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FW: Bindings Protocol

From: Jim Whitehead <ejw@ics.uci.edu>
Date: Tue, 25 Jan 2000 10:11:53 -0800
To: WebDAV WG <w3c-dist-auth@w3.org>
Message-ID: <NDBBIKLAGLCOPGKGADOJKEMJCMAA.ejw@ics.uci.edu>
Accidentally caught by the spam filter.

- Jim

-----Original Message-----
From: James J. Hunt [mailto:jjh@ira.uka.de]
Sent: Tuesday, January 25, 2000 1:30 AM
To: w3c-dist-auth@w3.org
Subject: [Moderator Action] Bindings Protocol



Dear WebDAV Contributors,

After having had read the recent postings on the proposed bindings protocol
specification, we would like to add our position on some issues that still
seem unresolved.  Some of this reiterates points that have already been
made, but it is necessary to restate these point to provide a consistent
view.  Some other points are new.

MEMBER URIs

The definition of WebDAV's internal member URIs seems to be one of the
key problems for the bindings protocol: creating a new binding to a
collection results in providing access to its members through
alternate request URIs and hence formally creates new internal member
URIs (as defined in WebDAV) for that collection, hence undesirably
changing its state.

Unfortunately, this is not a server-only issue: clients become aware
of internal member URIs, for example when applying a PROPFIND, i.e.
during client/server communication, which is the central scope of the
protocol.

The question is how can we make the requirement for internal member URIs
consistent with multiple paths to the same resource.  There seems to be no
requirement that the internal member URIs be stored in full in each
collection.  They are just need to be constructible for sending back to a
client.  If each collection and resource contains a list of all collections
that reference the given collection or resource, all internal member URIs
could be constructed.

When enumerating through a collection, each member of the collection would
be seen then just once, and the URI that was used to address the collection
could be used as the base for generating the URIs of the members.  The
situation is a little more complicated for deep iterations, e.g. PROPFIND
on a collection with depth infinity.  Here, if a resource is accessible
through more than one path from the root collection, the simple
implementation would return an entry or result for each path.  Note however
that path from outside this subtree need (and should) not be in the
enumeration.  (A recursive ls in UNIX does exactly this with hard links.)
We
would argue that this is acceptable.  The unique resource identifier can be
used to detect duplicates.

It is probably better to redefine the definition of internal members, but
that is outside our scope.  It would be better to make the above
implementation requirement instead of assuming some other definition.
Trying to compensate more than the above implementation suggests is
undesirable.

SOFT vs HARD

Another central topic is the discussion on soft/weak and hard/strong
links/bindings.  While an exact definition of these terms may slightly vary,
there seems to be agreement about the fact that the redirection reference
resources specification [RR], will cover the specification of some kind of
weak bindings, while DeltaV (particularly with revisioned collections) will
need strong bindings.  Thus hard link should be addressed in the binding
specification.

The binding specification claims that it is not acceptable
for moving a resource through one binding to disrupt another binding.
This seems sensible for use in future specifications like DeltaV that
will build upon the bindings specification.  In practice, this means,
that bindings probably will behave similar to, for example, hard links
in a file system. This should not be too hard to implement.

Moreover, future protocols like DeltaV seem to require that clients may
trace bindings on the server in both directions, from each binding source
(or path segment, as called in the specification) to its single destination
resource, and from each destination resource to its (possibly multiple)
binding sources.  Currently, the bindings specification allows to determine
whether two bindings are to the same resource (section 10).  In order to be

able to find all URI to a particular resource or collection, it is
necessary to implement the optional DAV property DAV:bindings.  This is not
only much more efficient than traversing the URI space of a server and
comparing bindings, but when links for other servers are allowed it may be
the only way to find all visible links (some links may not be visible to a
particular user because of access restrictions).

As for that, one could propose the bindings protocol to extend WebDAV
such that upon explicit request by the client (e.g. through an
additional header) the server returns not only a single member URI in
a single href element on PROPFIND requests, depending on the specific
request URI, but a set of member URIs that contains an arbitrary
single URI out of the set of all URIs for each binding resource that
immediately maps to the request URI.

RESOURCE INTEGRITY

There seems to be still some disagreement about the integrity of binding.
Our position is that binding should maintain integrity, but side effects
should be avoided whenever possible (Roy's no magic criteria).  This
dictates
that DELETE should only remove the given binding, not the resource or
collection itself.  That should (must) be deleted when no more references
exist.

One argument against this is that one can not positively remove a resource
from the server.  Permission restrictions aside, this is possible through
another mechanism, namely DAV:bindings.  By locking a resource and then
iterating through the results of DAV:bindings, the user should be able
to completely remove any resource from a server (again permission
restrictions aside).

Strong bindings *across* servers may, as mentioned in the current
version of the bindings specification, cause severe problems.
As host-to-host communication is the central issue for web protocols,
the bindings specification should thoroughly specify the behavior
between the corresponding servers rather than just mentioning some
problems.

LOCKS

According to [WebDAV], section 7.1, the bindings specification should
specify
how the BIND method interacts with a write lock.

ATOMICITY

Atomicity of DELETE would be very hard to implement, if at all possible,
e.g. on the basis of non-journaling file systems.  However, the idea behind
atomicity as used in the specification seems to be that a client can perform
a series of actions as a unity without being disturbed by other clients.
DeltaV, for example, wants to implement a CHECKIN operation with a workspace
or activity as request URL (but that is currently still on the TODO list).
This will probably require some kind of transaction model rather than an
atomicity model.  A transaction model that prevents other clients from
intermingling a series of operations should be sufficient. And this is
exactly what WebDAV locks are designed for.  For example, when deleting a
hierarchy of resources, a client may beforehand try to lock each resource
that contributes to the hierarchy and start deletion only if all locks could
be set.  Or a client may assert a write lock on a null resources on the
destination of a MOVE operation.  In other words, the binding specification
should not require atomicity of operations; instead, a transaction model
should be introduced, based on operation on DAV locks.  But transactions are
not in the scope of the bindings protocol; they belong either directly to
DeltaV or should be specified in a separate transactions protocol
specification.

Sincerely,

Juergen Reuter
James J. Hunt
Received on Tuesday, 25 January 2000 13:15:59 GMT

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