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Re: DELETE leaving a lock-null resource; was LOCK Scenarios

From: Geoffrey M. Clemm <gclemm@tantalum.atria.com>
Date: Sat, 25 Sep 1999 00:44:06 -0400
Message-Id: <9909250444.AA08814@tantalum>
To: w3c-dist-auth@w3.org

   From: "Yaron Goland (Exchange)" <yarong@Exchange.Microsoft.com>

   <gmc/> The proposed protocol would allow a server to lock the namespace,
   but would not *require* it to do so.  This lets a low-end server
   just lock the namespace, while letting a high-end server support
   versioning and multiple bindings while ensuring a low-end (or any)
   client access to his/her locked resource.  A win/win, I believe.

   <yg/> It is lose/lose if you are a client writer. When I write my
   client I MUST know EXACTLY what the behavior is. I can not have a
   situation where I am editing a document in Word, I lock the document
   and then the damn name changes! When I lock a resource I am also
   locking the name I am editing that resource under. It MUST NOT ever
   change. Any other behavior makes writing a simple client impossible.

First, let's be clear on what the proposal is:

When you issue a write lock request on a URL that maps to a resource
R, the state (i.e. body and properties) of R can only be updated by
PUT and PROPPATCH when the request is accompanied by the lock token.
Furthermore, until an UNLOCK is performed, a GET, PUT, PROPFIND, and
PROPPATCH to the original URL when accompanied by that lock token will
always apply to that resource, even if that resource has been MOVE'd
or DELETE'd subsequent to the LOCK request.

One way a server can implement these semantics is to fail any MOVE's
or DELETE's that would modify this URL mapping.  Another way would be
to just remember what resource the URL with that lock token maps to.
Same protocol, different implementation choices.

In either case, the desired effect, namely that I can:
  LOCK U => token-55
  GET U, token-55 => entity
  PUT U, token-55, body
  PUT U, token-55, body
  UNLOCK U, token-55
with the guarantee that all the GET's and PUT's will refer to the same
resource, and that only a client with token-55 can update U after the
LOCK request until the UNLOCK request has been issued.

I admit that the requirement that the token be used with the GET
statement is an incompatibility with the old spec, but it is by
no means complex for clients to handle.

   One of the really scary things I have heard both you and JimA say is how
   glad you are that WebDAV will offer many choices and allow for various
   gradations of service. THIS IS A MISTAKE! We are in the standards business,
   not the buffet business. Choice kills interoperability. That is what makes
   standard writing so damn hard. You can't give everyone everything and have
   interoperability so you have to screw over everybody. Hence the definition
   of compromise as "When no one gets what they want."

There are two very separate issues here.  The first is whether the protocol
should be compatible with a variety of server implementations.  I (and
I'm sure JimA) feel that is a good thing.

The second is whether a protocol should have many or any "optional"
parts.  If there's anything that JimA has been adamantly opposed to,
it is any "optional" protocol parts.  I share that view entirely (for
all the obvious reasons having to do with interoperability and
minimizing client complexity).  It is only with great anguish
(easily commensurate with that involved in making locking optional)
that we have broken the versioning protocol into "core" and "extended"
functionality.  But that is not what is going on here with locking,
so lets defer that discussion until we're dealing with the versioning
protocol.

For the locking protocol, the flexibility that is being discussed
is only of the first kind, i.e. there being one single (simple)
protocol definition, that admits a variety of implementation choices
by servers.

   <yg/>I would remind everyone of two very important WebDAV maximums:
   1) The spec is done when there is nothing left to cut.
   2) There are two types of features, mandatory and not in the spec.

<gmc/> I am a strong believer in both of these maxims.  In particular, the
main changes I want to make to 2518 is in the "cutting" area.  For
example, lock-null resources.  As for optional features, it is
only as a *very* last resort that a protocol is split into two
pieces, but as can be seen in the 2518, sometimes the last resort
is the only place you can go.

   <yg/> Just making locking optional required six months of debate and almost
   resulted in bloodshed.

<gmc/> Our experience with versioning as well, on the issue of breaking
the spec into "core" and "extended".

   <yg/> This is what I mean about thinking out of the box. Someone said "this
   resource is versioned." Once you have an affirmative command like that you
   can do anything you want. For example, you can say "A DELETE on a versioned
   resource without the magic foobar header deletes all links but the one to
   the history, not the resource."

<gmc/> The versioning example was just an example.  The decision by
the advanced collection team to propose that DELETE be "unbind" was
done independently of any needs of versioning.  John Stracke recently
posted a simple authoring example of why unbind is the "right"
semantics in the presence of multiple bindings.

   Yaron: A DELETE on a versioned resource without the magic foobar header
   deletes all bindings but the one to the history, not the resource.

<gmc/> The presence of magic foobar headers that significantly change the
semantics of basic operations is not in my view a recipe for success.
One then has to define the correct behavior of all those magic headers
when combined in various ways, and the likelihood of two servers doing
all the magic in compatible ways decreases severely.  Just look at all
the mail about the behavior of depth locking and URI protection and
null resource locking in the presence of multiple bindings.  Imagine
trying to make sense of this if there was a magical header that
changed the meaning for versioning, and another magical header that
changed the locking behavior of MOVE.

   Notice the difference. The default behavior for DELETE is still to nuke the
   resource. But in a case where we have positive affirmation that the behavior
   is to change then change away.

<gmc/> I have seen no convincing reason for having DELETE nuke the
resource.  For downlevel clients and servers that do not support
multiple bindings, there is no difference between unbind and destroy.
What incompatibility are you trying to avoid by making the semantics
be destroy?

   <yg/> Also notice that this DOES NOT mean that DELETEing a resource with
   references only deletes the reference. It only applies to the particular
   case of a versioned resource.

As above, special casing is not a path that leads to simplicity, either
for clients or for servers.

   <gmc/> In a file system, if you move a resource, it keeps its permissions.
   I don't consider a file system something provided by only a super high
   end provider, but they have no trouble moving permission 
   style information
   along with a resource (I know permissions are not locks, but analogous
   implementations can be used).

   <yg/> True but it doesn't keep its locks.

That's fine.  Then it just fails the move, and the client has to
unlock before moving.

But just for my own edification, I'd *really* be interested in knowing
what these file system locks are that you refer to.  What locks??
I've seen no user visible locks (in the WebDAV sense, with lock
tokens) in either Unix file systems or the Windows95 file system.  I
don't say that there aren't any, but I've never seen them exposed.  Is
this some internal implementation thing that is under the hood?  Did
I just miss them (I believe you've seen this question from me before :-).

   The file system has no way to maintain
   the lock across moves. This becomes especially important for the average
   case where you need to expose that lock through multiple protocols. These
   are the sorts of nitty gritty issues you only live and breath if you ship
   file systems for a living. The problem is that none of the active members
   seem to do that.

If a server can't support a move of a lock, it simply fails the move.
The client gets the error code back, unlocks the resource, and then
performs the move.  Easy for the client, easy for the server.
I continue to fail to see any problem here.

   > I need at least one example of such an unimplementable
   > feature of the protocol before I can address the issue.

   How about locks surviving moves? 

A server is free to refuse a move request it cannot honor for any
reason, whether because it cannot move locks, or because it cannot
perform cross-server moves, or whatever.  In the former case, the
client unlocks and retries the move.  In the latter case, the client
tries a copy/delete if that is OK.  Easy for server to detect and
implement, well defined error codes, simple client responses.

   How about allowing the name of a file to change when locked?

If you have the request-URL for the lock and the lock-token that was
returned, you are guaranteed to be able to keep using them to access
the locked resource.  That's the key use case, and it still works.

So I'm still waiting for something hard for the server or the client (:-).

Cheers,
Geoff
Received on Saturday, 25 September 1999 00:44:10 GMT

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