RE: [xml-dev] Concerned about the increasing reliance on XPath

["David Lee" <dlee@calldei.com>]

As a counter-contra-point I was of this same opinion in 2000 when I first
had to implement XML interfaces to a 4GL language ( Centura Desktop
Developer anyone ?  no ?) 
My 'intuitive' feeling was that XPath was an unnecessary overhead.
I have since been 'converted'.   My current opinion is that with modern
implementations, the major area that XPath suffers is that it requires
loading the entire document into memory to support the entire syntax.  Its
true that some work has been done with 'streamable xpath' that can avoid
this is some cases, but in general, the high level XML tools in general
(XPath, XQuery, XSLT etc) are based on an in-memory representation.
(ignoring XML Databases for the moment).   This can be a problem ... but the
solution rarely is avoiding xpath, and xpath IMHO isnt the bottlneck.
You'll see similar performance writing your own code against an in-memory
tree.

If you really have a need to avoid loading the document into memory you need
to resort to streaming API's.  Its not the xpath thats the performance issue
in these cases, its the loading-xml-into-memory that is.
Fortunately there is some awesome work going on to support streamable
subsets into XPath, XSLT and XQuery ... 

So to conclude, I would argue that reliance on xpath is not a bad thing at
all, its very good.
If you didn't have XPath you probably  couldn't implement your "simpler"
solution any more efficiently, and would loose in flexibility.

Side note <insert plug>  using Streaming API's doesn't always mean you have
to jump down a level of language.   In addition to the example given above
(streamable subsets of Xpath/XQuery/XSLT ) referenced above, xmlsh exposes a
very high level scriptable streaming API which I've used in production to
process huge amounts of XML (GB's) that wouldnt fit into memory nearly as
efficiently as a pure java implementation.   However the types of things you
can do with this API are limited to a narrow scope, mainly looking at the
current node, and skipping ahead to the next matching node.

Note this is still somewhat experimental and not fully documented yet but
you can get the idea here

http://www.xmlsh.org/FunctionsStAX

A simple example being 

http://www.xmlsh.org/FunctionStAXgetData
http://www.xmlsh.org/FunctionStAXgetAttribute


<end plug>

But again, this really only gives a significant performance gain when
dealing with large documents and very simple operations.





----------------------------------------
David A. Lee
dlee@calldei.com
http://www.xmlsh.org


-----Original Message-----
From: David Lee [mailto:dlee@calldei.com] 
Sent: Saturday, May 07, 2011 10:59 AM
To: 'Costello, Roger L.'; xml-dev@lists.xml.org
Subject: RE: [xml-dev] Concerned about the increasing reliance on XPath


Contra-point.
Do you have any tangible evidence that using XPath with simple expressions
requires significant processing ?

Its true that it requires the support infrastructure - and the same can be
said of any level of computation that requires a lower layer (which is prety
much all ).
But do you have support to your assentation that xpath requires large
computation for simple expressions  ?


----------------------------------------
David A. Lee
dlee@calldei.com
http://www.xmlsh.org

-----Original Message-----
From: Costello, Roger L. [mailto:costello@mitre.org] 
Sent: Saturday, May 07, 2011 9:52 AM
To: xml-dev@lists.xml.org
Subject: [xml-dev] Concerned about the increasing reliance on XPath


Hi Folks,

XPath is a fabulous language. It is incredibly powerful. It is a large, rich
language.

I have observed in increasing usage of XPath.

For example, in XML Schema 1.1 the new assert element uses XPath to express
constraints:

    <assert test="XPath" />

XPath gives a lot of power to the assert element.

But it also means that a lot of power is needed to evaluate the assert
element. 

To evaluate that tiny, innocuous assert element you need to implement the
entire XPath language.

Suppose the assert element was simplified. The only kind of assertion that
can be made is, "The value of the first child element must be greater than
the value of the second child element." Here's how we might use it to
express the constraint between a meeting's start time and end time:

<greaterThan>
        <element name="start-time" type="dateTime" />
        <element name="end-time" type="dateTime" />
</greaterThan>

Very little power is needed to evaluate this "assertion".  The assertion is
expressed entirely in XML markup.

We've lost an enormous amount of power/expressivity. But we've gained in
reduced cost of evaluation/processing/coding. 

While XPath is nice, it is:

    - not XML
    - requires huge amounts of processing (i.e., coding) wherever it's used

I am concerned about the increasing reliance on XPath. 

My Position: In some cases it would be better to ratchet down capabilities
and use XML markup rather than XPath.

I know many people will disagree with my position. Let's have a good
discussion of the issue. Here's the issue:

Issue: Is the increasing reliance on XPath a positive or negative trend?

(Implicit in this "issue" is an assumption that there is indeed a growing
trend toward using XPath)

/Roger

_______________________________________________________________________

XML-DEV is a publicly archived, unmoderated list hosted by OASIS
to support XML implementation and development. To minimize
spam in the archives, you must subscribe before posting.

[Un]Subscribe/change address: http://www.oasis-open.org/mlmanage/
Or unsubscribe: xml-dev-unsubscribe@lists.xml.org
subscribe: xml-dev-subscribe@lists.xml.org
List archive: http://lists.xml.org/archives/xml-dev/
List Guidelines: http://www.oasis-open.org/maillists/guidelines.php



_______________________________________________________________________

XML-DEV is a publicly archived, unmoderated list hosted by OASIS
to support XML implementation and development. To minimize
spam in the archives, you must subscribe before posting.

[Un]Subscribe/change address: http://www.oasis-open.org/mlmanage/
Or unsubscribe: xml-dev-unsubscribe@lists.xml.org
subscribe: xml-dev-subscribe@lists.xml.org
List archive: http://lists.xml.org/archives/xml-dev/
List Guidelines: http://www.oasis-open.org/maillists/guidelines.php