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Re: The DOM is not a model, it is a library!

From: Stephen R. Savitzky <steve@rsv.ricoh.com>
Date: 06 Oct 1999 09:32:29 -0700
To: "DOM Mailing List" <www-dom@w3.org>
Message-ID: <qczoxwqwki.fsf@congo.crc.ricoh.com>
keshlam@us.ibm.com writes:

> >face the wrath of hundreds users when live nodelists turn out to
> >be hopelessly inefficient, even though they were told up front that _this_
> >implementation is specialized and that getElementsByTagName is deprecated.
> 
> If _your_ users react this way,  then this is probably a stong hint that you
> either (a) haven't explained the issue well enough, or (b) made a bad guess
> about their needs and should consider retuning your implementation.

As an author of open-source software, I can't easily control the scope of my
user community.  It will be better in the long run to use a different API
that doesn't raise expectations that can't be met. 

This is especially true when a fully compliant implementation would break
the application.  

> Minimal storage space may not be compatable with best performance. To take
> an absurd example, consider a model which is singly-linked.  It could
> implement getParent by searching downward from all the root nodes until it
> finds a node which has the current node as a child. Obviously performance
> will be abysmal, but code written to the DOM API will run, and
> (eventually) generate the expected results, and that's all that DOM
> compliance promises.

That's nowhere near minimal enough.  I'm thinking of streaming applications
and processors with limited or no secondary storage, where the whole
document tree simply will not fit in memory no matter how far you shrink it.
But if you're always traversing the document in order, you can use a
TreeWalker and simply throw away a node after you've processed it.  At that
point it's gone; previousSibling of the current node returns null.

> As a more realistic example, consider a "proxy DOM" -- a DOM API wrapped
> around a storage representation which bears no resemblence to the DOM's
> structure at all. Allowing DOM access to a database would be a perfect
> example of this. It may be inefficient, especially when compared to the
> storage system's native API, and if performance is your primary goal you
> might not want to go this route. 

Consider the case where a huge document is being generated on-the-fly by
some computation.  You can't just restart that computation to go back a
node; things may have changed by then.

> The DOM is interfaces. It's only interfaces. What's going on behind those
> interfaces doesn't matter to the DOM as long as the expected results come
> back. 

This is simply false.  If the DOM were only interfaces it wouldn't be
specifying behavior, for example live nodelists.  And as I've pointed out,
even interfaces have implementation consequences, as in previousSibling,
ownerElement, and ownerDocument.  Sure, the underlying representation may
involve structure sharing, but once the application touches a node you have
to make a real object someplace and keep it around forever.  Otherwise
things like tests for equality don't work; they may not be part of the DOM
but DOM objects still have to behave like objects in the underlying
language.

> It _may_ matter to the DOM user -- but sticking to the standard interfaces
> serves their needs by allowing them to switch to another DOM if yours doesn't
> perform well enough, or letting them move their code to yours if your
> performance or features are a better fit than what they have been using.

Yes; and if my application breaks, or even becomes unuseably slow or runs
out of memory in five seconds, because it relies on an implementation with
non-strict behavior, they'll blame me for it.  Better not to use the DOM
interfaces at all.

> Note that if the right answer for you is to provide only DOM Level 1
> compliance, that's legitimate.

I can't even provide full Level 1 compliance.  

> It's up to you to understand the needs of your user community and the
> impact on your market; if you guess wrong, they'll tell you and/or go
> elsewhere.

Right.  I don't want somebody pulling the parse tree package out of my
open-source document-processing application and mistaking it for a DOM
implementation. 

> The same tradeoff exists for subset not supported by hasFeature. One model
> I've experimented with implements only a few of the most essential DOM
> interfaces. I don't claim it's a DOM, and it certainly won't run all DOM
> applications... but I can promise the user that code written against it
> will run on a DOM as defined by Level 1, Level 2, and (given the WG's
> caution about backward compatability) probably future levels.

That's essentially the situation I'm in, except that I can't make that
guarantee because not all code written against my subset will run on a full
DOM.  Some applications, like mine, may come to depend on nonstandard
behavior of the standard interfaces (like the fact that EntityReference
nodes have no children, or that EntityReference nodes are not automatically
expanded in the values of Attr nodes).  Or they may come to depend on
features of the implementation that aren't specified by the DOM, such as the
fact that my implementation of a NamedNodeMap can be cast to a NodeList.

> Significantly changing the syntax or semantics of C itself, or the
> abstract model presented by the DOM, runs the risk of causing breakage in
> both directions and probably deserves a warning to the user and a new name
> in recognition of that departure.

We're in complete agreement.  I believe that there are many applications
that are better served by a simple, custom parse tree implementation than
they are by the DOM; if any changes of any sort to the abstract model are
needed, it's far better to make a complete break at the start.  It may only
take a few days to implement a simple parse tree package; fully implementing
the DOM is a major undertaking, and rest assured that if you only do a
subset _somebody_ will come back in the future and demand the rest. 

That's why I'd like to see a specification that's general enough to cover
the widest possible range of applications, simple enough to provide basic
functionality without dragging in the rest of the DOM's baggage, efficiently
implementable in the most obvious way, and explicitly extensible so that you
can add what your application needs without raising the spectre of
portability.

It's not hard to do this.  You can base the whole Node API on the InfoSet,
and do all navigation using _unidirectional_ iterators and tree walkers so
that you never have to represent trees at all, and can throw away nodes
after you've seen them for the last time.  Bidirectional iterators and
generic tree walkers would be optional.  There was a draft of the DOM that
was somewhat like this, returning a NodeIterator instead of a NodeList
wherever it made sense; it's what I rebuilt my application around.

-- 
Stephen R. Savitzky  <steve@rsv.ricoh.com>  <http://rsv.ricoh.com/~steve/>
Platform for Information Applications:      <http://RiSource.org/PIA/>
Chief Software Scientist, Ricoh Silicon Valley, Inc. Calif. Research Center
 voice: 650.496.5710  front desk: 650.496.5700  fax: 650.854.8740 
  home: <steve@theStarport.org> URL: http://theStarport.org/people/steve/
Received on Wednesday, 6 October 1999 12:33:02 GMT

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