Hi everyone,
I've reviewed Andy's design objective of
IDOM, Lenny's view of old DOM and his proposal of redesign, and some users
feedback. Here is a "quick" summary and I would like to
call for a VOTE about the fate of these two interfaces.
1.0 Objective
==========
1. Define the strategy of Xerces-C++
public DOM interface. Decide which
one to keep, old DOM interface or new IDOM interface
2.0 Motivation
===========
1. As a long term strategy, Xerces-C++
shouldn't define two W3C DOM interfaces which simply confuses
users.
=> We've already got many
users' questions about what the difference, which one to use ...
etc.
2. With limited resource, we should focus our
development on ONE stream, no more duplicate effort
=> New DOM Level 3
development should be done on one interface, not both.
=> No more dual
maintenance: two set of samples (e.g. DOMPrint vs IDOMPrint), two parsers
(DOMParser vs IDOMParser)
=> To encourage more
users to develop DOM application AND implementation based on this
binding.
=> Such binding should
just define a set of abstract base classes (similar to JAVA
interface) where no implementation model is assumed
3.0 History
=========
'DOM' was the initial "W3C DOM interface"
developed by Xerces-C++. However the performance of its
implementation is not quite satisfactory.
Last year, Andy
Heninger came up with a new design with faster performance, and
such implementation came with a new set of interface
=> 'IDOM'.
Currently both 'DOM' and 'IDOM' are shipped
with Xerces-C++. 'IDOM' is claimed as experimental (like a
prototype) and is subject to change.
More information can be found in
:
http://xml.apache.org/xerces-c/program.html
4.0 IDOM
=========
4.1 Interface
==========
4.1.1 Features of IDOM Interface
--------------------------------------------------
e.g. virtual IDOM_Element*
IDOM_Document::createElement(const XMLCh* tagName) = 0;
1. Define as abstract base classes
2. Use normal C++ pointers.
=> So that abstract base class is possible.
=> Make it more C++ like.
Less Java like.
4.1.2 Pros and Cons of IDOM
Interface
----------------------------------------------------------
Pros:
1. Abstract base classes that correspond to the W3C DOM
interfaces
=> Can be recommended
as Apache DOM C++ Binding
=> More standard like, no
implementation assumed as they are just abstract interfaces using pure
virtual functions
2. (Depends on users' preference)
- someone prefers C++ like
style
Cons:
1. IDOM_XXX - weird prefix
'I'
Solution:
-
Proposed to rename to DOMXXXX which also
matches the DOM Level 3 naming convention
2. (Depends on users' preference)
- someone does not like
pointers, and wants Java-like interface for ease to use, ease to learn and
ease to port (from Java).
3. As the old DOM interface has been around for
a long time, majority of current Xerces-C++ still uses the old DOM
interface, significant migration impact
Solution:
-
Announce the deprecation of old DOM interface for a couple of releases
before removal
4.2 Implementation
===============
4.2.1
Features of IDOM Implementation
-----------------------------------------------------------
1. Use an
independent storage allocator per document. The advantage here is that
allocation would require no synchronization
=> Fast, good
scalability, reduced memory footprint
2. Use plain, null-terminated (XMLCh *)
utf-16 strings.
=> No DOMString
class overhead which is another performance contributor that makes
IDOM faster
4.2.2 Downside of IDOM
Implementation
-------------------------------------------------------------
1. Manual memory management
- If document comes from
parser, then parser owns the document. If document comes from
DOMImplementation, then users are responsible to delete it.
Solution:
- Provide
a means of disassociating a document from the parser
- Add a
function "Node::release()", similar to the idea of "Range::detach",
which allows users to indicate the release of the Node.
- From C++ Binding abstract interface perspective, it's
up to implementation how to handle this "release()" function.
- With Xerces-C++ IDOM implementation, the release()
function will delete the 'this' pointer if it is a document, else
no-op.
2. Memory retained until the document is
deleted.
- If you change the value of
an attribute or call removeNode many times, the memory of
the old value is not deallocated for reuse and the document grows and
grows
Solution:
- This in
fact is a tradeoff for the fast performance offered by independent
storage allocator.
-
There is no immediate good solution in place
5.0 old DOM
==========
5.1 Interface
==========
5.1.1 Features of old DOM Interface
-----------------------------------------------------
e.g. DOM_Element
DOM_Document::createElement(const DOMString tagName);
1. Use smart pointers -
Java-like
5.1.2 Pros and Cons of old DOM
Interface
--------------------------------------------------------------
Pros:
1. DOM_XXX - reasonable name
2. (Depends on users'
preference)
- someone wants
Java-like interface for ease to use, ease to learn and ease to port (from
Java).
3. Not that many users have migrated to IDOM yet, so migration
impact is minimal.
Cons:
1. Not abstract base class
- Cannot be recommended as Apache DOM C++
Binding
- Implementation (smart pointer indirection) is
assumed
Solution:
- This in fact is a tradeoff for the ease of use of smart pointer
design
- No
solution.
2. (Depends on users' preference)
- someone wants C++-like as
this is C++ interface
5.2 Implementation
===============
5.2.1 Features of old DOM
Implementation
----------------------------------------------------------------
1. Automatic
memory management
-
Memory is released when there is no more handles pointing to
it
- Use reference count to
keep track of handles
2. Use thread-safe DOMString class
5.2.2 Downside of old DOM
Implementation
--------------------------------------------------------------------
1. Performance is slow
- Memory management is the
biggest time consumer, and a lot of memory footprint.
- There are a whole lot of
blocks allocated when creating a document and then freed when finished with
it. Each and every node requires at least one and sometimes several
separately allocated blocks. DOMString take three. It adds up.
Solution:
- Lenny
suggests to use IDOM interface internally in
DOM implementation, patch in
Bugzilla 5967
- Then
the performance benefits of IDOM is gained but the memory retained problem in
IDOM implementation still remains to address.
- And
internally, we will have dual interface maintenance model as IDOM
interface is then used by DOM internally.
Vote Question:
============
I would like to call for a
vote:
==> Which INTERFACE
should be the Xerces-C++ public supported W3C DOM Interface, DOM
or IDOM? <===
Note:
1. The question is asking which "interface" to be officially
supported. Once the choice of interface is chosen, we can discuss how
to solve the downside of implementation as the next topic.
2. The one being voted will become the ONLY
Xerces-C++ supported public W3C DOM Interface, and is where the DOM Level 3
being implemented.
3. The API of the other interface will be
deprecated. And its samples, and associated Parser
will eventually be removed from the distribution
