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A proposal for Advanced Collection binding semantics

From: Geoffrey M. Clemm <gclemm@tantalum.atria.com>
Date: Sat, 1 May 1999 13:20:54 -0400
Message-Id: <9905011720.AA05533@tantalum>
To: w3c-dist-auth@w3.org

I've modified my proposal for advanced collection bindings semantics
based on the recent DELETE thread.  I'll attach what I've currently got.

Cheers,
Geoff

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- Advanced Collections and Bindings

The state of an advanced collection includes a set of "bindings".  A
binding is an association of a name (a string) with a resource.  A
legal binding name must satisfy the constraints for a URL path segment,
as defined by section 3.3 of RFC 2396.  (Note: if the definition of
"internal member name" is fixed in RFC 2518, we can just use that as
the binding name).

If a server maps a URL, "/xxx/" to an advanced collection resource R1,
a URL "/xxx/yyy" is an internal member of R1 if and only if the state
of R1 includes a binding of "yyy" to some resource.  Thus the bindings
of a collection effectively enumerate the internal members of the
collection.  All operations that add or delete internal members of an
advanced collection modify the bindings of that advanced collection.

For example, suppose http:/host/path/ is mapped to an advanced
collection resource R1, and http://host/path/index.html is not
mapped to a resource.  Then:

  PUT http://host/path/index.html HTTP/1.1

creates a new resource, R2, and adds a binding in R1 of "index.html"
with R2.


- The BIND method

The new BIND method is provided to allow direct manipulation of
bindings.  In particular, BIND can be used to create a new binding to
an existing resource, or to modify an existing binding (by associating
it with a different resource).

For example, suppose http://host/path/ is mapped to an advanced
collection resource R1, and R1 contains a binding of "index.html" with
resource R2, then the request:

  BIND http://host/path/index.html
  Destination: http://host/path/dummy.html

will add to R1 a binding of "dummy.html" with R2.  Note that this
results in two bindings to the same resource in the same collection.
If http://host/public/ is mapped to an advanced collection resource R3,
and R3 contains a binding of "test.html" with resource R4, then
the request:

  BIND http://host/path/index.html
  Destination: http://host/public/test.html

will change R3 by deleting the binding of "test.html" with R4 and adding
the binding of "test.html" with R2.


- Bindings vs. URL-Mappings

It is important to distinguish a "binding" provided by an advanced
collection from a "URL-mapping" provided by a web server.  In
particular, a binding is the part of the state of an advanced
collection that associates a a URL segment with a resource, while a
URL-mapping is maintained by a server from a URL to a resource.

The two concepts are related in that bindings induce URL-mappings.  In
particular, if there is a URL-mapping from a URL to an advanced
collection, then for each binding in that advanced collection, the
server MUST provide a URL-mapping from the URL extended by the binding
name to the resource associated with the binding.

But the inverse does not hold, namely, the fact that the server maps
two URL's to the same resource doesn *not* imply that this resource
must be an internal member of an advanced collection, *nor* does it
imply that there must be two bindings to that resource.  For example,
if the server maps http://host/todays-weather.html to the same
resource as http://host/5-1-1999-weather.html, this does not imply
that http://host is an advanced collection, nor that there are two
bindings to this resource.


- DELETE of an Advanced Collection Internal Member

To "delete" a resource, R1, that is an internal member of an advanced
collection, R2, means to remove the binding with R1 from the set of
bindings in R2.

Since there can be multiple bindings to a resource in the same or in
different advanced collections, the binding to be removed is
determined from the URL to which the DELETE request is applied.  In
particular, in the request "DELETE /xxx/yyy", then the binding that is
removed is the binding named "yyy" in the advanced collection to which
the server maps the URL "/xxx/".

For example, suppose http://host/public/ maps to a collection resource
R1, and R1 contains a binding of "index.html" with resource R2, then
the request:

  DELETE http://host/public/test.html HTTP/1.1

removes from R1 the binding of "test.html" with R2.

Note: This definition of delete is compatible with RFC-2068, but is
not compatible with wording in RFC-2518.  I support Jim Whitehead's
proposal that the wording in RFC-2518 be modified in order to allow
this definition of delete.


- MOVE of an Advanced Collection Internal Member

The move of a resource to an advanced collection creates a new binding
in that advanced collection to that resource.  The move of a resource
from an advanced collection deletes it from that advanced collection.

For example, suppose http://host/path/ is mapped to an advanced
collection resource R1, R1 contains a binding of "index.html" with
resource R2, http://host/public/ is mapped to an advanced collection
resource R3, and R3 contains a binding of "test.html" with resource
R4.  Then the request:

  MOVE http://host/path/index.html HTTP/1.1
  Destination: http://host/public/foo.html

removes from R1 the binding of "index.html" with R2, and creates a new
binding in R3 of "foo.html" with R2.

- Implementor's Note

If the binding with the existing resource cannot be made in the
destination advanced collection (such as when the destination
collection is on another server), a server can achieve the effect of a
MOVE by creating a new resource R5 which is a copy of R2, deleting the
binding of "index.html" with R1, fixing up all other bindings with R1
to be bindings with R5, and then creating a new binding in R3 of
"foo.html" with R4.  With this approach, if the bindings cannot be
fixed up, the MOVE request MUST fail.

Since most servers do not support cross-server bindings, a client
should expect a request for a cross-server MOVE of a resource with
multiple bindings to fail.
Received on Saturday, 1 May 1999 13:20:58 GMT

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