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Re: rfc2518 issue: DEFER_LOCK_NULL_RESOURCES_IN_SPEC

From: Alan Kent <ajk@mds.rmit.edu.au>
Date: Wed, 1 Aug 2001 12:19:17 +1000
To: w3c-dist-auth@w3.org
Message-ID: <20010801121917.A17914@io.mds.rmit.edu.au>
On Tue, Jul 31, 2001 at 01:44:18PM +0100, Hall, Shaun wrote:
> The purpose of LNRs are to "lock the name" (RFC 2518 sec 7.4), which is in
> line for PUTs and MKCOLs. If you drop support for MKCOLs, then you might
> confuse readers as you've diluted this statement....

Personally, I am not too stressed whichever way things go - I have not
implemented locking yet! :-)

Regarding LNR's being dropped - yes, it was suggested, but I think
enough people said "it should stay" so that it will not be removed.

There was another problem area brought up, which I can appreciate.
Its that LOCK for some implementations cannot be guaranteed to be
implemented fully and correctly. For example, consider WebDAV being
an interface onto a UNIX file system. (In my own case, its on to
a database containing web resource objects such as collections
and files. Sticking with a UNIX file system is easier to explain.)

So there is a WebDAV library sitting above the file system API.
Users of WebDAV always come in via the WebDAV library. Other
applications however can go directly to the file system.

  +----------------+ +----------------+ 
  |                | |                |
  | Application 1  | | Application 2  |
  |                | |                |
  +----------------+ +----------------+ 
           |               |
  +----------------+       |
  |                |       |
  |  WebDAV Layer  |       |
  |                |       /
  +----------------+      /
             \           /
           +----------------+
           |                |
           |   File System  |
           |                |
           +----------------+

If the WebDAV layer implements LOCK itself, then it can protect a
web resource against other WebDAV clients, but it cannot protect
itself against other applications going in via a different API.

The solution is for the WebDAV layer to translate the LOCK request
into a file sytem lock request. Then the lock will be honoured by
all applications (not just WebDAV clients).

Lock null resources however have unusual semantics in terms of what
most other systems implement. I don't think the UNIX file system,
for example, has the concept of locking a file name so other applications
cannot use it without a file being created.

There are possible solutions. For example, create a file, lock it,
then if the WebDAV client issues a MKCOL, delete the file and create
a directory instead and hope another application does not sneak in
with a race condition and create the directory first. (Deleting the
file I believe will delete the lock.) But this defeats the purpose of
a lock, doesn't it? It does reduce the chance of a race condition, but
it is not a guarantee.

To summarise: I can implement LNR in the WebDAV layer easily. But as
soon as the underlying data store is accessible by an means other
than WebDAV, the locking model (for a 100% correct implementation of
locks) must be pushed down into a common layer. I believe the problem
with the WebDAV LNR is that it is not a commonly implemented scheme for
locking by other systems, so it will be hard for implementors to
implement correctly according to the spec.

The choices (in my opinion) then are

(1) Its ok for implementors to not implement locking safely - its just
    an aid for applications, not something they should rely on.

(2) Locking is only to protect against other WebDAV clients and not
    against anything else

(3) WebDAV is only for use when the underlying storage model supports
    the LNR concept

(4) The locking model needs to change

I think (3) is not an appropriate choice for WebDAV as it restricts its
scope too much (I don't know of any other system that supports LNR).
I think (2) is silly - what is the purpose of locking if they don't work?
I think the real choices are (1) or (4). With the current WebDAV spec,
(1) is the only choice to me.

Alan
Received on Tuesday, 31 July 2001 22:19:56 GMT

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