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Re: Requirements for Collections

From: Judith Slein <slein@wrc.xerox.com>
Date: Tue, 24 Feb 1998 14:13:49 PST
Message-Id: <3.0.3.32.19980224171349.0096a100@pop-server.wrc.xerox.com>
To: John Turner <johnt@cgocable.net>
Cc: w3c-dist-auth@w3.org
Hi, John --

I've been assuming something like the DMA model of containment.  Here is a
quote from the DMA spec on the difference between direct membership in a
collection and membership-by-reference.  I can see that you understand
something different by direct containment.  Can you explain what it is, and
how it differs from membership-by-reference in your model?

"Direct containment. This models a 1:N, or one-to-many, relationship. A
containing object may contain multiple objects, but an object is directly
contained within at most one containing object. A containing object that
directly contains another object is the parent of that object. An object
that is directly contained within a parent object is a child of that
parent. Multiple directly contained objects are children. Direct
containment defines a strict hierarchy of objects with no cycles. Document
spaces implementing direct containment must prevent cycles. For example,
moving a parent into one of its children must be disallowed. DMA does not
limit the depth of the direct containment hierarchy, but a document space
may choose to impose a limit."
 
"Referential containment. This models an N:M, or many-to-many,
relationship. Objects that are referentially contained within a containing
object are referred to as containees. A containing object that
referentially contains another object is referred to as a container of that
object. A container may contain multiple containees. A containee may be
contained within multiple containers. Cycles in referential containment
relationships are not disallowed by DMA, because they could be expensive to
prevent. A document space may choose to disallow them, however, in which
case it is obligated to enforce this. DMA defines operations that modify
containment hierarchies in such a fashion that cyclical referential
containment can be supported."

At 04:51 AM 2/20/98 PST, John Turner wrote:
>I have a few comments on the requirements for collections.
>
>At 01:02 PM 2/20/98 PST, you wrote:
>> .
>> .
>> .
>>
>>1. A resource is a direct member of only one collection.  
>
>It seems to me that this requirement is mixing ideas.  Direct membership can
>be used to indicate that something is a part of the collection and should be
>treated as such.  A compound document for example.  Saying that it can only
>be a direct part of one document limits the usefulness.  There is no reason
>(except for complexity) that a document cannot have aliases in the
>namespace.  A server that implements the namespace directly from a file
>system would certainly have a harder time with aliases, but a server that
>has a document manager as backend could deal with it easily.  In addition,
>it would limit the means the DM server has to represent its documents in a
>web namespace.
>


Name:		Judith A. Slein
E-Mail:		slein@wrc.xerox.com
Phone:  	(716) 422-5169
Fax:		(716) 422-2938

Xerox Corporation
Mail Stop 105-50C
800 Phillips Road
Webster, NY 14580
Received on Tuesday, 24 February 1998 17:09:14 GMT

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