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RE: Question on uniqueness of exclusive locks

From: Jim Whitehead <ejw@ics.uci.edu>
Date: Tue, 4 May 1999 15:22:41 -0700
To: Henrik Frystyk Nielsen <frystyk@w3.org>, w3c-dist-auth@w3.org
Message-ID: <003301be967c$9fbfa280$d115c380@ics.uci.edu>

> Sorry if this has already been discussed but it seems to me that exclusive
> locks can be replicated using the refresh mechanism and hence
> aren't really guaranteed to be exclusive but rather sort of shared. Look
> this example:
> - While working in my office I get a lock on a resource and start edit it.
> I suddenly have to run home so I save the edits but don't want to release
> the lock as I don't want other people to start editing the document.
> - Later on at home I continue editing the document but the document is
> still locked by me at work. However, I can discover the identity of the
> lock using a PROPFIND and while I can not relock the resource it
> seems that I can do a refresh on it which again seems to give me a copy of
> the lock at home.

Actually, the refresh operation is used to update the start time for the
lock's timeout, not to get access to the lock token (and refreshing the lock
requires passing the lock token in the request -- see section 8.10.9).
PROPFIND on the lock discovery property can be used to retrieve the lock
token, but this does not refresh the lock in any way, it's just a request
for metadata on the resource concerning the active lock.

> - I then edit at home and save the edits back but don't release
> the lock as again, I don't want other people to start editing it.
> I now have two copies of the exclusive lock with two different
> revisions of the document.

No, the lock itself was never replicated. There is still only one lock,
which is on the same server as the resource you were editing.

When you were at work, your client had knowledge of the identifier of the
lock (the lock token), and when you went home, your client at home then
gained knowledge of the identifier of the lock by reading the lockdiscovery
property, but at no time was the lock ever duplicated.

> As there is no apparent link between a lock token and a
> specific revision of the resource (UUIDs at least don't do this), the lock
> can't help me detect the lost update problem and I may loose edits. In any
> case, the lock is no longer exclusive.

Locks don't help you *detect* the lost update problem, they help you
*prevent* the lost update problem.  Locks make it more difficult for lost
updates to occur, but they don't help a client detect lost updates.  You'll
still use If-Match with Etags to do lost update detection.

But, in your scenario, the only way you'll lose updates is if you yourself
decide to blow away your own data.  The only people who can modify the
locked resource in your scenario are those who you have given your
authentication credentials so they can authenticate as you.

In WebDAV, it is the combination of authentication credentials and
displaying knowledge of the lock by passing the lock token in an If header
that allows a write operation to occur on a locked resource.

The lock token alone is insufficient to grant write access to a locked
Authentication credentials alone are insufficient to grant write access to a
locked resource.

- Jim
Received on Tuesday, 4 May 1999 18:32:14 UTC

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