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Re: RFC-2518 LOCK-TOKEN: header

From: Geoffrey M. Clemm <geoffrey.clemm@rational.com>
Date: Wed, 26 Jan 2000 23:39:26 -0500
Message-Id: <10001270439.AA29116@tantalum>
To: w3c-dist-auth@w3.org

   From: jamsden@us.ibm.com

   But Geoff, the semantics you describe below would make locks useless (maybe
   your intent?!).

I believe locks are very useful for avoiding merge conflicts between
cooperating clients.  By "cooperating clients", I mean clients that
whenever they want to update a resource:

 - get an exclusive lock on the resource (LOCK)
 - get the body/properties of the resource (GET, PROPFIND)
 - update the body/properties of the resource (PUT, PROPPATCH)
 - unlock the resource (UNLOCK)

If a locking unaware client tries to update the resource, the locking
protocol prevents the update while the resource is locked, but has no
way of preventing the locking unaware client from getting the state of
the resource before or during the lock, and then unknowingly
overwriting updates from a locking client because it issues its update
after the locking client performed an UNLOCK.

This point is made in section 6.7 of rfc2518.

Now if rfc2518 stopped there, we'd be fine.  But unfortunately, it
doesn't, but tries to present locks as also providing authenticated
access control (e.g. "the right to update a resource").  The reason
this is unfortunate is that "merge avoidance between cooperating
clients" and "authenticated access control" require radically
different support and semantics.

In particular, for merge avoidance, a lock token is required, and
which principal is issuing the request is irrelevant.  On the other
hand, for authenticated access control, the principal is required, and
a "lock-token" is irrelevant.  Thus all the confusion in the working
group about when/where a principal is required and when/where a lock
token is required.

Furthermore, for merge avoidance, you want to protect the URL, because
the URL is what the client is holding as the "key" to identify the
resource being authored.  On the other hand, for authenticated access
control, it is the *resource* that is being controlled, not the URL.
Thus all the confusion in the working group about whether a resource
is being protected or whether a URL is being protected.

So whatever was intended by WebDAV locks, the protocol that is
specified in rfc2518 does a fine job of preventing merge conflicts
for cooperating clients, and is totally inadequate for providing
authenticated access control.  So I propose that anyone that cares
about authenticated access control become active in the WebDAV ACL
specification effort, and just use WebDAV locks for what they can
do, namely prevent merge conflicts for cooperating clients.

   If only the lock token was used to protect from overwrite
   conflicts, then any user could just do a PROPFIND, get the lock token from
   the DAV:lockdiscovery element, and then have at the resource!

Exactly.  Which is precisely what that user can do as soon as you UNLOCK
the resource.  If you intend on keeping the resource permanently
"writable only by a certain principal" (i.e. never issuing an UNLOCK),
then requiring a lock token is pointless, since it is the principal
that matters, and the lock token is irrelevant.  Furthermore, it is
the resource you want to control, and that control should survive
a MOVE, unlike a LOCK.

   The spec
   indicates that the user agent owning the lock, the one taking out the lock
   in the LOCK request, must be specified along with the lock token.

I see no protocol header or XML element in rfc2518 for this purpose.
As you've pointed out, the DAV:owner element does not provide this,
and as you have also pointed out, saying that something should be the
case without providing protocol elements for doing so is useless.

   It does
   not however say exactly what the user agent is. That's because ACLs weren't
   done yet, and the working group didn't want to define principal. So they
   just punted and left if up to the server implementation with
   server-specific ACL mechanisms until the ACL spec was done. This is why the
   principal element wasn't included in the DAV:activelock element. However, I
   think we've made enough progress in defining ACLs and principals that we
   can now safely include this information in DAV:activelock in which case all
   problems are resolved.

I've been basing my statements on what is in rfc2518, not on some
ACL functionality that might (and hopefully will!) appear in some
successor to rfc2518.

But I believe the ACL functionality should be orthogonal to the
locking functionality, and in particular, the ACL's should have no
notion of an "ACL token that must be included in an IF header", nor
should an ACL on a resource imply any "protection" of any URL to that
resource.  When this is done, I expect to see a DAV:acl-discovery
element defined, and with appropriate DAV:principal sub-elements.

Cheers,
Geoff
Received on Wednesday, 26 January 2000 23:39:33 GMT

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