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Re: WebDAV Bindings - Issue Yaron.AtomicDelete

From: Geoffrey M. Clemm <geoffrey.clemm@rational.com>
Date: Fri, 28 Jan 2000 10:09:59 -0500
Message-Id: <10001281509.AA00047@tantalum>
To: yarong@Exchange.Microsoft.com
Cc: w3c-dist-auth@w3.org, danlo@Exchange.Microsoft.com, brianan@Exchange.Microsoft.com

Like Jim, I'd like to thank Yaron allocating out some non-existent
free time for pursuing this issue!  A few comments:

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

   I explained to Brian and Dan that I needed to implement WebDAV over existing
   NTFS and FAT file systems and that it was critical that our WebDAV
   implementation allow for multiple simultaneous protocol access through
   NFS/Win32/SMB/FTP/WebDAV/etc.

I agree.  Note that this of course does not mean that a WebDAV
DELETE is anything like the operations called "delete" in any of
these other protocols.

Furthermore, I believe it is reasonable for a particular
protocol to provide access to only a subset of the repository.
In particular, it is reasonable to say that you can't point
WebDAV at the root of a file system, but only one level down.
This leaves a place to put "hidden" administrative or implementation
data (such as collections that have been DELETE'd but not yet
garbage collected).

   I asked them, given this requirement, if it
   would be possible to simulate an atomic delete by just moving the tree we
   wanted to delete and then begin the file-by-file delete.

   Both Brian and Dan agreed that the move operation is atomic and that this
   would allow one to make the entire sub-tree disappear from a particular
   point in the namespace in a single atomic act.

And as long as the place it was moved to was invisible to WebDAV access,
the "atomic" definition of DELETE is satisfied.

   However both Brian and Dan
   pointed out that the problems really begin once you have moved the tree. The
   both pointed out that if I insist on allowing multiple access to the files
   in question through WebDAV as well as FTP/SMB/NFS/Win32/etc. then it would
   be impossible to perform an atomic delete.
   The crux of the problem is that NFS, amongst others, does not necessarily
   refer to files using their path location. They often refer to files using
   File IDs which are completely independent of location. This means that the
   attempt to move the files from one part of the namespace to another in order
   to hide them won't work with NFS. NFS and similar systems will continue to
   be able to operate on the files, regardless of where they are moved to.

As long as these files are not visible to WebDAV clients, it doesn't matter
if other protocol operations continue to work on them.

   This means that even if the move on the sub-directory succeeds it is
   possible for a NFS client to come in and open (without the filesharedelete
   flag) or change the ACL on a file and hence prevent us from deleting the
   file.

That's OK, since the file is in the "invisible" location on the file
volume, it doesn't matter to WebDAV that some other protocol still has
access to those bits.  The file may also still exist in some backup
archive form on that host, and we don't care about that either.

   Even if we attempt to change the ACL on a file to prevent others from
   accessing the file another user with a File ID and a higher access can
   always override us. 

That's OK, as long as to a WebDAV client, it looked like the atomic delete
happened.

   The result is that the atomic move provided by the file system doesn't buy
   us much because it doesn't "hide" the files from everyone.

As indicated above, we can't guarantee that a DELETE will hide the files
from everyone.  In the case of multiple bindings, we are explicitly saying
that a DELETE doesn't even hide the file from WebDAV clients (i.e. they
can still see that file from any of the remaining bindings).

   This means that
   there is no way for us to be sure, once we begin actually deleting files,
   that we will successfully be able to delete all of this files.

If DELETE means that you no longer see the specified resource at the
specified URL, then we can successfully delete all the resources.  If
DELETE means that the bits are permanently erased from all possible
forms of access, then it is impossible to implement (in the presence
of backups, etc.).

   This then
   means that we can end up with a half deleted tree. A tree we can not
   subsequently move back after the failures because to do so would violate the
   atomic guarantee of an all-or-nothing delete.

By the "not visible at this URL" definition, a MOVE successfully performs
the delete.  Garbage collecting the space after the DELETE is not something
the protocol should (or could) define.

   So the bottom line is that we can't implement an atomic delete and allow for
   multiple simultaneous file system access through multiple protocols. Since
   our users absolutely demand multiple simultaneous file system access through
   multiple protocols we can not implement atomic delete.

I think the key questions here is:

Does delete mean "this resource is no longer visible at this URL" (as
proposed in the bindings protocol), or "the bits in this resource are
permanently removed from all possible access".

If we pick the former definition (which is the only one that is
compatible with the needs of multiple bindings and versioning), I
believe it is clear that atomic delete is feasible.

Cheers,
Geoff
Received on Friday, 28 January 2000 10:10:06 GMT

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