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An analysis of the proposal to place locks on URIs rather than re sources

From: Yaron Goland <yarong@Exchange.Microsoft.com>
Date: Wed, 23 Feb 2000 23:56:00 -0800
Message-ID: <7DE119D3D0E15543874F7561EECBDBED02619DE6@BEG.platinum.corp.microsoft.com>
To: "'Geoffrey M. Clemm'" <geoffrey.clemm@rational.com>, w3c-dist-auth@w3.org, "'yaron@goland.org'" <yaron@goland.org>
The following mind blowingly long nightmare of a post was never intended to
be posted. The reason is that I find that dropping 3.5 page long posts on WG
mailing lists is a really great way to make people's eyes glaze over.
However I still write up the really long posts because I find that it helps
me ensure that my own arguments are at least internally consistent. It also
allows me to project where arguments about an issue are likely to go. I then
use those projections to head off certain threads before they start and to
generally direct the discussion to the conclusion I would like to see. I
knew reading all that Plato would help some day.

However the changes in my employer are sucking away all my time. As such I
am dumping this nightmare on the group mostly because I know that Geoff will
read it and then call me up and ask lots of questions. I suppose I could
just have sent this to Geoff but there are a number of people on the list
who get really annoyed when I do that. They claim that they actually read
these outrageous monster posts. I think they are daft but I find it best not
to annoy the truly daft.

As such, without further ado, my thoughts on Geoff's original locking
proposal.

Execute summary: 

The proposal to put locks on URIs rather than resources introduces URIs as
first class objects into HTTP that are able to do things like answer
methods. This introduces a number of complications and contradictions into
the HTTP object model. As such, in order to best maintain the HTTP object
model and to enable it to be both robustly expressed and extended one must
understand HTTP in terms of the state of the resources. Part of the state of
a resource is the set of URIs under which it is accessible. A write LOCK
request is then understood as asking, in part, that one member of that set
of URIs only be removable by the lock owner.

Long Winded Examination of Proposal:

I oppose the proposal that locks be placed on URIs rather than on resources.
There is no such thing as a URI in the sense that URIs do not have
addressable existences. Only resources can be addressed and therefore locks
can only exist on resources. To attempt to turn URIs into something that can
have state one introduces an endless recursion. If state can be associated
with a URI then one must be able to access that state. Of course that means
one needs a URI to get to the state of the first URI. But since URIs can
have state that means that the URI that points to the URI with state can
itself have state which means one needs a URI to point to the URI that
points to the URI with state and so on.

Let's examine a manifestation of this problem. 

In his letter below Geoff states that the proposal to associate locks with
URIs rather than resources would result in a situation where "...a LOCK
request *never* requires a lock token". I though hard about this, trying to
understand exactly what it meant. Below I lay out my thinking process so
that if I have misunderstood the proposal my error can be found and
corrected.

Imagine user U takes out a depth 0 lock on collection C. Collection C has no
children. The ramification is that no one but U may add a new child to C.
User A then shows up and wants to create a new resource with the URL C/R.
Before creating the resource, however, A would like to first reserve the
name C/R. To this end A takes out a lock on C/R. To someone thinking in
terms of the current locking system we appear to have reached a
contradiction. No one but U may add a child to C but Geoff has clearly
stated that A can lock C/R without having to present a lock token.

What has happened is that the example illustrates a critical but subtle
difference between the two proposals. In the old system locks are on
resources. Therefore for A to lock the name C/R, A MUST have locked some
resource which claims to be accessible through C/R. This is where lock null
resources came in. Therefore the act of locking C/R created a new resource,
this new resource is a child of C, no one but U may add a child to C,
therefore A's lock request would be denied.

But in the new proposal locks aren't on resources, they are on URIs.
Therefore when A locks C/R it is not locking a resource, it is locking a
name. Names aren't members of collections, resources are. Therefore A can
lock C/R without running afoul of U's lock on C because C/R, even though it
is locked, is still not a member of the collection. When A will run into
trouble is when it attempts, through a COPY, MOVE, PROPPATCH, PUT, MKCOL,
etc. to associate the URL C/R with a resource. By associating a resource
with C/R A will cause that resource to become a member of C. Of course U has
reserved the right to create new members of C, therefore A's request will be
rejected. But this analysis seems to explain Geoff's statement that in the
new locking system one never needs a lock token to take out a lock.

The previous line of reasoning seems to produce at least two major issues. 

The first issue is that I believe that a lock of depth 0 on a collection
should reserve both the right to add new members and the exclusive right to
choose the name of the new members. That means that when U locked C not only
should it be impossible for anyone but U to add a new resource to C but U
should also have the exclusive right to choose whatever name it wants for
that new member. Therefore I would argue that A's lock on C/R should fail
because the act of allowing A to take out the lock would restrict U in his
choice of names for new resources of C. That is, C couldn't decide to create
a new child of C and call it C/R because the name was locked by A even
though C had locked C.

	The trivial solution to this issue is to state that a depth 0 lock
on a collection actually locks both the collection and any names which have
the collection's name as a base. Let us now look at an example where this
solution causes serious problems.

	Let us imagine again that we have a collection C, but this time it
already has a child resource called C/X. User U takes out a depth 0 lock on
C. Under the old locking system this would mean that no one but C could
delete C/X but anyone could, for example, issue a PUT against C/X. U's lock
only covered the adding and removing of members to C, not changing their
state.

	But under the new system user Z could not issue a PUT against C/X if
U has locked C with a depth 0 lock. The reason is that in order to reserve
for U the exclusive right to choose the name of the children of C it was
necessary to lock all names that began with C, including C/X. However the
very definition of the new proposal is that a lock is on the URI, not the
resource. Therefore the act of locking all URIs that began with the segment
C meant that the resources referred to by those children where themselves
locked. That is, by locking C we are forced to lock C/X in order to reserve
for C the right to control the name C/X. But in locking C/X we removed the
right for anyone but U to issue a PUT against C/X.

	The point here is extremely subtle and I apologize for my ham fisted
attempts to tease it out. What is happening is that the new proposal
actually calls for there to be two different types of locks. The first type
of lock is the one on the URI C. This is the lock that prevents anyone from
issuing PUT, DELETE, PROPPATCH, etc. against C. The second type of lock is
the ones on the URIs that begin with the segment C. This lock only prevents
DELETE and if the URI isn't associated with a resource,
PUT/MKCOL/COPY/MOVE/etc. The general rule in protocols is that there are
only three numbers, 0, 1 and infinity. That we need 2 different lock types
to cover this scenario therefore makes me nervous because 2 lock types will
almost certainly mean we actually need an infinite number of lock types to
cover a variety of different situations.

The second issue has to do with the problem of having state on URIs. Let us
use the previous example to illustrate the problem. U has locked C. A has
locked C/R. No resource is associated with C/R. So under the new proposal,
everything is fine. However, what happens when someone performs a PROPFIND,
depth infinity, on C? Do they see C/R? I imagine the answer is no because
C/R, given the reasoning before, is not a member of C. C/R will only become
a member of C when a resource is associated with C/R. However, if U were to
attempt to issue a PUT against C/R then U would get a lock error since the
name C/R is locked by A and therefore only A may associate a resource with
C/R. Of course A can't associate a resource with C/R because of U's lock on
C. But that isn't even the problem I'm worried about. Deadlocks are an
inherent problem in any distributed system. 

	What worries me is how do we figure out which URIs under C are
locked? You can imagine this to be a very common act for an administrator to
want to do. Today the way we handle this is through a recursive PROPFIND.
The reason this works is that one can only lock resources, all resources
which are bound to a name with C as a segment are children of C, all
children of C show up in a recursive PROPFIND. It is because locks are on
resources that we can always discover what under C is locked. The new
proposal, however, places locks on the URI, not the resource. As such the
PROPFIND can't tell you that C/R is locked because C/R is not bound to a
resource and only resources show up in PROPFIND.

	This is then the problem of having state on URIs. Because the URI
C/R has its own state, it is locked, we need a way to find that out. One way
to do this is to issue a LOCK request against C/R. But what does it mean to
issue a method against C/R? Methods are issued against resources yet there
is no resource associated with C/R, therefore who answers the method call?
In the object model I previous proposed
(http://lists.w3.org/Archives/Public/w3c-dist-auth/1999JulSep/0020.html) I
suggested that the resource that would handle the method sent to C/R is the
universal NULL resource. But that answer can't apply here. The reason is
that the universal NULL resource is, well, universal. It doesn't have any
state of its own. 

	Here we need state, that state is that the name C/R is locked. But
again, the lock is on the name, not the resource. There is no resource. So
who answered the method sent to C/R with a LOCK error? In HTTP the only
thing that answers methods are resources but there is no resource here, just
a name.

	The result is that now we need some way of talking to "names".
Without this mechanism there is no way to ask questions like "Which names
under C are locked?".

	This means that URIs are now resources. They must be, otherwise they
couldn't answer methods. If they can't answer methods then we can't get lock
errors. Therefore they must be resources.

	At least by making URIs resources we can ask questions like "which
URIs are locked". However I don't look forward to PROPFIND responses since
they will contain every single possible name. After all, PROPFIND returns
all the resources that are members of C. All URIs are resources. Therefore
all possible URIs under C are members of C and therefore will be returned by
PROPFIND.

	Of course, URIs can't be resources. Because the whole new locking
proposal is that locks aren't on resources at all. Ack.

In conclusion (yes, it's over!!!!!) I believe that in order to best maintain
the HTTP object model and to enable it to be both robustly expressed and
extended one must understand HTTP in terms of the state of the resources.
Part of the state of a resource is the set of URIs under which it is
accessible. A LOCK request is then understood as asking that one member of
that set of URIs not be removed from the table during the duration of the
LOCK. In other words, resources have state, URIs don't.

			Yaron

> -----Original Message-----
> From: Geoffrey M. Clemm [mailto:geoffrey.clemm@rational.com]
> Sent: Thu, December 30, 1999 2:33 PM
> To: w3c-dist-auth@w3.org
> Subject: Re: Creating a lock-null in a locked collection
> 
> 
> 
> As an addendum to Yaron's response, I'll note that there is a proposal
> that WebDAV locking is better understood as locks on URL's rather than
> on resources.  This not only makes the current WebDAV locking behavior
> easier to motivate (e.g. a resource "loses" a lock after a move
> because it is no longer identified by the locked URL), but removes the
> need to define a "lock null resource".
> 
> One effect of this proposal would be that the answer to Joe's question
> would change, i.e. that a LOCK request *never* requires a lock token.
> A LOCK request can fail (such as when it would collide with 
> an existing
> exclusive lock), but no failure would be resolvable through 
> the addition
> of a lock token.
> 
> FYI, the current state of the lock proposal is:
> 
> A lock can be placed on a URL with a LOCK request.  This URL is called
> the "lock base".  If the lock is Depth:N, then a lock is induced on
> any URL that consists of the lock base extended by N or fewer
> segments.  Only an authorized holder of a lock token for a locked URL
> can modify either the state of the resource identified by the URL, or
> which resource is identified by the URL.  A URL MAY NOT be locked
> with two exclusive locks with the same locktype.
> 
> Cheers,
> Geoff
> 
>    From: "Yaron Goland (Exchange)" <yarong@Exchange.Microsoft.com>
> 
>    The depth 0 lock controls the ability to add level 1 
> children to the
>    collection.
>    A lock null resource is a resource and hence is a member 
> of whatever
>    collections it exists within.
>    Therefore to create a lock null resource as a level 1 
> child of a collection
>    with a depth 0 lock one must have the depth 0 lock.
> 
>    In other words, yes, you must submit the token on the LOCK 
> on /foo/bar.
> 
> 			   Yaron
> 
>    > -----Original Message-----
>    > From: Joe Orton [mailto:joe@orton.demon.co.uk]
>    > Sent: Thu, December 30, 1999 6:45 AM
>    > To: w3c-dist-auth@w3.org
>    > Subject: Creating a lock-null in a locked collection
>    > 
>    > 
>    > I have an depth 0 write lock on /foo/. If I do LOCK /foo/bar 
>    > to create a
>    > lock-null resource, do I need to submit the locktoken 
> for /foo/ in the
>    > LOCK request? Or only in the PUT to /foo/bar later?
>    > 
>    > (Or is it decided that lock-null resources are going to go away?)
>    > 
>    > joe
>    > 
> 
Received on Thursday, 24 February 2000 02:56:30 GMT

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