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Re: Advanced Collections & the WebDAV Object Model

From: Geoffrey M. Clemm <gclemm@tantalum.atria.com>
Date: Mon, 30 Aug 1999 16:36:22 -0400
Message-Id: <9908302036.AA01395@tantalum>
To: yarong@Exchange.Microsoft.com
Cc: w3c-dist-auth@w3.org
I enthusiastically support the production of a "WebDAV object model",
and apologize for the delay in responding to Yaron's original message.
I'll make a few general comments in this message, and then break up
my responses into separate threads to avoid confusion.

   From: "Yaron Goland (Exchange)" <yarong@Exchange.Microsoft.com>

   When I do return from vacation I will follow up this paper with one looking
   at the details of advanced collections as modeled by this paper and I will
   finish out the paper.

If Yaron has time to do this (and do his real job and get any sleep :-),
I will be both impressed and grateful!

   Note that bind becomes more about replication then NR(N1) = NR(N2) mapping

That would suprise me, since the intent for BIND was explicitly to give
clients control over the NR mapping, in particular, allowing the client
to request that NR(N1) = NR(N2).

   and yes, the model for collections will work (I think) for versioning.

That is of course something that I care a great deal about.

I'll just copy the rest of Yaron's message here verbatim, for anyone
whose mail threading system only keeps recent messages.

Cheers,
Geoff

   The WebDAV Object Model

   by
   Yaron Y. Goland
   July 15, 1999

   Disclaimer

   Weasel words are always annoying but the following weasel words are
   important so please read them.

   There is no standardized formalization of the HTTP or WebDAV object models.
   Neither RFC 2616 nor RFC 2518 explicitly define their object models. Both,
   in fact, seem to spend quite a bit of time trying to not talk about their
   object models and rather to focus on presenting themselves as
   request/response protocols.

   This is unfortunate as it has lead to a lot of confusion regarding how these
   protocols should be extended. The author of this paper had some very
   definite ideas about how the HTTP and WebDAV object models worked as he
   helped to author RFC 2518. His ideas changed over time and the content of
   this paper is more rigorously defined than anything the author had actually
   thought of when helping to write RFC 2518. That is also unfortunate as RFC
   2518 probably would have been clearer had the author written this paper
   several years earlier.

   Therefore the contents of this paper represent the opinions of the author
   and the author alone. Anyone who references this paper as being in anyway
   authoritative will be making a fool of themselves. However it is the
   author's hope that this paper will lead to the creation of a normative
   document which formally and rigorously defines both the HTTP and WebDAV
   object models.

   The author of this paper assumes that the reader has read and understood RFC
   2616, RFC 2617 and RFC 2518, in that order.

   The Larry Masinter Disclaimer - The following document is not complete. I
   apologize for wasting your time with its meaningless prattle.

   Introduction

   The goal of this paper is to provide an object model that will completely
   describe the behavior of the URI namespace and HTTP resources as seen by
   HTTP clients. This model is completely unrelated to how one would actually
   implement an HTTP server. Rather it provides a robust model against which
   extensions to RFC 2616 and RFC 2518 can be judged so as to ensure that such
   extensions are consistent with the functionality already present in those
   two specifications. It is the expectation of the author of this paper that
   as future extensions are approved this paper will be extended to include
   them.

   [Will add a short summary of each major section.]

   1	Namespace/Resource Space

   1.1	In the beginning...

   There exists the namespace. The namespace is the set of all possible URIs.

   There exists the resource space. The resource space is the set of all
   currently existing resources.

   There exists a function NR() which maps from the namespace to the resource
   space. 

   There exists another function RN() which maps from the resource space to the
   namespace.

   NR() is read as "Namespace to Resource space mapping function". The input to
   NR() is N, which represents a single member of the namespace. The output of
   NR() is R, which represents a single member of the resource space.

   RN() is read as "Resource space to namespace mapping function". The input to
   RN() is R, which represents a single member of the resource space. The
   output of RN() is the set n which contains zero or more members of the
   namespace.

   NR() is a one-to-one function. For each value in the namespace there exists
   one and only one value that it can map to in the resource space. 

   RN() is a one-to-many function. For each value in the resource space there
   are zero to an infinite number of values it can map to in the namespace.

   For every N in the namespace there must be a value R as the output of NR(N).
   By default all NR(N) map to the NULL resource.

   For every R, RN(R) will output a set n but n may be empty.

   Thus while every N has an R not all Rs have Ns.

   1.2	Form without...

   A resource is defined as an object able to accept requests and send
   responses. How a resource is actually implemented, how a namespace
   communicates with it and all other issues pertaining to the underlying
   nature of a resource are, by default, left completely undefined. Thus all we
   know about a resource is what we are able to learn by sending it requests
   via a member of the namespace and getting a response.

   1.3	Call me Ishmael...

   RFC 2616 introduces the http URL subset of the URI namespace set. The basic
   form of the http URL is http://dest/segment/segment/...:port. Dest is either
   a domain name or an IP address.

   To send a message to a http name one first discovers the IP address for a
   domain name in dest or if an IP address is already provided then the message
   is sent directly to that IP address. A TCP port can also be specified, port
   80 is the default.

   The message form provided by RFC 2616 places the segment information inside
   of the message. So RFC 2616 processing can be thought of as a multiple level
   multiplexer. The first level of demultiplexing is based on the IP address
   and port. The second level of demultiplexing occurs when the message is sent
   to the specified IP address/port and the segment information is provided.

   1.4	Sprechen Sie...

   As a bit of a side note, the request-URI of a RFC 2616 message can accept
   any arbitrary URI. Therefore it is possible to use HTTP to speak with any
   member of the namespace. That is the theory.

   The reality is that trying to put non-HTTP URLs in an RFC 2616 message is a
   dangerous business. For example, if one makes a RFC 2616 request with the
   request URI foo:bar and get back a 404, does that mean that NR(foo:bar) ==
   NULL?

   The answer is both yes and no, which is the problem. The answer is yes that
   for the specified IP address and port the processing system present there
   thinks that foo:bar is NULL but that doesn't mean that there isn't some
   other system someplace which could actually translate foo:bar.

   An example may help explain things better. Imagine one makes an HTTP request
   for ftp://foo/bar. Some HTTP systems can proxy this request properly and do
   something "intelligent." The rest will just reject it. So the value of
   NR(ftp://foo/bar) depends on who you ask.

   This problem doesn't exist with http URLs because there is a well defined
   master location. If you go to the specified IP address and port whatever you
   get in response is the definitive response from NR(http://whatever...). In
   the case of non-http URLs there does not, by default, exist a RFC 2616 based
   mechanism for determining if one is talking to the definitive processor for
   a particular non-http name.

   1.5	Send in the methods...

   RFC 2616 defines a protocol called HTTP. HTTP provides a mechanism for
   sending requests, called methods, to a member of the namespace and receiving
   a response. The exact mechanism defined by HTTP is specified as:

	   1)	A client sends a method formatted as specified by RFC 2616
   to the member of the namespace specified by the request-URI.
	   2)	The namespace member N performs the function NR(N) and
   passes the method to the resulting R through an unspecified mechanism.
	   3)	R will return a result to N through an unspecified
   mechanism.
	   4)	N will then pass that response in the format defined in RFC
   2616 to the requestor.

   The key concept is that only resources can perform computations. The
   namespace only exists to forward requests to and send replies from
   resources.

   The format of a RFC 2616 request consists of a method name, a set of
   name/value pairs called headers and a byte stream called the body. In
   general RFC 2616 requests are called methods.

   The format of a RFC 2616 response consists of a response code, a response
   comment, headers and a body.

   It is possible for a resource to know the name of the namespace member who
   sends it a request. The resource may change its response based on the value
   of the namespace member who sent the request. So, for example, the foo
   method sent to N1 and N2, both of which map to R1, may end up with two
   different responses even though they were sent to the exact same resource.

   RFC 2616 also defines a set of standard methods - OPTIONS, GET, HEAD, POST,
   PUT, DELETE, TRACE and CONNECT. 

   The following summaries of the method's functionality are meant to
   concentrate on the effect and interaction of the method with the namespace
   and resource space. Thus many details are left out. Note that the "code"
   listed below is executed inside of the resource. Also note that if the
   resource doesn't support the particular method then an error will be
   returned.

   The following are not really namespace related methods - 

   OPTIONS(N) - Will return descriptive information about the functionality of
   R. Note that R may be aware of the value of N and therefore could return
   different information based the value of N.

   GET(N) - Will return a body whose value may or may not be chosen based on
   the value of submitted headers.

   HEAD(N) - Returns the headers, without the body, that would have been
   returned had the request been a GET(N).

   POST(N) - A well defined way to blow holes in firewalls.

   TRACE(N) - Ping for HTTP.

   CONNECT(N) - Just weird, don't worry about it.

   The following alter the value of NR() -

   PUT(N,G):
   R = NR(N);
   If (R != NULL) {
      R.GET = G; // Future responses to GET will be G
      return;
   }
   R' = R.CreateNewResource();
   NR(N) = R';
   R'.GET = G;
   return;

   The previous doesn't probably describe content negotiation, which will be
   discussed later in the paper.

   DELETE(N): If NR(N) = R is the NULL resource then the method will fail. If R
   is not the NULL resource then R is to be deleted from the resource space and
   all n in the output set to RN(R) are to have their NR(N) set to the NULL
   resource.

   1.6	31 Flavors...

   Resources, as has already been implied, come in a number of different
   flavors. 

   The first flavor of resource is the NULL resource. The NULL resource will
   reject all methods and their namespace representative will return a 404 for
   them.

   NULL should be thought of as a base type upon which many different resources
   inherit from. For example, a very common inheritor of NULL adds support for
   the PUT method. The PUT supporting NULL resource will accept PUT methods,
   will create a new resource and will set the GET output properly.

   RFC 2616 doesn't require that the NULL resource support OPTIONS so it isn't
   always possible to determine what sort of NULL resource one is talking to.

   1.7	Moving on up...

   RFC 2616 does not provide for very much control over the output of NR(). PUT
   can be used to create a new R as a side effect and DELETE can be used to
   destroy a R, but that is it.

   For many applications more control is needed. Specifically, support for copy
   and move functionality. Copy and move allow one to change the NR() output in
   a manner that doesn't require clients to understand the fundamental nature
   of the underlying resources.

   The COPY and MOVE methods are defined by RFC 2518.

   COPY(Ns,Nd) - 
   Rtemp = NR(Nd);
   If ((Rtemp != NULL) && (Overwrite == FALSE))
      return(FAIL);
   Create(Rd);
   Rs = NR(Ns);
   Rd.State = Rs.State; // Just copying state information from Rs to Rd, the
   resources are independent of each other.
   NR(Nd) = Rs;
   If ((Rtemp != NULL) && (RN(Rtemp) == Empty Set)) // THIS IS OPTIONAL
      Destroy(Rtemp);

   MOVE(Ns, Nd) - 
   Rtemp = NR(Nd);
   If ((Rtemp != NULL) && (Overwrite == FALSE))
      return(FAIL);
   NR(Nd) = NR(Ns);
   NR(Ns) = NULL;
   If ((Rtemp != NULL) && (RN(Rtemp) == Empty Set)) // THIS IS OPTIONAL
      Destroy(Rtemp);

   1.8	Good Fences Make Good Neighbors...

   RFC 2518 also provides for locking support that has some interesting effects
   on the relationship of the namespace to the resource space.

   LOCK(N,LockType) -
   R = NR(N);
   LockToken = R.Lock(LockType);
   return(LockToken);

   UNLOCK(N, LockToken) - 
   R = NR(N);
   return(R.UnLock(LockToken));

   The key point about locking to understand is that resources are locked, not
   namespace entries. The type of the lock then effects how the resource
   handles other types of requests.

   For example, if NR(N1) = NR(N2) and a write lock is executed on N1 then it
   will be impossible for a non-lock owner to delete N2 because N2 points to
   the same resource and write locks prevent deletion of the resource by
   non-lock owners.

   [Add an example of processing methods on locked resources in order to drive
   home the point that locks effect both the resource and the name under which
   the lock was sent in. So you could move N2 but not N1 if you had a write
   lock on N1. Actually, this should be a section on write locks in
   particular.]

   2	Hierarchy
   2.1	What We Have Here is a Failure to Communicate...

   The http URL namespace, as previously defined, consists of a series of
   segments that are separated by the "/" delimiter. This means that the http
   URL namespace is hierarchical. Thus hierarchical terminology applies.

   For example, http://foo/ is the parent of http://foo/bar.
   http://foo/bar/blah/ is the child of http://foo/bar/.

   An important subtlety of the http URL is that http://foo/bar and
   http://foo/bar/ are not necessarily the same resource. Therefore
   http://foo/bar/ and not http://foo/bar is the parent of http://foo/bar/blah
   and http://foo/bar/blah/. Also note that http://foo is not a legal http URL.

   2.2	From Little Acorns...

   By default two resources related to each other through the namespace
   hierarchy do not necessarily have any semantic relationship. So just because
   R1 = NR(http://foo/) is the parent of R2 = NR(http://foo/bar) in the foo
   part of the http URL namespace this does not necessarily mean that R1 and R2
   have any other relationship.

   Nevertheless, in many circumstances the fact that R2 was made the child of
   R1 in the foo subset of the http URL namespace does have semantic meanings.
   A common example is the use of WebDAV to implement a file hierarchy.

   As such, a mechanism is needed to enumerate a resource's children in a
   particular namespace in order to have a complete list of all the resources
   which have some relationship implied by their membership in some namespace.

   The trivial solution to this problem would be to introduce the SillyChild()
   function which takes a member N of the namespace as an argument and returns
   a set c of children. However SillyChild(N) would have an output that
   consists of every possible legal combination of characters since every
   possible URI in the universe is contained in the namespace.

   What is needed is the following Child() function:
   Child(N) -
   Set c = NULL;  //c is a variable of type set that is initially empty
   for every child C of N
      if (NR(C) != NULL)
	 add C to c;
   Return(c);

   In other words, one needs a way to list all children that map to a resource
   that does not inherit from NULL.

   RFC 2518 tackles this problem by introducing a new type of resource called a
   collection resource. Collection resources implement the Child function
   through the PROPFIND method. The actual functionality of the PROPFIND
   function is a bit more complex and will be discussed in detail later in this
   paper.

   Note that for a resource R for which NR(N1) == NR(N2) == R the answer R will
   give for PROPFIND when addressed through N1 will be different than the
   answer given for N2 assuming N1 != N2. This is because the child
   relationship is a function of the namespace and not of the resource space.

   This is a very subtle but important point so I will repeat it again - The
   child relationship is based on the naming relationships that exist within
   the namespace.

   That is why the same function can give totally unrelated answers for the
   PROPFIND method depending upon the name used to address the resource.

   Once one can list the children of a collection resource it is very temping
   to allow one to manipulate entire hierarchies of the namespace based on
   their child relations. RFC 2518 introduces the Depth header to provide this
   functionality. The Depth header, when used on a collection resource, allows
   one to specify that a method should apply to the resource and to its
   children for the number of levels specified by the Depth header. The Depth
   header current supports 0, 1 and infinite levels. Note that the Depth header
   is to be ignored if it is not used on a collection resource.

   RFC 2518 provides Depth support for the COPY, MOVE and DELETE methods.

   The following is an example of the COPY method enhanced to support Depth.
   Note that everything before the comment "Here is where we add depth support"
   is unchanged.

   COPY(Ns,Nd) - 
   Rtemp = NR(Nd);
   If ((Rtemp != NULL) && (Overwrite == FALSE))
      return(FAIL);
   Create(Rd);
   Rs = NR(Ns);
   Rd.State = Rs.State; // Just copying state information from Rs to Rd, the
   resources are independent of each other.
   NR(Nd) = Rs;
   If ((Rtemp != NULL) && (RN(Rtemp) == Empty Set)) // THIS IS OPTIONAL
      Destroy(Rtemp);
   // Here is where we add depth support
   if (Rs == Collection Resource)
      switch (depth) {
	 case 0:
	    return;
	 case 1:
	    depth = 0; //We assume all further COPY calls get this new value
	 case infinity:
	    break;
	 default:
	    //We ignore illegal depth values
	    return;
      }
      for every child C of Ns
	 if (NR(C) != NULL)
	    COPY(C,Nd+Last_Segment(C));

   Nd+Last_Segment(C) is used to build up the destination of the new
   destination argument. Nd is the base and the Last_Segment function takes the
   last function of the URI name of C and concatenates it with Nd.

   The point of providing this code is to drive home the point that depth based
   functions such as COPY, MOVE and DELETE are based on relationships
   established in the namespace and not on any relationships established in
   resource space.

   [Add in code for MOVE and DELETE.]

   Having introduced a new resource called a collection RFC 2518 also
   introduced a means to create such a resource through the MKCOL method.

   MKCOL(N) -
   R = NR(N);
   If (R != NULL)
      return(error);
   If (R != MKCOL supporting NULL resource)
      return(error);
   Create Resource R' as a collection;
   NR(N) = R';

   2.3	A little taxonomy...

   To understand the following conversation one needs to understand that RFC
   2518 divides the world into two types of resources, DAV compliant and
   non-DAV compliant.

   The DAV compliant world is divided into two parts, level 1 and level 2. 

   A level 1 DAV compliant resource support all the RFC 2616 methods plus COPY,
   MOVE, PROPFIND and PROPPATCH.

   A level 2 DAV compliant resource support everything a level 1 DAV compliant
   resource supports plus LOCK and UNLOCK.

   Level 1 and Level 2 DAV compliant resources also come in two flavors,
   collection and non-collection resources. A collection resource enforces the
   namespace consistency requirements defined below and support the Depth
   header.

   DAV Compliant Resources	Level 1	Level 2	
   Non-Collection Resource	RFC 2616 + COPY + MOVE + PROPFIND + PROPPATCH
   Non-Collection Level 1 Resource + LOCK + UNLOCK	
   Collection Resource	Non-Collection Level 1 Resource + Consistency
   Requirements + Depth	Collection Level 1 Resource + LOCK + UNLOCK	

   The purpose of the previous was to drive home the point that in the world of
   RFC 2518 there are two classes of resource, DAV and non-DAV compliant.

   2.4	A foolish consistency is the hobgoblin of little minds...

   [For completeness I should talk about namespace consistency. I'm not going
   to, but I really should.]

   3	Inheritance

   [We need to talk generally about the idea that a resource can inherit from
   multiple types of resources such as a DASL/Level 2/Bind enabled/Versionable
   resources.]

   Neither RFC 2616 nor RFC 2518 provide generic mechanisms to create arbitrary
   types of resources. There is no way, for example, to specify that one wants
   a DAV compliant level 1 non-collection resource.

   In the same way there is no way to specify what sort of NULL resource one
   wants. For example, a user may want to specify that N and all its children
   will point to a type of NULL resource that will only produce DAV Level 2
   compliant resources.

   The only method current specified which sort of provide for the previous
   functionality is MKCOL. However even MKCOL doesn't allow the user to specify
   if the resulting collection should be Level 1 or Level 2.

   4	WebDAV Properties
   PROPFIND
   PROPPATCH
   5	Content Negotiation
   [Talk about the "client proposes - server disposes" model. Also talk about
   the difference between content-location and location. Focus on the ambiguity
   Henrik pointed out in content-location and content negotiation. You have the
   news paper problem and the language problem and the way you use
   content-location in each case is inconsistent with the other.]
Received on Monday, 30 August 1999 16:37:17 GMT

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