Subject: RE: Status code for creating lock-null resource

From: "Tim Ellison" <[email-masked]>
Date: Fri, 15 Jun 2001 21:03:52 +0100

This is cool.

> A lock-null resource is a null resource that has been locked.  A lock null
> resource has diferent properties that a null resource. Specifically, it
has
> at least the lock discovery properties, and it is a member of its parent's
> collection.

Now I'm confused again.  A lock-null resource (i.e. a URI that has no
resource binding) is-a locked resource?  How did you make that leap?

Is it fair to think of LOCK on a (unmapped) URI as creating a resource just
as I think of PUT creating a resource?  If not, why?

> > If a lock null resource is not a resource, then what is it?
>
> Since a lock null resource has state, I would claim it is a resource. By
the
> act of a client taking out a lock, they have likely made a mapping of a
URL
> to a conceptual resource, and are int he process of fleshing out the
> computer representation of the conceptual resource.

Agreed.

Tim

---

From: "Dan Brotsky" <[email-masked]>
Date: Tue, 26 Jun 2001 23:09:37 -0700

I'm definitely in the camp that thinks lock-null resources should go 
away in the next revise of 2518, and I very much like the way this 
discussion went, which I would summarize as:

0. we don't talk about "null resources" any more, just unmapped-URIs.

1. lock-null resources are just "regular resources" that are required 
to respond to certain methods in certain ways.

2. creating a lock-null resource should return 201

So far I would say a simple rephrasing might be:

a. The result of a LOCK call on an unmapped-URI is that the URI gets 
mapped to a no-content resource that responds to certain (non-write) 
methods with 404 or 405.

But the problem with this rephrasing is that:

3. GET on the resource returns 404 or 405 not 204 (as would be 
expected for a "regular" no-content resource).  [Aside: 404 really 
makes no sense here, given that the resource is required to appear as 
a member of its parent collection - PROPFIND returns 207 but GET 
returns 404?  But that's a different issue.]

4. OPTIONS and other non-write methods return 404 or 405 for no 
particular reason.

5. UNLOCK on the resource returns the URL to an unmapped state.

Note that restrictions #3 and #4 are unmotivated, as far as I can 
tell; the only "special" aspect of this resource (aside from #5) is 
that the server really can't know whether it's a collection or not. 
[Aside: The DAV:resourcetype property really needs another value in 
these cases, such as "indeterminate".  But that's another issue.]

Which brings us to #5: UNLOCK (essentially) deletes the resource.  I 
think I understand the motivation for this: a client that locks the 
URI but then decides not to write it doesn't want to have to do a 
delete in order not to leave an empty resource mapped to the URI. 
But my problem is that the spec *mandates* this behavior, rather than 
leaving up to the server (or allowing the client to request it 
specially as part of the UNLOCK).

Essentially the spec is requiring that a LOCK/UNLOCK combination with 
no intervening write operations do *nothing* to a URI or the resource 
mapped to it.  (This is a mandate that has been picked up in deltaV, 
as well.)  But while I can understand that servers might want to work 
this way, and that clients might want to request this, I can see no 
reason to mandate this.

In fact, my experience with building workflow and document-management 
servers leads me to entirely the other conclusion: servers need the 
discretion to interpret LOCK as a statement of intent in the 
workflow, one that an UNLOCK does not necessarily "undo."  In 
particular, LOCK is the only way in WebDAV for a client to create a 
no-content resource (as opposed to one with content-length 0), and 
just because the client doesn't create the content before unlocking 
doesn't mean that the client wants the created resource (and live 
properties such as its creator) to be expunged from the server.

As we revise 2518, I think we should give servers much more 
discretion about how they handle LOCK/UNLOCK pairs, and possibly we 
should give clients a way of indicating in the UNLOCK that "the lock 
was a mistake, undo it if you can" (much as deltaV allows undoing a 
checkout).  If we do so, I think lock-null resources (and their 
concomitant problems) can be done away with completely, and LOCK can 
simply be seen as creating a no-content resource when applied to 
unmapped URIs.  Servers who wish to preserve the lock-null kinds of 
behavior for those resources are welcome to, and they will still be 
in compliance with the spec.

     dan
-- 
Daniel Brotsky, Adobe Systems
tel 408-536-4150, pager 877-704-4062
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