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Re: BINDing using a weak reference

From: Eric Sedlar <esedlar@us.oracle.com>
Date: Fri, 10 Dec 1999 00:12:10 -0800
Message-ID: <3850B5D9.615291B7@us.oracle.com>
To: "Geoffrey M. Clemm" <geoffrey.clemm@rational.com>
CC: w3c-dist-auth@w3.org
If you have an orphaned cycle, the reference count doesn't get decremented to zero,
so you don't know to set a deleted flag.  You don't know what is going to be
deleted until you do a graph traversal.  Let's say we have the following files, ID
= 1 & Id =2.  /a identifies resource 1.  /a/b identifies resource 2.  We bind /a
into /a/b, creating a cycle (I can now get to resource 1 with /a/b/a).  The
refcount on /a is 2 (the BINDing from root and the one from /a/b).  The refcount on
/a/b is 1 (the BINDing from /a).  Now I unlink /a from root. Both resource 1 (the
former /a) and resource 2 (the former /a/b) now have refcounts of 1.  However, both
should be deleted as I now have an orphaned cycle (RFC2518 specifies that deletes
are recursive to infinity).  There is no way to detect this condition without the
garbage collection.  Until the garbage collection runs, resources 1 & 2 will still
show up on hits.

Garbage collection on in-memory objects (like in a Java VM) is bad enough.  Doing
it on disk is incredibly awful.  Many applications find the need to issue lots of
bulk DELETEs (delete *.* in this directory, for example).  Each of these will
require a separate path traversal if you are doing this synchronously.

The problem with WebDAV BINDings as currently spec'ed, as well as UNIX hard links,
is that they have both navigation and persistence semantics.  The reason people
want to create cycles of BINDings / links is for navigational reasons, not for
persistence reasons.  If you have a "smart" weak link that follows the referenced
object and is removed when the referenced object is removed, you have all the
functionality that most use cases require.

--Eric

"Geoffrey M. Clemm" wrote:

> Why not just add a "deleted" attribute to a resource, set this to
> be "true" when a resource is deleted, and then automatically add
> "!deleted" to search queries?
>
> Cheers,
> Geoff
>
>    From: "Eric Sedlar" <esedlar@us.oracle.com>
>
>    Of course you will build an index.  That's the problem, because you are
>    locating the data without traversing the collection hierarchy to find the
>    data.  Let's say you find a hit using that index.  That item may actually
>    have been deleted because you have deferred garbage collection, and there is
>    no way to update the index to remove that item until the actual resource has
>    been deleted.  If you were traversing the hierarchy as well, you wouldn't
>    find that item if it would be garbage collected later because there wouldn't
>    be a path to find it.
>
>    ccjason@us.ibm.com wrote:
>
>    > >>
>    > I can tell you
>    > that if most Oracle customers are offered the choice of having a bunch of
>    > great neat content management functionality at the price of decreasing
>    > search performance, they will stick with the search performance.  Nothing
>    > irritates users more than waiting for search results.
>
>    > >>
>    > On the comment I quoted above...Are we talking about text searches?
>    > You did mention text searches above.
>    > If performance were important,
>    > wouldn't you build an index and just use the index to service queries?
>    > ... just like most search systems do?   If you use an index,  the amount
>    > of time users wait for search results is not really relevant to your
>    > point.  The performance will be the same either way.
Received on Friday, 10 December 1999 03:20:51 GMT

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