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Comments to DOM

From: Arthur Rother <arthur.rother@ovidius.com>
Date: Mon, 14 Jun 1999 16:14:08 -0400
Message-Id: <4.1.19990614154709.0093d3e0@mail.jps.net>
To: www-dom@w3.org

I'm writing a DOM interface to our sgml/xml transformation engine called
MetaMorphosis. There will be a C++ binding and a COM binding. The DOM
interface is enriched with methods and properties that allow the usage of
MetaMorphosis' transformation language. This allows more powerful
navigation and node selection, as well as the possibility of transforming
one document into another using MetaMorphosis scripts.

Trying to write the DOM interface I have the following comments with
respect to the DOM specification. 
As far as I understood, the DOM is not intended to be the most easy to use
interface to XML documents, but a general view reflecting the structure of
XML documents. Nevertheless I think, from a users point of view some more
programming friendly features would be useful.

If you think about a COM binding, the underlying implementation is often in
C++. I don't know how this is with python or perl. But calls from e.g. VB
over COM to C++ takes a big amount of calculation time. Therefore the best
strategy would be to have some powerful functions that do a lot of work
with as few calls to the DOM API as 

Now DOM has been there for some time now, and this input is be a bit late.
But I am curious about what the DOM minds say about these, or why they have
choosen differently at the time. Probably, these things were discussed before.

1. The node types 
It would have been more convenient, if the types of nodes would have been a
2^n enumeration, instead of a n enumeration. for example:

// NodeType 
const unsigned short ELEMENT_NODE = 1; 
const unsigned short ATTRIBUTE_NODE = 2; 
const unsigned short TEXT_NODE = 4; 
const unsigned short CDATA_SECTION_NODE = 8; 
const unsigned short ENTITY_REFERENCE_NODE = 16; 
const unsigned short ENTITY_NODE = 32; 
const unsigned short PROCESSING_INSTRUCTION_NODE = 64; 
const unsigned short COMMENT_NODE = 128; 
const unsigned short DOCUMENT_NODE = 256; 
const unsigned short DOCUMENT_TYPE_NODE = 512; 
const unsigned short DOCUMENT_FRAGMENT_NODE = 1024; 
const unsigned short NOTATION_NODE = 2048; 
const unsigned short ALL_TYPES = 4095;

This way, the node types can be ored bitwise. In the current DOM spec, an
orable node type is not needed, but for many extensions on the DOM, it
would have been very helpful. (I know there is a similar idea for the
iterators, but not as convenient). The reason for this is that a few
extensions to the DOM interface would make navigating much easier and more
efficient. Consider the following case.

first = myNode->nextSibling(TEXT_NODE|ELEMENT_NODE);

To write something like this with the current DOM implementation one would
need several calls to the interface which would decrease the speed of the
program. (The default for node types would always be ALL_TYPES, to have the
same behaviour as DOM).
Same for 

kids = myNode->childNodes(TEXT_NODE|ELEMENT_NODE);

Compare DOM version

if (myNode->nodeType == ELEMENT_NODE || myNode->nodeType == TEXT_NODE) {...}

with my suggestion

if (myNode->nodeType == ELEMENT_NODE|TEXT_NODE){...}.

The latter needs one call less to the DOM API.

In my extension to the DOM interface, I have many more uses for these kind
of node type values. So I had to implement my own node type handling next
to the one specified by the DOM (ugly). 
I know that in the above examples, information such as childNodes or
nextSibling are no longer properties, which in an abstract sence they are.
This against my use of node types. But in most implementations querying for
these properties will lead to some more or less complex commands anyway, so
they are not going to be implemented as 
properties (except for maybe in COM as pseudo properties). 

2. Keys to nodes 

With the following comment I can also see, why it might be against some DOM
policy. Still for my purposes, it is quite needed. What I am missing is
some kind of key (a long value) mechanism for retrieving nodes not by their
relationship to other nodes, but by using a key value. 
The objects described in DOM are not the nodes, but are interfaces to
nodes. This means, that there is some other database, in which the document
is stored. This means, that any interface object would have a mechanism to
point into the database. This is most likely to be some sort of key (for
example a pointer address or offset) of type/size long. Having access to
this key would be a great thing. I know you cannot make any assumptions on
what the key means. So it is a dangerious thing to play with, but by this
you can preserve handles to the nodes in the document using the key, not 
the entire interface. 

One of the main reasons for needing such a feature, is when writing a
powerful, lets say transformation engine, using the DOM, one would have to
recompile the application to have it work on another DOM. Using this key
feature, it would be possible to write wrappers over other DOM
implementations, where no recompilation is neccessary. Instead, a
specialized document factory is implemented, with options to create
interfaces on other DOM's. For example:

(c++ snip)
MyDOM.Document *myInterfaceOnNetscapeDom = NULL;
myInterfaceOnNetscapeDom = MyFactory.openNetscapeDOM();

Is there any DOM implementation, that does not use any sort of key
internally to refer to the actual node? 
In for example C++ one could take the address of the interface object. But
this of course does not work always, because in most implementations one
can have several instances of an interface on one and the same node.

otherInterface = thisInterface->firstChild->parentNode;

otherInterface and thisInterface will most likely be two different objects,
but the same node.

So, to get access to the key, one might want to define:

interface Node { 
readonly attribute long key; 

In addition to this, a method like

interface Document { 
Node nodeFromKey(long key) const; 

is needed, to get an interface to an object using it's key. The key of a
document object would enable to access a document using a key (a mechanism
for determing, if the document is still in memory is required for this).
Using both document and node key would also allow testing, whether two
nodes are the same. 
It is not required that the key values remain constant after starting an
application again or reloading a document. 
I miss this feature.

Some more question 
What is actually meant with DOM compliant. Does it mean, one should be able
to exchange a library, recompile the program and no compiler error should
(Excluding maybe some implemenation dependend document factories)

And can I allow a document to contain a forest, i.e. more than one document
Can I allow another pi before the Document element as in: 
<?xml version="1.0" encoding="utf8"?> 
<?mypi some pi text here?> 
Can I implement attributes like nextSibling, firstChild and childNodes as
methods and allow an optional argument, for filtering certain node types?

Am I then still DOM compliant?

Best regards, 
Arthur Rother 
Ovidius GmbH
Received on Monday, 14 June 1999 17:25:49 GMT

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