash) may respond as if the operation were
invoked on it, and should return a content-location header with
http://foo.bar/blah/ in it.  In general clients SHOULD use the "/" form of
collection names.

A resource MAY be a collection but not be WebDAV compliant.  That is, the
resource may comply with all the rules set out in this specification
regarding how a collection is to behave without necessarily supporting all
methods that a WebDAV compliant resource is required to support.  In such a
case the resource may return the dav:resourcetype property with the value
dav:collection but MUST NOT return a DAV header containing the value "1" on
an OPTIONS response.

------------------------

7.5 Write Locks and Collections

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 a resource which has a
URI which is an existing 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.

If a lock owner causes the URI of a resource to be added as an internal
member URI of a 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.

------------------------

8.1 PROPFIND

The PROPFIND method retrieves properties defined on the resource identified
by the Request-URI, if the resource does not have any internal members, or
on the resource identified by the Request-URI and potentially its member
resources, if the resource is a collection that has internal member URIs.
All DAV compliant resources MUST support the PROPFIND method and the
propfind XML element (section 12.14) along with all XML elements defined for
use with that element.

A client may submit a Depth header with a value of "0", "1", or "infinity"
with a PROPFIND on a collection resource with internal member URIs.  DAV
compliant servers MUST support the "0", "1" and "infinity" behaviors. By
default, the PROPFIND method without a Depth header MUST act as if a "Depth:
infinity" header was included.

{the next three paragraphs are unchanged}

A client may submit a propfind XML element in the body of the request method
describing what information is being requested.  It is possible to request
particular property values, all property values, or a list of the names of
the resource's properties.  A client may choose not to submit a request
body.  An empty PROPFIND request body MUST be treated as a request for the
names and values of all properties.

All servers MUST support returning a response of content type text/xml or
application/xml that contains a multistatus XML element that describes the
results of the attempts to retrieve the various properties.

If there is an error retrieving a property then a proper error result MUST
be included in the response.  A request to retrieve the value of a property
which does not exist is an error and MUST be noted, if the response uses a
multistatus XML element, with a response XML element which contains a 404
(Not Found) status value.

{the next paragraph was changed}

Consequently, the multistatus XML element for a collection resource with
member URIs MUST include a response XML element for each member URI of the
collection, to whatever depth was requested. Each response XML element MUST
contain an href XML element that gives the URI of the resource on which the
properties in the prop XML element are defined.  Results for a PROPFIND on a
collection resource with internal member URIs are returned as a flat list
whose order of entries is not significant.

{the last two paragraphs are unchanged}

In the case of allprop and propname, if a principal does not have the right
to know whether a particular property exists then the property should be
silently excluded from the response.

The results of this method SHOULD NOT be cached.

----------------------

8.6 DELETE

8.6.1 DELETE for Non-Collection Resources

If the DELETE method is issued to a non-collection resource whose URIs are
an internal member of one or more collections, then during DELETE processing
a server MUST remove any URI for the resource identified by the Request-URI
from collections which contain it as a member.

8.6.2 DELETE for Collections

{the following paragraph was unchanged}

The DELETE method on a collection MUST act as if a "Depth: infinity" header
was used on it.  A client MUST NOT submit a Depth header with a DELETE on a
collection with any value but infinity.

DELETE instructs that the collection specified in the Request-URI and all
resources identified by its internal member URIs are to be deleted.

If any resource identified by a member URI cannot be deleted then all of the
member's ancestors MUST NOT be deleted, so as to maintain namespace
consistency.

{the following paragraph was unchanged}

Any headers included with DELETE MUST be applied in processing every
resource to be deleted.

When the DELETE method has completed processing it MUST result in a
consistent namespace.

{the following paragraph was unchanged}

If an error occurs with a resource other than the resource identified in the
Request-URI then the response MUST be a 207 (Multi-Status).  424 (Failed
Dependency) errors SHOULD NOT be in the 207 (Multi-Status).  They can be
safely left out because the client will know that the ancestors of a
resource could not be deleted when the client receives an error for the
ancestor's progeny.  Additionally 204 (No Content) errors SHOULD NOT be
returned in the 207 (Multi-Status).  The reason for this prohibition is that
204 (No Content) is the default success code.

-----------------------

8.8 COPY Method

The COPY method creates a duplicate of the source resource, identified by
the Request-URI, in the destination resource, identified by the URI in the
Destination header.  The Destination header MUST be present.  The exact
behavior of the COPY method depends on the type of the source resource.

{the following paragraph was unchanged}

All WebDAV compliant resources MUST support the COPY method.  However,
support for the COPY method does not guarantee the ability to copy a
resource. For example, separate programs may control resources on the same
server.  As a result, it may not be possible to copy a resource to a
location that appears to be on the same server.

-------------------------

8.8.3 COPY for Collections

{the following paragraph was unchanged}

The COPY method on a collection without a Depth header MUST act as if a
Depth header with value "infinity" was included.  A client may submit a
Depth header on a COPY on a collection with a value of "0" or "infinity".
DAV compliant servers MUST support the "0" and "infinity" Depth header
behaviors.

A COPY of depth infinity instructs that the collection resource identified
by the Request-URI is to be copied to the location identified by the URI in
the Destination header, and all its internal member resources are to be
copied to a location relative to it, recursively through all levels of the
collection hierarchy.

A COPY of "Depth: 0" only instructs that the collection and its properties
but not resources identified by its internal member URIs, are to be copied.

{the following paragraph was unchanged}

Any headers included with a COPY MUST be applied in processing every
resource to be copied with the exception of the Destination header.

The Destination header only specifies the destination URI for the
Request-URI. When applied to members of the collection identified by the
Request-URI the value of Destination is to be modified to reflect the
current location in the hierarchy.  So, if the Request-URI is /a/ with Host
header value http://fun.com/ and the Destination is http://fun.com/b/ then
when http://fun.com/a/c/d is processed it must use a Destination of
http://fun.com/b/c/d.

When the COPY method has completed processing it MUST have created a
consistent namespace at the destination (see section 5.1 for the definition
of namespace consistency).  However, if an error occurs while copying an
internal collection, the server MUST NOT copy any resources identified by
members of this collection (i.e., the server must skip this subtree), as
this would create an inconsistent namespace. After detecting an error, the
COPY operation SHOULD try to finish as much of the original copy operation
as possible (i.e., the server should still attempt to copy other subtrees
and their members, that are not descendents of an error-causing collection).
So, for example, if an infinite depth copy operation is performed on
collection /a/, which contains collections /a/b/ and /a/c/, and an error
occurs copying /a/b/, an attempt should still be made to copy /a/c/.
Similarly, after encountering an error copying a non-collection resource as
part of an infinite depth copy, the server SHOULD try to finish as much of
the original copy operation as possible.

{the following 2 paragraphs are unchanged}

If an error in executing the COPY method occurs with a resource other than
the resource identified in the Request-URI then the response MUST be a 207
(Multi-Status).

The 424 (Failed Dependency) status code SHOULD NOT be returned in the 207
(Multi-Status) response from a COPY method.  These responses can be safely
omitted because the client will know that the progeny of a resource could
not be copied when the client receives an error for the parent.
Additionally 201 (Created)/204 (No Content) status codes SHOULD NOT be
returned as values in 207 (Multi-Status) responses from COPY methods.  They,
too, can be safely omitted because they are the default success codes.

---------------------------

8.9 MOVE Method

The MOVE operation on a non-collection resource is the logical equivalent of
a copy (COPY), followed by consistency maintenance processing, followed by a
delete of the source, where all three actions are performed atomically.  The
consistency maintenance step allows the server to perform updates caused by
the move, such as updating all URIs other than the Request-URI which
identify the source resource, to point to the new destination resource.
Consequently, the Destination header MUST be present on all MOVE methods and
MUST follow all COPY requirements for the COPY part of the MOVE method.  All
DAV compliant resources MUST support the MOVE method.  However, support for
the MOVE method does not guarantee the ability to move a resource to a
particular destination.

{the final two paragraphs are unchanged}

For example, separate programs may actually control different sets of
resources on the same server.  Therefore, it may not be possible to move a
resource within a namespace that appears to belong to the same server.

If a resource exists at the destination, the destination resource will be
DELETEd as a side-effect of the MOVE operation, subject to the restrictions
of the Overwrite header.

----------------------------

8.9.2 MOVE for Collections

A MOVE with "Depth: infinity" instructs that the collection identified by
the Request-URI be moved to the URI specified in the Destination header, and
all resources identified by its internal member URIs are to be moved to
locations relative to it, recursively through all levels of the collection
hierarchy.

{the following three paragraphs are unchanged}

The MOVE method on a collection MUST act as if a "Depth: infinity" header
was used on it.  A client MUST NOT submit a Depth header on a MOVE on a
collection with any value but "infinity".

Any headers included with MOVE MUST be applied in processing every resource
to be moved with the exception of the Destination header.

The behavior of the Destination header is the same as given for COPY on
collections.

When the MOVE method has completed processing it MUST have created a
consistent namespace at both the source and destination (see section 5.1 for
the definition of namespace consistency). However, if an error occurs while
moving an internal collection, the server MUST NOT move any resources
identified by members of the failed collection (i.e., the server must skip
the error-causing subtree), as this would create an inconsistent namespace.
In this case, after detecting the error, the move operation SHOULD try to
finish as much of the original move as possible (i.e., the server should
still attempt to move other subtrees and the resources identified by their
members, that are not descendents of an error-causing collection).  So, for
example, if an infinite depth move is performed on collection /a/, which
contains collections /a/b/ and /a/c/, and an error occurs moving /a/b/, an
attempt should still be made to try moving /a/c/. Similarly, after
encountering an error moving a non-collection resource as part of an
infinite depth move, the server SHOULD try to finish as much of the original
move operation as possible.

{the following two paragraphs are unchanged}

If an error occurs with a resource other than the resource identified in the
Request-URI then the response MUST be a 207 (Multi-Status).

The 424 (Failed Dependency) status code SHOULD NOT be returned in the 207
(Multi-Status) response from a MOVE method.  These errors can be safely
omitted because the client will know that the progeny of a resource could
not be moved when the client receives an error for the parent.  Additionally
201 (Created)/204 (No Content) responses SHOULD NOT be returned as values in
207 (Multi-Status) responses from a MOVE.  These responses can be safely
omitted because they are the default success codes.

--------------------------------

9.3 Destination Header

Destination = "Destination" ":" absoluteURI

The Destination header specifies the URI which identifies a destination
resource for methods such as COPY and MOVE, which take two URIs as
parameters.  Note that the absoluteURI production is defined in [RFC2396].

---------------------------------

12.13.2 set XML element

Name:	set
Namespace:	DAV:
Purpose:	Lists the DAV property values to be set for a resource.
Description: The set XML element MUST contain only a prop XML element.  The
elements contained by the prop XML element inside the set XML element MUST
specify the name and value of properties that are set on the resource
identified by Request-URI.  If a property already exists then its value is
replaced. Language tagging information in the property's value (in the
"xml:lang" attribute, if present) MUST be persistently stored along with the
property, and MUST be subsequently retrievable using PROPFIND.

----------------------------------

That's it!

- Jim
Received on Wednesday, 4 November 1998 03:34:08 GMT

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