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RE: Status code for creating lock-null resource

From: Clemm, Geoff <gclemm@rational.com>
Date: Fri, 29 Jun 2001 10:59:22 -0400
Message-ID: <3906C56A7BD1F54593344C05BD1374B103770D0B@SUS-MA1IT01>
To: w3c-dist-auth@w3.org
   From: Dan Brotsky [mailto:dbrotsky@Adobe.COM]

   But in the workflows I described, the user concept is not "intent
   to create" per se, it's "intent to edit, creating if necessary".
   From a user point of view, I'm being asked to work on resource
   "foo," creating it if necessary.  I don't want to have to tell my
   client to do something different in the two cases.

I understand and agree with that, but in general, a user gesture
will invoke several client requests (sometimes a very large number).
Your client will be doing at least 2 requests for this gesture
anyway (i.e. LOCK/GET), so having it do 3 requests (i.e. PUT/LOCK/GET)
is not qualitatively different.  In somewhat more detail:

PUT(URL)/if-does-not-exist
LOCK(URL)
if (lock-failed)
  then report-error; break;
GET(URL)
PROPFIND(URL, appropriate-authoring-properties)

Notice that the only difference in this sequence and the one you
suggest is the initial "PUT(URL)/if-does-not-exist".  That additional
request seems to be a relatively minor imposition on the client,
compared to the complexity of dealing with the variety of ways that
servers end up handling LNRs.

   I just want to
   tell it "I want to edit 'foo'; do the right thing."  Which, if we
   allow LOCK to create foo if necessary (as it does currently), is
   always done by the sequence LOCK / GET (but only if the GET is
   allowed to succeed on the LOCKED created resource).

Yes, I agree that this is a good argument against the current LNR
behavior of returning a 404 on a GET.  In general, this is a very good
argument for why a client can be written more simply if there is a
"real" resource that holds the lock, instead of some "null" resource
with special behavior.

   I actually think (from your next message) that we agree about what 
   should happen.  And I actually believe that LNRs can be changed 
   fairly easily to be "empty" rather than "null" resources; all we have 
   to do is remove the method response restrictions AND take away the 
   "auto-delete" on UNLOCK behavior.

Yes, but if we are going to implicitly create a real resource, I think
it is a simpler and more consistent model to just have the client
explicitly create the resource.  In particular, the "auto PUT with an
empty body" doesn't handle the case where you want to create a
collection there.  That's what motivated the "auto-morph" behavior
of a LNR.  If the client just creates the kind of resource it
wants there (PUT, MKCOL, MKWORKSPACE, whatever), you can do what you
need without having to have a special "LNR" that has the ability to
morph into another kind of resource (thus requiring special code in
MKCOL, MKWORKSPACE, etc.).

   A no-content resource is one which returns 204.  (By the way, some 
   servers do this for directory resources when "default docs" and 
   "directory listings" are turned off.)  The response means "there's a 
   resource here but it has no body (and thus no mime type).

   A 0-length resource corresponds to a 0-length file in a file system; 
   it has a MIME type but no octets.

   No-content resources and 0-length resources both show up in listings 
   and both respond to GET.

A problem with distinguishing no-content resources from 0-length
resources is that most repositories have no such distinction, and
it is a non-trivial amount of work to layer this distinction on
existing repositories.  Which means you want to require it only if
it really gives you significant benefit.  But either the no-content
has the auto-morph capability (which means every "create" method
has to have a special case for it), or it does not, in which case
it does not handle any of the LNR use cases that are collections or
other types of resources not created by PUT.

   >   It might be just as
   >hard for servers to handle no-content resources (if they currently only
   >handle 0-length resources) as it is to support a lock-null resource.

   That's a good point.  But I'm not arguing against LNRs cause I think 
   they're hard to implement :^).  I don't like their conceptual model, 
   and I think we don't need to introduce them at all.

I'm somewhat different ... I believe they are a pain to implement,
I'm fairly neutral on their conceptual model, but then agree with
you on the conclusion, i.e. that we don't need to introduce them at
all.  

   >   At any
   >rate, I wouldn't support adding a completely new concept (no-content
   >resources) even if we did remove the lock-null concept.

   No-content resources are not new; they're in HTTP 1.0 and 1.1.

Yes, but the distinction between a no-content and zero-length resource
is made by many repositories, and is not a feature currently required
by 2518 servers, so requiring it for locking behavior would be
problematic.

Cheers,
Geoff
Received on Friday, 29 June 2001 10:52:56 GMT

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