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RE: collection with ordered members

From: Yaron Goland <yarong@microsoft.com>
Date: Wed, 29 Oct 1997 01:35:26 -0800
Message-ID: <11352BDEEB92CF119F3F00805F14F48503F9F203@red-44-msg.dns.microsoft.com>
To: "'Jim Davis'" <jdavis@parc.xerox.com>
Cc: w3c-dist-auth@w3.org
I have spent 50 minutes writing responses to this letter. Every one
comes out mean spirited. So I am going to make one point and then I am
going to steal a step from Larry Masinter.

Your cache implementation would not help improve performance because the
underlying storage, be it a file system, a database, whatever, is
usually accessible through multiple independent protocols. As such the
cache file would have to be revalidated upon every use, negating its
usefulness. There are additional problems, such as the relatively large
size of index, the cost of efficiently implementing the cache file (it
would effectively have to be a database), etc.

Now for a page from the Larry Masinter handbook of IETF discussion lists
- Please submit a draft proposing the alterations to the DAV protocol
you would like to see adopted. We are arguing in the air and there are
so many vague assertions being made that it is difficult to give
concrete responses.

Anyway, it is 1:30 in the morning and I am going to sleep. I will be
happy to continue to participate in this thread once a draft has been
submitted.

		Good Night,
			Yaron

> -----Original Message-----
> From:	Jim Davis [SMTP:jdavis@parc.xerox.com]
> Sent:	Tuesday, October 28, 1997 4:53 PM
> To:	Yaron Goland
> Cc:	w3c-dist-auth@w3.org
> Subject:	RE: collection with ordered members 
> 
> As I see it, you raise two objections to ordered collections. (If you
> have
> others, I missed them, for which I beg your pardon.)  Let me try to
> refute
> them as best I can.  First an outline, then details.  
> 
> 1. "The promise to maintain order is a serious burden to
> implementers."  
> 2. "The feature ... breaks older clients".
> 
> I also have the impression that you see a third objection but I can't
> find
> a quote that states it well.  (I use quotes to ensure I am not
> attacking a
> straw-person.)  I think your objection might be
> 
> 3. It has never been done before.
> 
> So before I proceed, are these your three objections, or did I
> misunderstand them?  Are there others?
> 
> Either of the first two would be severe.  I hope to show that neither
> one
> applies.  I hope my formal tone won't be taken as harsh, I am simply
> trying
> t be as logical as I can, and to ensure I address every argument you
> make.
> 
> Re 1 (expensive)
> 
> A. It is not a serious burden to implementers
> 
> You object that "Most DAV servers will most likely be implemented on
> top of
> file systems", and, as I concede, file servers don't allow you to
> control
> the order by which a directories components are retrieved.  Fair
> enough.  But
> 
> 1. DAV requires several other features that file systems don't
> support.  
> 
>   It requires properties and it requires external members.  I know of
> no
> file system that allows one to store arbitrary properties with a file
> system object.  Maybe the Macintosh does this, but who uses that?
> Unix
> does not, and, while the CIFS spec mentions extended attributes, I
> can't
> see any trace of this in NT4.  Nor is it clear what one writes into
> the
> file system to store an external member.
> 
> 2. component order can be implemented straightforwardly and cheaply.
> 
> One method is to write a file, something like "order.dat", that
> records the
> order.  Indeed, this can be a cache of the data that would be returned
> by
> an INDEX on the collection.  Those operations that would add new
> elements
> would have to edit this file, but that's not hard.
> 
> I can't tell from your message, but you might have the impression that
> I
> (we) are asking for the ability to request an *arbitrary* sorting of
> the
> members at run time. If this were so, that would be expensive, since
> no
> cache would work.  You write: 
> 
> >... the server has to read in the entire directory, keeping it in
> >local memory, sort according to the requested order, and then return
> it.
> 
> But that is not what I am asking for.  What I am asking for is ONE
> order.
> 
> 3. Some DAV servers will be implemented on top of document management
> systems that do support ordering directly.  Furthermore, I would claim
> that
> those who want really large collections are MORE likely to use such
> implementations.  (By analogy, it's said that Apache is the most
> widely
> used Web server.  But I am not sure that it is as widely used by those
> with
> truly huge Web sites.)  If DAV does not provide means to control
> order,
> then it make DAV much less interesting to those who need the ability.
> They
> will seek other protocols.  
> 
> 1 B. Even if it were expensive, it could be made optional.
> 
> I propose that an implementation be free to return members in any
> order it
> likes, and free to reject requests with headers that refer to
> ordering.
> From my standpoint, this won't hurt a bit.  If the Brand X server does
> not
> support it, I'll just use one that does.
> 
> You have objected before that optionality is bad, but we have plenty
> of
> other optional areas in DAV, at least according to the current
> available
> draft.  PATCH supports VTML, optionally.  Support for locking is
> optional
> (level 2), and servers are allowed to support "any combination of
> exclusive
> and shared locks".  Versioning is optional (level 3) .  In this case,
> I
> don't even care whether there's a specified means to detect whether a
> server supports ordered collections.  I'll settle for getting an
> error.
> 
> Re 2 (incompatible)
> 
> You write
> 
> >the feature ... breaks older clients. An older client may fully
> >understand the compound document format being used but because it was
> >written before DAV it would not be able to put the pieces back
> together.
> >Whereas if it could simply do a GET on the resource and get back the
> entire
> >document, then both DAV and non-DAV HTTP aware clients can interact
> with
> >compound documents. Of course, that requires the server to be aware
> of the
> >particular compound document format in use which reduces the
> situation to
> >the case I mentioned before, which requires discovery for the
> particular
> >formats the server supports, thus arguing for putting this feature in
> as an
> >extension and not as part of the base DAV spec.
> 
> This is, I think, an elaboration on the idea that a client would write
> a
> compound document to the server with a PUT (perhaps as a MIME
> multipart, or
> perhaps not), that the server would understand the document format,
> disassemble it into pieces, store it in a collection, then serve up
> the
> pieces (piecemeal) via GETs.
> 
> If that's the idea, I think it's a strawperson position, or at least,
> it's
> not what *I* want.  If someone else does, let them speak up.  I have
> not
> proposed this, and I have no interest in it.  In my view, a client can
> either PUT and GET a compound document in a format that it
> (privately)
> understands, or do a MKCOL and a set of ordered PUTs, but not both.
> 
> Re 3: (never done before)
> 
> One can find support for ordered collections in many large scale
> document
> repositories (e.g. Documentum), in the DMA API, and in the POP and
> IMAP
> mail APIs.  
> 
> It is also my impression that DAV has some features never seen before,
> as
> well.  Are State Tokens not an innovation?  (and a clever one, I might
> add)
>  And Lock Tokens?
> 
> In summary, ordered collections can be implemented as cheaply as other
> key
> features in DAV, offer no significant performance degradation, are
> entirely
> optional, fully compatible with all existing applications, are already
> well
> established (in other protocols).  They would make DAV significantly
> more
> useful for certain applications, at only a very small cost in
> complexity.
> 
> If you find my reasoning unsound or based on falsehood, I would be
> pleased
> and in your debt to be corrected. If your objections arose from a
> misunderstanding of the idea, I apologize for proposing it so poorly
> before, and if I now have your support, I could quickly generate a
> formal
> description.
> 
> with all best regards,
> 
> Jim
> 
Received on Wednesday, 29 October 1997 04:36:52 GMT

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