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Simplifying RFC-2518 Locking: A second proposal

From: Geoffrey M. Clemm <gclemm@tantalum.atria.com>
Date: Mon, 18 Oct 1999 08:15:48 -0400
Message-Id: <9910181215.AA21066@tantalum>
To: w3c-dist-auth@w3.org
First, thanks to JimW for pointing out several areas of poor word
choice in the original proposal.  I didn't mean to eliminate the ability
to refresh a lock, and didn't mean to say that the live properties were
not part of the state of a resource ... just that they were not part
of the *lockable* state of the resource.

   From: Edgar Schwarz <Edgar.Schwarz@de.bosch.com>

   <jc>  I suspect Geoff didn't mean it, but I think he said (by using the
   word "identify") that the server "tells" the client where the resource
   has moved to. </jc>

   <es> My idea was that the server will tell the client in this case just the
   new URI. Then he can forget the mapping because the client now knows
   the new URI and can reaccess it. Also the client can signal his user
   that the resource has moved. If it is moved again before accessing it
   (a rare event I think) the client will just get a new URI again.  Do
   you see any problems ? </es>

<gmc>
I actually see any problem with this ... I just wasn't thinking
along those lines (even though after reviewing the mail archives, it's
something that both you and Jason have brought up).  And the more that
I (finally! :-) do think about it, the more I like it.

Returning a 302 is enshrined HTTP behavior (for good reason).  We're
even defining a new resource type (redirect references) just so that
we can give the client the ability to specify this useful functionality.
So clients in general will be designed to handle this case.

Jason raised the issue of a "race" condition (i.e. every time you try
to access your locked resource, you get back a new "302" because some
other client keeps moving it).  My response is that if your client
just updates it's location table (as opposed to trying to move it back),
there is no race condition.  The probability of someone trying to rapidly
move it around to a different locations is so low that I really don't
think that we need to be concerned with that possibility.

Finally, I think that the 302 approach gives us the best of both
worlds.  An implementation can chose to just fail any DELETE or MOVE
that would make a locked resource no longer appear at the locking URL.
Clients need to handle DELETE's and MOVE's failing anyway (e.g. the
collection containing the resource is locked).  Alternatively, an
implementation can chose to allow the DELETE or MOVE, as long as it
is tracking an alternative URL that it can give back to the client for
the 302 response.  Again, clients need to handle 302's anyway, so this
is again straightforward for a client to deal with.  Flexibility for server
implementors with little/no additional cost to clients.

   <es> BTW, I also don't want depth locks and lock null resources complicating
   my client. </es>

<gmc> In my earlier proposal, I included static depth locking as a
"compromise" for the depth locking advocates.  But as JimW (as depth
locking spokesman :-) points out, that's not what depth locking advocates
want ... they want dynamic depth locking.

So it's time to rev the lock simplification proposal: Add in the 302
behavior that Edgar advocates, and take out depth locking altogether.
A couple of notes about depth locking:

(1) I don't see client writers clamoring for it ... the only strong
voices in its support appear to be the 2518 authors.  It would be great
if the client writers on this list would weigh in here with some
opinions/guidance.

(2) The reality is that depth locking is not provided in one of the
key WebDAV platforms (Office 2000).  As fun as it is to bash
Microsoft, the Microsoft file system guys probably know as much or
more than anyone about the proper use of file locks for concurrent
access management.  The fact that they chose not to support them is in
my opinion significant.  If nothing else, it is significant in that
any client that wants to work against the Office 2000 server had
better not expect a depth lock request to succeed.  Obviously this
can't be our sole reason for taking them out, but if it seems like a
50/50 call, it is a factor.

So without more ado, here's the modified simplified locking proposal.

Cheers,
Geoff

***** Simplified RFC-2518 Locking, Version-2 *****

A LOCK/UNLOCK request places/removes a lock on exactly one resource
(the resource identified by the request-URL).  A LOCK/UNLOCK request
on a URL that is not mapped to a resource will fail with a 404.

Only an explicit LOCK request ever adds a lock to a resource, and only
an explicit UNLOCK request ever removes a lock from a resource.  In
particular, locks are not deleted as a side effect of a MOVE or a
DELETE, and locks are not added as a side effect of a MOVE due to
"inheriting a lock" from a collection.

A request fails with a 423 if the request would modify the state of a
write-locked resource for which you don't hold (and specify) the lock
token.  The state of a basic resource that is affected by a lock is
its body and its dead properties.  The state of a collection that is
affected by a lock is its set of bindings, its body, and its dead
properties.  A write-lock request on a resource that is already
write-locked will fail with a 423 unless it is an explicit "refresh"
of an existing lock.

When a Lock-Token header is included in any request, if the locked
resource is no longer available at that URL, the server MUST return a
302 indicating a new URL at which the locked resource is available.  A
server MAY refuse to perform a MOVE/DELETE that would cause a locked
resource to no longer be available at the locked URL.

***** End: Simplified RFC-2518 Locking, Version-2 *****
Received on Monday, 18 October 1999 08:15:53 GMT

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