Re: Locking: Implementation Considerations

I believe that in the context of advanced collections (multiple
bindings to a single resource) and versioning (multiple workspaces
providing different views of the same URL hierarchy), Depth:infinity
locking will be undesireable both for clients (does the wrong thing)
and for servers (too expensive to implement).

From the server side, Judy has described the difficulty of
implementing Depth:infinity locking in the context of multiple
bindings.  With the introduction of versioning, with rule-based
selection of each versioned collection on the URL path to a versioned
resource, the cost of maintaining Depth:infinity locking increases to
an even more unacceptable level.

From the client side, a Depth:infinity write lock on a collection will
lock all properties of all resources in the workspace's view of that
collection.  This effectively prevents labeling and creating
successors of those resources by another user in *any* workspace,
effectively eliminating many critical aspects parallel development.

In contrast, Depth:0 locks work just fine, since normally you would apply
them only to the working resources you have checked out, and those are
not visible in other workspaces anyway.

So what conclusion can one draw from all this?

One possibility is to say that locking is largely unneccessary in the
presence of versioning (after all, everything is read-only unless you
explicitly check it out into your workspace), so versioning servers
just won't bother with locking at all.  This would have an unfortunate
interoperability result on Class-2 clients try to work with versioning
servers.

Another possibility is to downgrade Depth:infinity locking to a MAY,
thereby warning clients that they are likely to find this not supported,
and to turn the default lock into Depth:0, instead of Depth:infinity.
The latter will cause an interoperability issue with existing class-2
servers, but since there aren't many of those yet, if we move fast, this
would probably be OK.

Cheers,
Geoff

   From: "Slein, Judith A" <JSlein@crt.xerox.com>
   Date: Thu, 5 Aug 1999 17:13:56 -0400 
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   One of the issues we've been talking about is what should happen if you MOVE
   a resource into a locked collection.  What lock should be on the resource
   after the MOVE?

   I think the question is whether collection locks with Depth: infinity should
   be inherited statically or dynamically.  Should a collection lock with
   Depth: infinity affect just those resources that are in the collection at
   the time the lock is created (static inheritance), or should the lock affect
   whatever resources come into the collection while it is in force (and stop
   applying to any resources that are removed from the collection) (dynamic
   inheritance)?

   Static inheritance suggests that the lock would be maintained on the
   collection and also maintained on each resource in the collection to depth
   infinity.  It would be painful to create this lock, and painful to remove
   it, and while it is in force it would be necessary to keep track of the
   MOVEs out of the collection in order to be able to remove the lock correctly
   in the end.  However, if every lock is maintained on each resource it
   affects, it is easy to tell whether a given resource is locked.

   Dynamic inheritance suggests that the lock would be maintained only on the
   collection.  It is easy to create and remove such a lock.  But it is
   difficult to tell whether any given resource is locked when someone attempts
   a PUT, MOVE, etc.  Especially once the BIND method is available, you would
   have to trace from the resource in question upward through all the bindings
   on all the collections in the hierarchy to find out whether the resource is
   locked.

   Currently, section 7.7 of RFC 2518 requires dynamic inheritance of locks.

Received on Wednesday, 18 August 1999 17:33:59 UTC