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Re: Locking URIs rather than Resources

From: Eric Sedlar <esedlar@us.oracle.com>
Date: Fri, 07 Jan 2000 15:44:56 -0800
Message-ID: <38767A78.FC32B010@us.oracle.com>
To: ccjason@us.ibm.com
CC: w3c-dist-auth@w3.org
You make a bunch of comments about lock sets and Depth locks, and you don't
seem particularly comfortable with any of this stuff.  This is all coming from
Document Management requirements for compound documents.  I think the correct
solution to these issues comes from understanding that requirement.  Here is a
counterexample to your dynamic lock preference...

Let's say that I have a "book" entity that I want to manifest as a collection
resource (with some additional XML properties describing the book's state).
In the collection is a set of HTML resources corresponding to each chapter of
a book.  Let's say that this book is a programmer's guide  for a software
product, and a couple of these chapters are about "Getting Started".  Now we
add an administration manual (another collection somewhere else) and I want to
share the Getting Started (GS) info between books, so I want to move those
chapters into a third collection containing shared resources between the
manuals.  I have the following collections now:

/manuals/prguide
/manuals/common
/manuals/admin

When I started working on the programmer's guide (PG), I depth:infinity lock
the entire collection with a shared lock, to make sure nobody messes with the
chapters while I am working.  When I move the GS info out of the locked
collection I want to retain these locks, since they are still part of the
manual.

The automatic assumptions Jason proposes regarding when locks are removed in
the face of BINDs, MOVEs, etc. are not going to be correct in all cases.  If
I depth lock a collection, I have a perfectly valid example of when I want to
retain the lock when members of a collection are moved out.  There is no
recourse in this situation, since reestablishing the lock introduces a race
condition.

Therefore, I think it is better that the client (i.e. the application) have
specific control of the locking semantics, and that the server not try to do a
lot of funny stuff with guessing what the client wants.

---------

Other miscellaneous responses:

* the negative depth proposal, along with a header to allow name locking
separate from resource locking, are for the functional completeness of the
model.  The problem with locks is that you often want to get a bunch of them
at once (and have the server handle any deadlock detection issues).  Another
way to handle this is to allow a list of URLs to be LOCKed to be attached to
ANY request.

* when I refer to DELETEing a resource in my proposal earlier, I mean a DELETE
that removes the last binding, deleting the associated resource.

--Eric


ccjason@us.ibm.com wrote:

> >    * a LOCK token must be associated with the locked resource and URL
>   > Umm?  This doesn't sound like a requirement.  At least not a
> fundamental
>   > one.  Feel free to explain what you mean if you think this is an
> important
>   > point.
>   >
>
>   I had this as a requirement for compatability with RFC 2518.  Are you
>   suggesting that lock tokens are superfluous and should be removed?
>
> No.  Just that lock tokens are an implementation of a requirement, not
> a requirement in and of themselves.  If the same can be achieved in some
> other way, that's fine.  You seem to be shooting for compatibility in which
> case LT are the mechanism to use.  -- I didn't actually understand the
> point of your statement either.  That's fine though.  What's most important
> is what you say below.
>
>    It wasn't clear to me in general if shared locks were the default rather
>    than exclusive, but it seems natural that this should be the case for
> both
>    name and resource locks.
>
> I hadn't considered the possibility of name locks being exclusive.  The
> need
> for this wasn't one of our requirements.  We just wanted to protect a URI
> from
> having it's mapping change.  This was purely for the sanity of the client.
>
> My initial vote for resource locks is that there be no default.  One must
> specify what one wants.
>
>    As far as treating name locks as a formal entity,
>    that is a better way of describing what you are doing when you want a
> lock
>    on a URL not associated with a resource--you're locking only that PART
> of
>    the collection resource corresponding to that name in its set of
> bindings,
>    you aren't locking some resource that that name will be associated with
>    later.
>
> I'll just try harder to assimilate this view.
>
>    I think that people who have worked with locking before understand the
>    difference between locking an aggregate object and an individual item in
>    that aggregate, as there are many examples of that in software today
>    (locking a file vs. locking a byte range in that file, table locks vs.
> row
>    locks in a SQL database, etc.).  I don't think people are comfortable
> with
>    the idea of treating namespaces as lockable entities separate from the
>    entities the names refer to, since that isn't generally done.
>
> Actually, only Geoff's most recent proposal couched it that way.  All
> others
> have tended to say that a predetermined set of bindings are "protected".
> The
> term "locked" wasn't used... and after it was set up, it wasn't based on
> namespace... just bindings.
>
>   Once you
>   agree that the set of bindings are part of the state of a collection, you
>   are treating the collection resource as an aggregate that can be locked,
> as
>   well as one or more bindings it contains.  The model I propose is
> simpler,
>   in that only one kind of object WebDAV requests can refer to--the
> resource,
>   without treating URLs as something separate.
>
> A good thing. URL's are just a side effect.  I forget the word Geoff used
> to
> use to refer to this.
>
>   > I agree with the semantics, but I'm uncomfortable with creating a new
>   > concept
>   > called a "lock set".  This also rings of "implementation".
>   > Hopefully this can be avoided.  Anyway...
>
>   The lock set is just a group of locks associated with a single lock
> token.
>   Given that the Depth: infinity header effectively gets many locks (rather
>   than one big lock) and associates them with a single token, you were
> already
>   doing this before, just without a name.  It just seemed to me to be
> easier
>   to understand if you have a term for describing what a lock token
>   identifies.  This is just a terminology issue.
>
> I am still uncomfortable speaking of sets of locks.  Also, that's not what
> we had before except in one proposal that someone made (twice) but that
> proposal didn't last long.  People liked the dynamic nature of this.
> That's
> not to say that it had to be based on namespace.  Just parentage.
>
> Perhaps I shoudln't say this, but with the advent locking of collections,
> do
> we still need depth locks?  I forget why this was important and what set of
> people felt it was important.  I know Jim Whitehead was one of the people
> and I believed the others were document management folks that wanted to
> to lock composite entities.  Anyway, I just wanted to check if we still
> need them.  If so, I'd like to explicitly note this in the locking doc I'm
> doing.
>
> Re: Depth locking...
>
>   >    An RFC 2518 LOCK
>   >    request applied to a collection will work in the same way, except
> that
>   >    if
>   >    the depth header is specified, name and resource locks will be given
>   >    recursively for each contained resource.
>   >
>   > Hmmm.  Or perhaps just resource locks.  No, that won't work either.
>   > Okay...
>   > You're going to have to explain some of the
>   > complications of this.  Like, are all of these resources still locked
> if
>   > you
>   > remove the binding that put them under the influence of the original
> lock
>   > request?
>
>   No.  The LOCK protocol request is a shorthand way of requesting a set of
>   names and resources to lock.  Deleting one of the names or resources you
>   have locked removes the locks, since you have nothing there remaining to
>   lock.
>
> The DELETE only removes the binding.  The resource may still exist and
> be exposed at other URI's.  More below.
>
>   The validity of the relationships between the set of names that are
>   locked and the resources they apply to is not maintained.  Here's a
> boundary
>   case to illustrate the issue:
>
>   Let's say I have resource ID 1 bound by foo.html, ID 2 bound by bar.html,
>   and ID 3 bound by werf.html, all in the same collection.  Now, I issue 3
>   LOCK requests on each URL, and get lock tokens T1, T2, & T3.  Now (using
> a
>   temporary URL) I MOVE everything around so that foo.html refers to
> resource
>   ID 2, bar.html refers to ID 3, and werf.html refers to ID 1.  LOCK token
> T1
>   still refers to foo.html and resource ID 1, even though foo.html now
> refers
>   to resource ID 2, and if I UNLOCK with token T1, foo.html and resource ID
> 1
>   are unlocked.
>
> Yucky!  A very nice clean model, but it doesn't serve the client very
> well if the methods will act this way.  Letting a LT's name locks get
> separated
> from it's resource locks seems like a bad idea and probably serves no
> purpose.
> In addition, the client can't release one type of locks (name) without
> releasing the
> other (resource)... unless the client creates two lock sets explicitly.
>
>   >  I have to provide token T2 to UNLOCK resource ID 1.
>
> I think you mean you have to use T1 to unlock ID 1, but you have to use T2
> to
> unlock the name/binding/slot that currently maps to ID 1.
>
>   > And does adding a resource to a depth locked collection, create a
>   > resource and name lock for the newly bound resource?
>
>   I would say that the default for this would be no, since again the
>   relationships between the names and resources is not something that the
>   server enforces--only clients can do that via use of locks.  Since the
>   collection is locked, you would have to provide the lock token to insert
> the
>   new resource into the locked collection anyway.
>
> The thinking on the list has been that they want it automatically extended
> to
> the children.  Folks have questioned this on occasion and the folks that
> value
> depth locks have always expressed a preference for the locks automatically
> being extended to children.  In addition, they liked having the lock stop
> applying to any child resource that leaves the locked tree.
>
>   However, there may be some need for combining the PUT and LOCK into the
> same
>   request to avoid race conditions.  I can see this being an issue with a
>   number of protocol requests (BIND & LOCK, etc.).  There probably needs to
> be
>   a way to add a lock as a part of any WebDAV request.
>
> Perhaps folks would find this acceptable.  That is, if a client is adding
> to a
> locked collection, the client can specify that the lock be extended to what
> it is
> adding to that collection. -- Or perhaps we'd make the default to extend
> any
> depth locks affecting the collection into which a resource is added.  Noone
> has mentioned actually wanting it NOT to be added.
>
>   > And do name locks remain
>   > if a subtree is separated from the tree where the original LOCK request
>   > was submitted.
>
>   Yes.  That is why each name in the path (logically) is locked separately,
> to
>   simplify the task of tracking all of these types of issues.
>
> When stated as I did above, people have expressed a preference to having
> the
> inherited locks stop acting on the resources that move/delete away.  But
> if it was the resource
> itself that was originally locked (not via inheritance) many folks like it
> to
> continue to affect the resource even after movement/unbinding.
>
>   > If you delete a locked binding, is the name lock automatically deleted?
>
>   If you DELETE a locked resource, the resource lock is removed from the
> lock
>   set, leaving only the name locks.  I.e. I Lock /a/foo.html.  Then I
> delete
>   /a/foo.html.  I still have the name lock, so nobody else can put
> something
>   at /a/foo.html (I'm reserving the namespace like an LNR).
>   I think this will
>   be needed for doing certain types of tree manipulations as an atomic
> unit.
>
> Reserving the namespace can be a good thing.
>
> As for unlocking the resource that was deleted... remember, DELETE just
> removes a binding, and does not necessarily destroy the resource.  It may
> or may not having other bindings and mappings.   I don't disagree that
> this might be the right thing to do, but if we do decide to unlock that
> resource, we have have to cross our fingers when we say the resource lock
> is on a resource.
>
>   > Thus we get the benefit of LNR's without having to create a pseudo
>   > resource
>   > there.  Presumably PROPFIND will not pretend there is a resource there
>   > either.
>   > This sounds very good.  It's the main positive point I see in this
>   > proposal.
>
>   Right.  This is the main goal of my proposal, to create an abstraction
> that
>   is general.
>
>   >    3) the activelocks property displays all name locks active for this
>   >    collection as well as all resource locks.
>   >
>   > BTW, Would activelocks on a collection would reveal name locks on
>   > descendents
>   > rather than the name locks of bindings to the collection?  Hopefully.
>
>   Yes.  The activelocks property (as a property of the collection) should
> only
>   reveal state belonging to that collection.  Since the list of bindings to
>   descendants is the state of the collection, activelocks would list any
> locks
>   on those names.
>
> Yea!  So "LNR"s will reject PROPFIND requests just like ordinary null
> resources.
> Good stuff.
>
>   >    4) the "locktype" property is used to distinguish between name and
>   >    resource
>   >    locks.  If locktype "resource" is specified, only the specified
>   >    resource
>   >    lock is granted in that lock set.  If locktype "name" is specified,
> the
>   >    names from root to the resource are locked, if the Depth header is
> not
>   >    specified.
>   >
>   >
>   >    If the URL "/a/b/c.html" is given, negative depth values can be
>   >    given to lock fractions of the path.  E.g. if Depth:0 is specified,
> the
>   >    name
>   >    "c.html" is reserved in collection /a/b, but no other locks are
>   >    granted.
>   >    If
>   >    Depth:-1 is specified, the name "b" in "/a" is also reserved (etc.)
>   >    Only if
>   >    locktype is not specified does the LOCK request behave as it point
> #2
>   >    (granting resource & name locks simoultaneously), basically for
>   >    compatability with existing applications.
>   >
>   > Is all this needed? I thought the point in locking the name was to
> insure
>   > that
>   > the locked resource remained accessable to the client at a known URI?
>   > "-1"
>   > doesn't achieve that and may prevent trivial implementations on top of
>   > some
>   > respositories.  I'm also not sure I understand the motivation (at least
>   > given our
>   > current capabilities) of a client locking a resource without also
> locking
>   > the
>   > namespace.  Currently it doesn't seem helpful to lock a resource if you
>   > can't
>   > get at it subsequently.
>
>   I can see the value of allowing a URL reference that is derived from a
>   resource ID, like "/resources/1234" or something in some implementations
>   (Oracle's implementation will have this).  Resource ID is constant
>   regardless of BINDings.  I recommend adding something like this to the
>   protocol.
>
> I can see the value in that too.  It sounds like something that could
> neatly
> go in a separate proposal.  It doesn't impact anything else.
>
> Still though... I have my doubts that all of the above concerning negative
> depths is worthwhile. And in fact for platforms like IIS that want to use
> native capabilities, I don't think they'll be albe to do anything other
> than
> lock all of the bindingss up to the root.
Received on Friday, 7 January 2000 18:37:46 GMT

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