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Re: Locking a Resource or Locking a URL?

From: Roy T. Fielding <fielding@kiwi.ics.uci.edu>
Date: Tue, 09 Mar 1999 15:27:52 -0800
To: "Slein, Judith A" <JSlein@crt.xerox.com>
cc: w3c-dist-auth@w3.org
Message-ID: <9903091527.aa01746@paris.ics.uci.edu>
>Yaron (I think) suggested at one point that we treat references the way
>WebDAV treats source.  So we would introduce a new property on the reference
>similar to the DAV:source property on ordinary resources.  In order to apply
>any method to the reference, a client would have to get the URI from this
>new property and submit the request to that URI.

That's pretty much what I meant.  Let's say we have three URI -- A, B, C --
which have the following relationship

     A is any resource
     B is a direct reference resource whose target is A
     C is a resource that defines the mapping B --> A

C is therefore a resource that generates side-effects on other resources,
a common thing in HTTP.  The above is, in fact, how most servers are
configured today, though few allow remote editing of the config file C.

All of the common passthrough methods on B get applied to A.  The others
(all namespace operations: COPY, MOVE, DELETE, ...) result in an error
response saying either "I am a reference, go to C to change my definition",
or 401/403 if that information is protected and the credentials are missing.

>This mechanism would replace the No-Passthrough header currently in the
>advanced collections protocol.

Yes.  I'm not keen on using a header field to modify the target of a
method, for three reasons:

  1) it messes-up caching of normal requests, just like content negotiation;
  2) it places the client in the position of guessing the implementation
     on the server side;
  3) it limits the generation of direct reference names (B) to individual
     references rather than the more interesting computed reference sets.

>This would work if the default behavior for every method was to apply to the
>target.  Then any request submitted to the URI that identifies the reference
>would get applied to the target, and to apply the request to the reference
>you would send it to the URI in DAV:ref.  
>
>But currently there are exceptions, for which the default behavior -- the
>one we want down-level clients to get -- is to apply the request to the
>reference.
> 
>We always want to make it possible for reference-aware clients to override
>the default behavior, and have the request applied to the reference or the
>target or both.  No-Passthrough lets the client have the method applied to
>the reference; looking up the value of DAV:reftarget and submitting the
>request to it allows the client to have the method applied to the target.
>
>Are you suggesting that each reference contain instructions for the default
>behavior of each method? So the protocol would specify a syntax for setting
>up these instructions, and each individual reference could have a different
>default behavior?  Or that the server would decide on the default behavior
>and the client could somehow discover what it is?

I'm not clear as to why it would need to be set on a per-reference basis.
I can think of a good reason not to allow it: consider a direct reference
whose target is a direct reference whose target is A.  If all references
pass through the same methods, the semantics are consistent.  Otherwise
it gets messy.

Regardless, it is safer to assume that the server decides what methods
cannot be passed-through.  The protocol then only needs to define the
error response for "I am a reference, go to C", and the simplest of many
possible exchange formats for a representation of C (the instructions
telling the server to map B-->A).  This is identical to the content
negotiation problem, where B is the negotiation handle, {A,...} is the
set of variants, and C is the instructions for mapping requests of B
to one of the variant set.

Note that all of the properties of interest to the author are on the
resources A and C -- resource B is simply a polarized window to A.
I have a vague recollection that the reason people wanted to pass-through
by choice rather than by default was to be able to access properties
of the reference.  This design solves that problem.  It also works for
redirect references.

The other advantage of this design is that it takes into account the
direct or indirect references that are not authorable.  That is, the
server can show them as part of a collection (part of its namespace)
and have an interoperable answer for a client that tries to COPY or
MOVE or DELETE those names.  I think this is cleaner than having the
server pretend each reference is an independent entity with its own
properties.

....Roy
Received on Tuesday, 9 March 1999 18:32:28 GMT

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