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RE: Review: Redirect Reference Resources -- Part one

From: Slein, Judith A <JSlein@crt.xerox.com>
Date: Mon, 14 Feb 2000 13:51:54 -0500
Message-ID: <8E3CFBC709A8D21191A400805F15E0DBD245A6@crte147.wc.eso.mc.xerox.com>
To: "'Juergen Reuter'" <reuterj@ira.uka.de>, WebDAV WG <w3c-dist-auth@w3.org>
Cc: jjh@ira.uka.de

Thanks very much for your detailed review of the first 6 sections of
Redirect References.  I am sure that most of your suggestions for improved
language and focus in the spec will be accepted.  I'll just comment here on
a few of the deeper issues that you raise.

-----Original Message-----
From: Juergen Reuter [mailto:reuterj@ira.uka.de]
Sent: Monday, February 07, 2000 8:51 AM
Cc: reuterj@ira.uka.de; jjh@ira.uka.de
Subject: Review: Redirect Reference Resources -- Part one

> If the client is aware that it is operating on a redirect reference 
> resource, it can resolve the reference by retrieving the reference 
> resource's DAV:reftarget property (defined in Section 12.1 below), whose 
> value contains the URI of the target resource.  It can then submit 
> requests to the target resource. 

Why not using the HTTP Location header field to resolve the reference?
This can be (and usually is) done even by clients that are unaware of
the above.  The true reason for introducing a DAV:reftarget property
seems to be the possibility to *modify* the URI of the target resource
rather than resolving the reference.

You are right that we need to step back and consider why we introduced the
DAV:reftarget property to begin with:

There is no other way to find the redirect references to a given resource
except by doing a search on this property.  (Even with DAV:reftarget, you
can only discover the local redirects to a given resource.)

A referencing-aware client can operate more efficiently with this property.
For example, if it needs to perform some operation on the targets of all of
the redirect references in a collection, it can do a PROPFIND to retrieve
all the DAV:reftarget properties at once, and submit requests directly to
the target resources, instead of performing two round-trips for each
redirect reference.

At the moment the spec prevents the use you suggest, since it says that
DAV:reftarget is readonly.  The rationale for this was that changing the
value of DAV:reftarget (changing what resource it points to) is effectively
to create an entirely new redirect reference, so we would do better to
require clients to do a new MKRESOURCE with the desired DAV:reftarget.

But since the server is (probably) not going to maintain DAV:reftarget, the
client might just be wanting to update it to keep it pointing at the same
resource.  So maybe it should be a client-writable property.

> A redirect reference resource is a new type of resource. To distinguish 
> redirect reference resources from non-reference resources, a new value 
> of the DAV:resourcetype property (defined in [WebDAV]), DAV:redirectref, 
> is defined in Section 13.1 below.

Just wondering: is there a standardized registration procedure for new
DAV:resourcetype property values, e.g. something like MIME
registration procedures as defined in RFC 2048?  If not, that might
substantially impair interoperability.

There is no standardized registration procedure.  Should we make this an
issue on the RFC-2518 list?

> The body of the new resource is empty.

What is the body of a resource?  For standard data containers, the
intended meaning seems to be clear, but what is the meaning for other
resource types?  In this case, this probably should read "The entity
body of a response returned from a GET request on the new resource
MUST be empty.", or the term "body of a resource" should be defined
somewhere else.  (N.b.: WebDAV also uses the term "body of a resource"
in sections 8.10.2 and 17.5 without prior definition).  Anyway, for
clients that do not support automatic redirection, it is sensible to
provide some text/html response body to be returned from a GET request
that can be shown to the user.  Hence, I move to take away the
restriction that the body must be empty.

You are right that something needs to be done about this language.  Joe
Orton also pointed out that there is confusion here and in our discussion of
PUT with redirect references about what is allowed and what the result is.

MKRESOURCE is defined not just for this spec, but also for DeltaV, and the
plan is for the two specs to contain identical definitions (Section 5.1 of
redirect references should be identical to what appears in the DeltaV spec).
The intent is that the MKRESOURCE method lets you create a resource and set
its properties, but it doesn't provide any way to do the equivalent of a
PUT.  So we need to find a way to express that without preventing the useful
behavior you describe, of responding to GET with a response body explaining
that the resource you are looking for is located somewhere else.

> The properties of the new resource are as specified by the 
> DAV:propertyupdate request body, using PROPPATCH semantics.


This leads to a new question: is MKRESOURCE atomic (from all WebDAV
client's perspective)?  Or can another client access the new
resource's properties before they fully have been initialized?

If an error is encountered while initializing the properties, that
means that some properties may have been initialized, while others
still are undefined; atomicity would at least in that case be broken.
However, if MKRESOURCE could atomically create and (read & write) lock
a resource, this would prevent other clients from accessing the newly
created resource until the creator decides to remove the lock.  Hence,
I would like to put up for discussion the following question: Should a
MKRESOURCE request may contain a DAV:lockinfo XML element and those
request headers defined for the LOCK method in order to request atomic
creation and locking of a new resource?

I would rather not take on this issue in the context of redirect references.
Whatever the semantics of PROPPATCH are in RFC 2518, that's what you get
here.  If RFC 2518 provides a way to ask that a resource be read-locked
while its properties are being set, it can be used here.  Otherwise, not.

I would actually be just as happy myself to have MKRESOURCE do nothing more
than create a resource, and make setting its properties be a separate
operation.  But I know that others feel strongly that MKRESOURCE needs to do
Received on Monday, 14 February 2000 13:52:19 UTC

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