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Re: Simplifying RFC-2518 Locking: A proposal, lock remapping

From: <ccjason@us.ibm.com>
Date: Sun, 17 Oct 1999 19:52:13 -0400
To: "Geoffrey M. Clemm" <gclemm@tantalum.atria.com>
cc: w3c-dist-auth@w3.org
Message-ID: <8525680D.0082C031.00@D51MTA03.pok.ibm.com>

   That mapping table would need to be global (at least to the workspace).  It
   would need an entry for every URI locked by a depth lock if we plan to
   remap each of those independently.

  Which is reasonable with static depth locks, and quite the pain with
  dynamic depth locks, but I digress ... (:-).

On the contrary... You don't digress.  I was only evaluating it based on your
static proposal... which I will comments on when I get some time.


    i2) This proposal breaks our model whereby all webdav
    resources have parents at their analagous parent URI.  This would not
    be the case here unless URI protection were added.

  I believe that there is no more breakage in this regard than
  is introduced by multiple bindings.  You have a new name for a resource
  *other* than the name it used to have (just like adding a binding).

Hmmm.  "Interesting" way of looking at it.


  Unlike a normal binding, this new name (the <lock-token,URL> pair)
  is stored off in some server maintained place where clients can't
  mess with it.

Ummm.  I'm not sure I follow you.  I am talking from the client's perspective
that the URI parentage is not continuous.   I'm not concerned about servers.  I
expect them to be more sophisticated than clients.  (I don't know what the folks
that spent so much time defining parentage care about though.  I just know that
they must value continuous parentage to have put so much time in on it.)

  Now it's true that this new name effectively exists in a non-WebDAV
  namespace, but WebDAV was carefully designed to ensure that these
  non-WebDAV namespaces can co-exist with the WebDAV ones.
  The fact that this new name is bound to the same resource that
  exists in WebDAV space should cause no confusion.

I don't think I'm following you.  (At the same time... I do follow you but I
feel like I'm being con'ed. :-))


    That is... a whole
    tree of URI's could be locked, but if someone does a MOVE above the
    root of that URI at which the lock request was originally directed,
    then there will be a gap in the parentage tree.

  The WebDAV resource space will have no gaps.  The <lock-token,URL> name
  is still valid.  It's true that you can't treat the new name as forming
  a tree of any interesting kind, but that's not what it is there for.
  It's a way to guarantee that a client has a reliable handle on the
  resource it locked (e.g. for a subsequent PUT/UNLOCK).

I guess you could say it has no gaps if you say that a client is to forget the
URI where it found/locked the resource and only be concerned about the resource
itself now and it's special new compound name and access method.    But if a
client is showing a tree view of the server (ala Windows Explorer for example)
and you need to show the resource you have a lock on... you can't really show
the tree if it's parents have gone away.  Or could you? :-)  You might be able
to figure out a way to show it... but I suspect this will require twisting the
mind of the user in a way that it doesn't need to be twisted. -- BTW, I don't
care about tree views much, but there are a couple folks down the hall from me
the worship them.  And we already know of one large software company that makes
heavy use of them in a popular OS.

    I personally don't care about this,
    but a lot of folks went through a ton of time
    defining and refining how webdav resources must have a parent at their
    parent URI.  A ton.  They must feel this is a valuable feature.  --

  As do I.  If I were violating it, I'd be concerned.  I believe we
  are just "supplementing" it with some additional reliable names.

You were one of those folks. :-)   I'll give this "new namespace" idea some time
to twist my mind.  I do have the feeling that I'm being brain washed.  (Recent
discover! The sun rises in the west.  People have just been holding their
compasses upside down. :-))

Another thought... if we're just going to couch this as a "new namespace" why
don't we just tell folks to use (locally) locatable guid's?  (Never mind.  I
know the answer(s).)



     i4) For depth locks, we'd need to define behaivor.
     Given /a/b/c and a depth lock placed on /a/b/.  It's pretty clear what
     the redirection does for an access to /a/b/.  But what about /a/b/c?
     Does it refer to the resource that was originally at that location?

  If you use the Lock-Token header, it will be the resource that was at
  /a/b/c when you applied the lock.

So if you lock /a/b/ and then replace /a/b/c with another resource... one that
isn't locked... what happens when you do a GET on /a/b/c with a lock token?
It sounds like you get what was originally there... not what's there now.
What happens if you do it without a lock token?  I'm guessing you get a
failure since I guess there's nothing there in the normal URI namespace.
How do you access the new c member of /a/b/?  Add it to the lock before
you move it there?  Will that work?  Or perhaps use another lock on it
and then move it there... and address it at it's old URI with that second
lock?  It sounds like that might work... but will the MOVE really work?
(I think this might have been what JimA was refering to about DELETE and
MOVE.)   Will the MOVE know that you are adding it to the ***locked***
collection... not to the URI that is now null?

    Or does it refer to the current "c" member of the collection that was
    orginally at /a/b/.

  No, the Lock-Token is just there to guarantee access to the original resource.
  If you want to play with the current members of the collection, you'd
  just BIND that collection into some working collection, and then
  issue your requests against that new binding without the
  Lock-Token header.

Ah... And lock that "working" collection in turn so that you could handle
someone
moving/deleting that collection in turn.   Or possibly do this all from the
start so that you wouldn't have to react.

  LOCK  /a/b/  at depth (since we might want to protect /a/b/c and /a/b/d, etc)
      ; let's call the resulting token tokB
  Check that /a/b/ is really the resource you expected there.  Possibly
      check children too if it's important for the action.
  MKCOL /tempthing  to the resource or collection we want to work with.
  LOCK /tempthing depth 0   ; call the resulting token tokTemp
  Check that /tempthing is the empty coll that you just created.  (Too bad we
don't have
       lock null resources)
  BIND  /tempthing/b/   /a/b/  providing tokA for /a/b/

  ; now to remove /a/b/c from /a/b/  I'm not quite sure how to do that.  I think
  ;    it is...  It's not clear what DELETE /a/b/c will do, but I think the
  ;    following will work.
  DELETE /tempthing/b/c  providing the tokTemp token but definitely not the tokB
token.
  ; But no... that won't work if /tempthing has moved... since the remapping
works in
  ; a resource by resource manner.  Adding the tokTemp token won't insure that I
get
  ; the right c resource deleted... or even the right b parent.  Because neither
of
  ; those uses the tokTemp lock.  I'm stuck.  There is no way to delete c out of
b.
  ; Not until we discuss (i6) below.

  ; but let's pretend the DELETE above works.  Let's see how many more methods
are
  ; needed...

  work gets done here on the locked area of the namespace.

  ; Now lets assume work is done.  Now to clean up...

  UNLOCK tokB
  DELETE /tempthing/
  ; but what if it moved.  Will adding a token fix this?  We need to figure
  ;  out how DELETE with this resolution figures out what parent we're talking
about.
  ;  I think this was JimA's point.  (see (i6) below)
  UNLOCK tokTemp  /tempthing/
  ; presumably it will use the token to figure out what resource I'm refering to
  ;  in addition to figuring out what lock I'm removing.

This is in contrast to the following with URI protection...

  LOCK /a/b/ at depth
  Check if what's at /a/b/ is really what you were planning to lock.  Also check
       children if you like.
  DELETE /a/b/c
  do some other work
  UNLOCK tokA

This looks a lot easier for the client.  Sure the first one didn't need to go to
so much trouble.  It just did so so that it had deterministic behavior.  The
latter recipe achieves deterministic behavior at much less expense for the
client.


    i5) We'd have to work out when the redirection is
    triggered.  It's probably not sufficient to say it's triggered when
    there is no resource at a URI.  It would need to also deal with the
    situation where a different resource is now at that location.  But
    that does create another ambiguity rlated to (i4).  Ah... it's a bit
    of a mess.  I'm not going to get into it here.

  I think you're still thinking of it as a "<lock-token,URL> -> new-URL"
  mapping ... it's not.  It's "<lock-token,URL> -> resource".  The server
  stores this information in its lock table at the time of the lock request,
  and doesn't change it until the UNLOCK request is made.  The server
  knows when to use the lock table when it gets a Lock-Token header.
  Otherwise it just ignores the lock table.

I see what you're saying.  The remapping is not triggered on finding a
null resource at a URI.  It's triggered by the existance of the
special header in the request.  If the header is there, remapping is
enabled.  If it is not, then it isn't.

Now let's add another issue that was revealed as I was going through the
scenario in (i4)...

(i6) So what does it mean to DELETE /a/b/c with a token for /a/b/c?  How does
it know what parent to act on?  In this case is the client expected to provide
a token for the parent instead?  I take it this would apply to BIND and MOVE
also.
What if the parent doesn't have a token.  In the example above... I didn't
lock the / URI because that would have caused problems for other clients
pulling the same trick.  So what would the DELETE operator be for /tempthing/?


   and there is no special case URL mapping that would somehow add lock
   tokens to namespace resolution calculations, some of which happen
   outside my server's domain.

  As long as the same server handles the same segment of the URL space
  (e.g. nobody messes with the Apache config file), all requests for
  Lock-Token based URL's will go to the same server.  Sure, if the
  client holds onto the lock token long enough for that to no longer
  apply, then it won't find the resource.  The server just handles that
  like a timeout, (probably it would have timed out anyway by then :-).

I think you may have misunderstood his question.  I think he was refering
to the situation I had in (i4) above where I really didn't have a reliable
way to denote a new resource at /a/b/c since the token only refers to the
destination, not to specific subsegments of the URI.  Of course we
can refine things to handle this, but it's getting more complicated.

I think you two were hinting at cross server bindings.  The thought of that
on top of what we've already said sure is boggling.

    <ja> It seems like if we have to do this, we're still missing something. I
    think the solution is that if the user is conserned about someone
    moving his locked resource, then he should lock the parent
    collection. That's what locking collections is for, to control the
    namespace. </ja>

   Lacking URI protection, he'd also lock all the collections to the
   root.

  No, just the subtree being worked on.  If you ever "lose" the sub-tree
  you were working on, you can use your "lock-token" and the original
  subtree root URL to find your sub-tree, bind it somewhere so you can
  keep working on it.  If somebody moves *that* away, heck, you've still
  got your lock token, and you can rebind to it as often as you need to.

Whoops on both of us.  I think JimA was way ahead of us.  I think he was
thinking about also needing to lock the parent to unambiguously deal with
BIND, DELETE and MOVE.  See (i6) above.

You are correct that they wouldn't need to lock to the root.  Just to
and including the parent.  (I assumed Jim was thinking of an alternative
to the remapping scheme... that's why I said what I said.)

My hat goes off to JimA... it was pretty sharp of him to realize we have
a problem with the parent being ambiguous for some operations.

J.
Received on Sunday, 17 October 1999 19:48:20 GMT

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