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Re: BINDing using a weak reference

From: Eric Sedlar <esedlar@us.oracle.com>
Date: Wed, 8 Dec 1999 12:57:46 -0800
Message-ID: <00ea01bf41be$e1280030$79442382@us.oracle.com>
To: "Geoffrey M. Clemm" <geoffrey.clemm@rational.com>, <w3c-dist-auth@w3.org>
The reason you give that repository-wide searches are not likely is that you
won't want to search all of the revisions?  Most of the revision selection
mechanisms are not dependent on position in the hierarchy, and so I could do
some simple SQL like:

SELECT ... FROM labels, resources where labels.name = <blah> and
labels.resource-id = resources.resource-id and resources.body
contains(keyword1, keyword2, ...)

This would select all the documents with a particular label with the
keywords requested.  The same type of thing can be done with typical
text-search engines.

Let me ask another question though:

* let's say I create versioned resource /foo/bar.txt.
* I bind another name to that resource, so now we have /werf/bar.txt
pointing to the same resource
* I label the currently selected revision of /foo/bar.txt with some tag

I assume that a revision selection rule specifying that label for
/foo/bar.txt would also apply that selection if it encountered /werf/bar.txt
as well, correct?

From a performance standpoint, I would like to point out that the types of
indexes that work for hierarchy-independent searches tend to be MUCH faster
than traversing hierarchies.  Database indexes (or should I say indices?) as
well as text-search engines will both see a significant performance slowdown
if you force queries to traverse the URL hierarchy for all access.  Plus you
will have to do a bunch of development on all of these search engines to
handle the join with the hierarchical information.  I STRONGLY recommend not
REQUIRING hierarchical access to WebDAV-managed content.  You will break
most of the search technology currently in use on the web.  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.

For this reason alone, I think it is worth adding the concept of weak
BINDings to the Advanced Collections spec, since I think a lot of server
implementors will want to avoid dealing with cyclic references that impact
object persistence and avoid garbage collection.

--Eric

>
> Before diving in, I'd like to thank Eric for becoming active
> in the working group!  Getting this kind of in depth analysis
> is of immense value as we try to nail down the spec for last call.
>
>    From: "Eric Sedlar" <esedlar@us.oracle.com>
>
>    Are you assuming that WebDAV servers will generally be standalone
>    repositories, or do expect existing repositories (e.g. ClearCase,
>    MSExchange, Oracle RDBMS) will implement WebDAV in addition to other
>    protocols they currently use to access information therein?
>
> The latter.
>
>    Do you
>    expect WebDAV servers to be only used for managing development content
>    or also to manage production content?
>
> Both.
>
>    It would seem like you would
>    want the broadest audience possible, and that would include existing
>    servers and handling both development and production content.
>
> Definitely.
>
>    If you
>    implementing WebDAV on an existing server breaks their existing
>    guarantees, they won't be able to support WebDAV, which seems like an
>    undesirable outcome.
>
> The challenge is that different server implementations (e.g. file
> systems, relational databases, document mgmt repositories, versioning
> repositories) have very different guarantees, so defining a protocol
> that allows clients to interoperate with all of them inevitably leads
> to choices that are not optimal for any particular existing server.
>
> The example of the Unix file system was intended to illustrate how
> your (very reasonable) definition of strong/weak bindings are problematic
> when applied to a different server implementation (i.e. the Unix file
> system).
>
>    It seems to me that the most common type of WebDAV application would
>    be managing content on a web site, and the most common type of search
>    (as is most common today) would be the Yahoo-like one box keyword
>    search across the entire resource space, or perhaps contrained across
>    a certain class of resources, like Amazon's "Search all books"
>    functionality.  If I want to implement this functionality against a
>    resource space managed by WebDAV, I'm screwed if I defer garbage
>    collection.
>
> A WebDAV site will commonly contained the source resources from which
> derived resources are computed, as well as historical information
> (e.g. previous revisions of existing information).  In the case of
> Yahoo-like searches, unless they are constrained to a particular tree
> (and in the case of versioning, a particular tree with a particular
> target-selector header), many/most of the "hits" will be incorrect.
> The "search all resources" approach works when only "deployed"
> resources exist at a site ... with WebDAV support, there will be much
> more available, and naive searches that don't restrict themselves
> to a particular hierarchy will no longer be effective.
>
>    >    Also,
>    >    deferring the garbage collection associated with the unlink breaks
the
>    >    transactional guarantee when people COMMIT their work.
>    >
>    > There is no COMMIT functionality in WebDAV, so this would not be an
issue.
>
>    Are you assuming that WebDAV can only be implemented against
>    non-transactional content repositories?
>
> No - we are just assuming that WebDAV must be implementable
> on either a transactional or a non-transactional server, which
> means that no cross-request transactional behavior can be
> assumed or required.
>
>    If so, you should state that
>    in the charter for WebDAV.  I think you would be unnecessarily
>    restricting the number of vendors interested in WebDAV if you did
>    this.
>
> If there is any problem implementing WebDAV on a transactional server,
> please flag it!  My point was just that WebDAV cannot assume cross-request
> transactional behavior, so an argument based on what is needed to support
> cross-request transactions would not apply to the WebDAV protocol.
> Perhaps I misunderstood your point about COMMIT?
>
>    >    <es> locking a URL locks the namespace, not the resource.
>    >
>    > I assume this only applies to weak bindings (i.e. there must be some
>    > way to lock the resource itself, so I assume that is done through a
>    > strong binding?).
>
>    I'm jumping to conclusions here based on the conversations you have
>    been having with Yaron Goland.  Given that some applications will want
>    to reserve the entire pathname when they LOCK it to prevent it from
>    being moved, it seemed like a dichotomy would have to be introduced
>    where there are two separate types of LOCK operations: one that locks
>    the name separately from another request that locks the resource it
>    refers to.
>
> Currently, that is not the case for RFC-2518.  A lock is applied to the
> URL and results in both a lock on the resource identified by the URL,
> and the protection of the mapping of that URL to that resource.
>
>    >    The resource the locked URL points to may actually be deleted
while
>    >    the URL to it is locked.   In this case the resource
>    >    is deleted when the lock is released.  (Think of the lock like an
open file
>    >    descriptor, or in database terms, a snapshot in a read-only
transaction that
>    >    was started when the lock was acquired).  This has the same effect
as if
>    >    someone deleted that resource the instant the lock was released.
>    >    </es>
>    >
>    > This could be hard for a server implement if it didn't have
underlying
>    > transaction support.
>
>    Well UNIX doesn't go through a lot of hoops to maintain storage for
>    open files.  However, another way to do this is to increase the
>    reference count on the resource by one for each BINDing and LOCK
>    against the resource.  Why do you think this would be hard?
>
> Well, the creation of those .nfsxxx files that sometimes don't get
> garbage collected properly are some pretty ugly hoops (:-).  But more
> to the point, RFC-2518 currently states that a locked resource can
> only be deleted by a holder of the lock token, and that if you do hold
> the lock token and issue a DELETE, the DELETE applies immediately and
> is not deferred until the lock is removed.  So the DELETE semantics
> you describe would not be compliant with RFC-2518.
>
> Cheers,
> Geoff
>
>
Received on Wednesday, 8 December 1999 15:57:10 GMT

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