Subject: Optional Backpointers from Targets to References

From: "Slein, Judith A" <[email-masked]>
Date: Tue, 15 Sep 1998 16:11:28 -0400

There has been no discussion following Jim Davis's mailnote "Proposal:
optional backpointers for ACR" of August 19.  Unless there are
objections, this proposal will be incorporated in the Advanced
Collections protocol specification, and a corresponding requirement will
be added to the Advanced Collections requirements.

The requirement will state:

There is an interoperable means of navigating from a resource that is
the target of multiple references up the collection hierarchies from
which it is referenced.

The protocol specification will include:

The definition of a new, optional property DAV:references, which
provides backpointers from a target to all resources that refer to it.

This property is live on servers that choose to support it.  It may be
present on resources on other servers, but in that case will be dead.

Clients can use this property to discover which resources reference the
target resource, and by parsing the URLs of the references, can discover
the collections where those references reside and navigate up the
collection hierarchy.

Judith A. Slein
Xerox Corporation
[email-masked]
(716)422-5169
800 Phillips Road 105/50C
Webster, NY 14580

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From: "Roy T. Fielding" <[email-masked]>
Date: Fri, 18 Sep 1998 18:31:31 -0700

I doubt that unrestricted backpointers to collection parents will be
implementable on general web servers, but I have no objection to defining
an optional backpointer property provided that it is also mentioned that
such a property is not a very good way to implement scalable systems.
I would expect any system that implemented such a thing would restrict
the backpointers to references within the same object repository
(whatever serves as the server backend).

The same functionality can be accomplished via a DASL query, regardless
of how the server backend works, which has the advantage of specifying
the scope of the query.  So, I don't think directly specifying
a property to hold that information should imply that the property is
maintained live.  OTOH, I'm not sure how the response to such a DASL
query would look like (i.e., if it would need DAV:references itself).

....Roy