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Re: app/foo and app/fooresult

From: Jim Whitehead <ejw@ics.uci.edu>
Date: Mon, 3 Feb 1997 10:18:39 -0800
Message-Id: <af1bc6cc05021004a2d5@[128.195.185.27]>
To: w3c-dist-auth@w3.org
Cc: marc@ckm.ucsf.edu
At 12:03 PM 1/30/97, Marc Salomon wrote:

>Is there a rationale for defining complementary MIME types for requests and
>responses to a method besides the ability to unambiguously determine the nature
>of the message out of context?  Are you that much more likely to retain the C-T
>header than the HTTP request/response line?
>
>If a lock request is in the form of:
>
>LOCK foo HTTP/1.x
>Content-Type: application/lock
>
>[lock request entity]
>
>and the response is in the form of:
>
>HTTP/1.x 20x OK
>Content-Type: application/lockresult
>
>[lock response body]
>
>The main reason that I can see for creating MIME types for request and reponse
>is that you are missing the HTTP request or status lines but manage to keep the
>Content-Type header value around.  Contrast this to the message/http content
>type that doesn't need to a separate IMT for the request and reponse as its
>form is unambiguous.

Excellent question.

There are several choices for where method parameters can be placed.

1. In the URL, via munging (e.g.,
http://foo.bar.com/junk.html?type=exlusive_write)
2. In headers, which often end up being method-specific (such as LockEntry,
which if used as a header would have limited applicability outside LOCK and
CHECKOUT).
3. In the message body.  In this category, we had the choices of a) using
the same content type for all methods (e.g., application/davparams) or b)
having method-specific content types.

Combinations of the above choices are also available.  Combining approaches
#2 and #3 offers the advantage that headers which have utility across
methods can be placed into headers, where appropriate.  This combination
approach is primarily what was followed in the current WEBDAV draft, with
the "Overwrite" header being an example of a header which has utility
across many methods (however, the overwrite header may not be necessary,
since existing headers may cover its functionality).

When the design team decided how to package method parameters, there were
several concerns:

a) the agreement made at the Palo Alto meeting (from the minutes):

* The group reached rough consensus that new DAV functionality should be
specified with new HTTP methods, with request parameters in the message
body. However, this was subject to the caveat that existing HTTP/1.1
headers should be used where appropriate and consistent with existing
meaning and usage. Also, this design choice was not meant to preclude
the definition of new headers, if they are the best design choice.

This reflected a sentiment expressed at this meeting that method-specific
headers were not desirable within HTTP -- headers ideally should apply to
all methods, not just a single method.

b) the desire to accomodate potential email access to DAV functionality
(this group is not actively working on how this is done, but wishes to make
it easy to do so in the future)

Affecting the thinking of the design team were the leftover bodies from the
approach of using POST with all parameters in the message body, which was
in previous drafts.  At the Palo Alto meeting the group decided to define
separate methods rather than do all DAV capability via POST.  This was due
to the consistent feedback from HTTP server authors (I polled Robert
Thau/Apache, Anselm Baird-Smith/Jigsaw, and Jim Cunningham/Netscape), that
methods were far preferable than a POST based approach.  However, in the
POST approach all parameters were in the message body, and this colored our
thinking.

The other fact that affected the design team was our lack of knowledge
about the message/http content type.

With the design team's biases out on the table, I still find the current
approach for parameter packaging (parameters primarily in bodies with
specific content types, with a few new headers which apply to many methods
-- consistent with the agreement reached at the Palo Alto meeting) to be
better than alternatives.  Alternatives, and their advantages and
disadvantages are discussed below:

1. URL munging.  While this has the advantage of working with downlevel
clients, URL munging is bad for many reasons.  First, URLs are opaque
strings which identify resources.  Once you do parameter munging, an entry
in the namespace isn't the identifier for a resource, but rather an
identifier for some arbitrary computation.  Also, the munge then adds extra
semantics to the URL, which then makes the URL non-opaque.  Never seriously
considered for DAV because it breaks the semantics of URLs, this technique
is used by some existing versioning systems.

2. All parameters in headers.  This approach has the advantage of not
requiring many new content types, and is closest to existing HTTP/1.1
practice.  However, this approach does create a feature interaction
problem.  Every time a new header is introduced into HTTP, it is necessary
to detail the interactions between that header and existing methods.  Also,
in the future, every time a new method is added to HTTP, all existing
headers must be checked for applicability or interactions with the new
method (in fact, there is a definite need to go through all of the HTTP/1.1
headers and decide whether they apply to the DAV methods, and what they
mean if they do apply).

This problem is mitigated by placing the method parameter information in
the message body.  With all method parameters in the body, the parameters
are *modularized*, and are in a location where the server can immediately
know the information relates to a specific method.  This allows a server to
immediately reject a method-specific parameter if the client sends it as a
header.  And, if a new method is introduced, the parameter attributes do
not need to be examined in the light of this new method.  If the new method
wants to use existing parameter attributes, it can, but it isn't forced to
do a time-consuming evaluation of the new method versus all existing
headers.  Similarly, the method parameter attributes for all DAV methods do
not need to be examined to see if they affect existing HTTP/1.1 methods,
which would be the case if they were defined as headers.

The design team also believed that method-specific content types which
encapsulate  method parameter attributes would be conducive to email access
to DAV functionality.  However, given that the HTTP message would be
packaged into a message/http content type (which could carry the method
parameter attributes as headers as easily as parameters in the message
body), this does not end up being a substantial advantage for the
parameters in message body approach.

However, the email usage case *does* provide support for using
method-specific content types rather than using an application/davparams
content type.  Since email may deliver messages out-of-order, by having the
method name in the content type, a client can reconnect a method response
to a method request.  The design team worked to make sure the response
carried enough information so that a client could reconnect a response it
to a request, even if it was received out-of-order.

Having the request information packaged in a special content type also has
the advantage that a request can be formulated once, stored in an
intermediate location, then reused verbatim in potentially many requests.
However, I cannot easily come up with a convincing usage case for this
capability.

Thus, the most compelling argument in favor of packaging method parameters
into the message body and defining new content types is modularity, and the
information hiding that results from this modularity.  This prevents the
need for an exhaustive header interaction analysis for new method parameter
headers.  The compelling argument in favor of method-specific content types
is reconnecting out-of-order responses with initial requests, which may
occur in email usage of DAV.

This post represents my understanding of the design team's rationale for
their choice of method parameter packaging.  I welcome your comments on our
choice.  Certainly, if the group decides on an alternate approach, then the
opinion of the group will hold.

- Jim
Received on Monday, 3 February 1997 13:48:04 GMT

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