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Re: LOCK Scenarios

From: <jamsden@us.ibm.com>
Date: Wed, 18 Aug 1999 09:29:24 -0400
To: w3c-dist-auth@w3.org
Message-ID: <852567D1.004A398E.00@d54mta03.raleigh.ibm.com>

A couple misunderstandings here.

1) I think Judith suggested that you were suggesting that
depth and singleton locks at the destination be treated
differently. At least that was my initial reading.  I've
not found evidence of you saying this though.  As it turns
out it looks like she missed the point of the distinction
between your expectations and Kevin's.  I think the
point is not singleton vs. depth... or singleton vs
inherited.  The point is whether
a LOCK that is rooted (singleton or not) at the destination
URI survives.
depth only applies to the parent of the destination resource when determining if
the new destination gets the lock or not. I do not even consider the resource
type or what kind of lock it has with respect to the COPY or MOVE operation
because it doesn't matter, this lock is not retained. However, if the new parent
collection is depth infinity locked, the destination will inherit the lock
(regardless of what lock the deleted resource might have had), and if the
destination is also a collection, it will have a depth infinity lock too. Just
like it would if you had created the collection with MKCOL or a resource with
  BTW, I propose we use the phrases, "rooted at the URI"
vs "inherited at the URI/resource" when talking about
these locks.  As far as vocabulary goes, there are also
a few other cases that need vocabulary.  I'll save that
for a seperate note.
I don't know what either of these phrases mean. A resource is locked or
unlocked. A resource inherites any depth infinity lock of its parent collection,
no matter how it became a member of that collection.

2) It sounds like you also think that having support for
retaining a lock rooted on URI below a deleted URI is too
messy  to support.  If so, we agree.
That's why I made the proposal
above.  It eliminates everything that I'd consider a
"special case".  It does still leave the issue of if
the destination URI remains locked.  And lets the client
decide that.  I
think this is more of a feature than a special case to
handle.  The client simply declares if he wants to retain
the lock or not when he issues the DELETE.  Depending on
the implementation, the server might even find it *easier*
to support the keeping of the lock.  (AKA no code at all.)
But the proposal would be for the server to support both
behaviors at the DELETE request URI.
I don't think the destination URL retains the lock of the resource that used to
be at that destination unless it just happens to have the same depth infinity
locked parent collection. But this would be a new lock with the timeout reset.

3) 8.10.5 says a DELETE destroys a lock at the destination.
I think this is not optimal.  A scenario is...  /a/b  exists
and is a resource... not a collection.  We want to replace it
with a new resource that is a collection.  But the spec says
that MKCOL can not replace am existing resource (or
collection).  It requires that a delete be performed first.
Now if one wants to accomplish this atomically (with
LOCKS), right now one has several choices.
      a) LOCK /a/.  It would probably be a depth 0 lock since
  there would be no point in locking at depth except possibly
  to prevent someone from tossing in another lock immediately
  after in the /a/b/... tree.    But if they did chose to use
  a depth lock, they'd also risk being unable to get the lock
  if someone for example has a conflicting lock somehwere in
  a /a/c/ tree.  It's a trade off. Depth or not, the other
  disadvantage of it is that they are also locking out any
  operations on /a/.  The point is that if they decide to lock
  /a/, they are locking more than they need to and may
  interfere with other principles... or be blocked themselves.
      b) LOCK /a/b.  But unfortunately with the current spec,
  the initial DELETE will destroy the lock.  So this isn't
  really a choice unless we want to reissue the lock... to
  create a null lock which is better, but still risks problems
  if another entity beats us to the punch.  It's a race condition.

Now if we altered 8.10.5 to allow support for not deleting the
lock, we could act surgically to insure that we get exactly the
safe atomic behavior we want.  (Ignoring a tangential caveat.)
We lock /a/b possibly at depth.  Do the delete specifying that
we want the lock retained.  Then do the MKCOL /a/b/ and any
other operations we require.  When done, release the lock. Once
again ignoring an unspecified caveat, we know that as soon as
we get the lock that we're going to be able to acomplish what
we set out to do without another principle interfering.

If we don't want this, then I think we need to evaluate why we
support lock-null resources.  I'd think the arguments for each
of these are the same.
There are a large number of situations in authoring environments where
transaction semantics are required. WebDAV doesn't (yet) support transactions,
and I don't think we should attempt to come up with a lot of special cases (like
lock-null resources) supported by the protocol to overcome this important
missing function. Rather let's propose an extension that does support
transactions. Might be pretty hard with a stateless server though.

BTW, my final comment above has prompted me to check into the
history of destination lock deletion and lock-null resources...


Here Yaron's solution to atomicity in COPY.  It's not to support locks
on the destination.  It's to simply change the spec to say COPY should
be implemented atomically.  No explanation.  Just his recomendation.
Sounds reasonable.


There was a time where deleting a resource didn't necessarily delete its lock.
   Nov 96.


But deleting the destination lock was in there April 97.  I could find
no explanation for the change.  It looks like they/Yaron decided to start
from scratch and this was the result. -- I believe this is also where
lock-null resources came in.  No earlier references to this.  No documentated
rationale behind it.  Yaron?
Received on Wednesday, 18 August 1999 09:30:57 UTC

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