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Subject: Re: [wsbpel] Issue 11 - why <copy create="yes"> is instrincally interwinedwith XPath
I think we all understand that binding each query language requires
clarifying many language specific issues; I just don't see how that
argument plays against the "create" proposal. Alex made specific
observations about how the particular XPath binding should be clarified; I
don't see anything substantial that would prevent the proposal from being
refined to deal with them.
If, for example, we need to clarify that only XPath expressions that
returns a single node are valid, then, well, we have to do that using XPath
specific language - I don't get what is the big deal. The fact is that many
activities already depend on the use of a query language, and that usually
means that the execution semantics cannot be fully determine until one
language is selected, even if the overall general behavior of the activity
is common across languages.
As I sadi before, I think this proposal is our best chance to get something
reasonable done for issue 11. IMO, the only acceptable alternative would be
to close with no change.
Paco
Alex Yiu
<alex.yiu@oracle. To: chris.keller@active-endpoints.com
com> cc: wsbpel@lists.oasis-open.org, Alex Yiu <alex.yiu@oracle.com>
Subject: Re: [wsbpel] Issue 11 - why <copy create="yes"> is instrincally interwined
05/10/2005 12:58 with XPath
AM
Chris,
It is of course possible to come up with a similar semantics in some other
non-XPath languages. (I need to stress that I use the word "similar" and
"some", not "same" and "all")
But, the question is: How does one come up with one single, portable and
universal description which is applicable to all programming languages that
we can think of?
The BPEL base specification MUST provide a universal, clear and
expression-language-independent description to state:
what are the expected behavior of this "create" feature
what expression language patterns are supported and what are not
supported with this feature.
Without this description, this "create" feature is basically NON-portable
to other expression language.
For javascript, we basically need to re-describe the semantics in some sort
of Javascript terminology by saying something like:
getElement(“a”) should return one node, while getElement(“b”,10)
returns zero node.
[Disclaimer: I am no Javascript expert. My terminologies used above
are probably wrong. Let me switch to Java and XQuery for other
examples.]
Chris, few questions for you (some of them are already stated in my
previous email) before we move forward:
[A]
How do you resolve "Parent Location Ambiguity" problem that I stated in my
previous email without referencing to any XPath terminology? How do you
resolve that problem with an expression-language independent description?
[B]
Even if we stay within the pure XPath world, it was not clearly enough
stated that which XPath expression patterns are supported and which are not
supported. If not supported, why?
Can we support "/abc//def/ghi[3]", as long as "/abc//def" returns one
element node? If not, why?
Do we support "/abc/def/ghi[@abc='fff'][3]"?
Do we support "/abc/def[@abc='fff']/ghi[3]"? If not, why?
And, how do you describe what XPath expression patterns are supported or
not supported without hardwiring your proposal to XPath terms and concepts?
And, did you already use quite a few of XPath terms in your proposal to do
that?
[C]
Instead of covering the infinite universe of programming languages, could
you come up with a single, universal but clear description to at least
cover the following cases?
(1) someone uses Java with JAXB embedded within BPEL:
See:
http://java.sun.com/developer/technicalArticles/WebServices/jaxb/index.html
http://java.sun.com/developer/technicalArticles/WebServices/jaxb/CollectionType.java
e.g.:
<to>
javaVariableForCollectionType.getBooks().getBook().get(3)
</to>
How do we say:
"javaVariableForCollectionType.getBooks().getBook().get(3)" return null
while
javaVariableForCollectionType.getBooks().getBook()" return a non-null
java.util.List object?
WITHOUT referencing to Java terminologies?
(2) someone uses XQuery embedded within BPEL
e.g.:
<to>
{ (for $b in $collectionTypeVar1/book
where $b/price > 200
order by $b/price ) }[3]
</to>
Is the "where" and "order by" clause in XQuery supported, when this
"create" feature is used?
Please note: XPath does not have the order-by capabilities, while XQuery
has.
Another important note:
NOT All programming languages have the same XML capabilities. Even within
one programming language, there may be multiple ways to access XML data.
Example #1: There are other ways to expose XML data in Java other than
JAXB. How about just pure DOM API?
Example #2: Javascript with E4X extension has more XML capabilites than the
original javascript.
http://www.ecma-international.org/publications/files/ECMA-ST/ECMA-357.pdf
Here is an example of E4X syntax: "order.item.*[1]". Is that supported?
Thanks!
Regards,
Alex Yiu
Chris Keller wrote:
Hi Alex,
<to variable=”x” part=”y”>
<query create=”yes” queryLanguage=”jscript”>
getElement(“a”).getElement(“b”,10)
</query>
</to>
A context for this script execution would know create was turned on
and then it would create the 10th element called “b” under “a” if it
did not exist and return the new “b” element. If create was not
turned on it could throw a fault.
Thereby the “create” could be applied to a non-xpath language if the
binding makes it so.
- Chris
From: Alex Yiu [mailto:alex.yiu@oracle.com]
Sent: Monday, May 09, 2005 9:46 PM
To: wsbpel@lists.oasis-open.org
Cc: Alex Yiu
Subject: Re: [wsbpel] Issue 11 - why <copy create="yes"> is
instrincally interwined with XPath
Hi all,
As I mentioned in the last conf call, here are details on why <copy
create="yes"> is instrincally interwined with XPath - i.e. no
feasible way to define this feature in an expression-language
independent fashion.
Summary of why <copy create="yes"> is a bad idea:
It is intrinsically XPath dependent (more details are provided
below), while BPEL itself should be expression language
neutual.
<copy> itself is somewhat schema-aware [See
mismatchedAssignmentFailure], while "create" version of <copy>
is not:
As the new node will just always be appended to the
parent, regardless of the related schema design, as the
current proposal stands.
The current form <copy> itself is the pure replacement
logic, one can compare the XSD type info associated with
from-node and to-node, when XPath 2.0/XQuery 1.0 data
model is used. If mismatched, it can trigger
mismatchedAssignmentFailure fault. However, for the
"create" version, this feature is lost. That creates some
hidden and profound asymmetry, which are surprising to
users.
The create version of <copy> in the standard is NOT
schema-aware, it is almost useless. And, customers immediately
force vendors to do add sort of schema tricks. It defeats the
whole point of standardization. It is a very slippery slope.
Bad for standardization.
Details of XPath-Only / XPath-Dependent Concern
Parent Location Ambiguity
The proposal says: "Newly created nodes are always appended as the
next child of a parent."
However, it does not address one big ambiguity issue: Where is the
parent?
Consider this example:
<abc>
<def id="d1"> <ghi id="g1"> </def>
<def id="d2"> <ghi id="g2"> </def>
</abc>
/abc/def/ghi[1] points to "g1"
/abc/def/ghi[2] points to "g2"
Now, if someone tries to use this "create" feature to copy to
"/abc/def/ghi[3]", which <def> is the parent? "d1" or "d2"????
One thing the proposal SHOULD say is: by "popping off" the
bottom-most-XPath-child axis token in XPath expression parse tree,
(i.e. "/abc/def/ghi[3]" => "/abc/def"), the resultant XPath
expression must be evaluated to be ONE single element node. Then, it
solves the ambiguity of parent location. This is a MUST-HAVE
clarification.
Please note that: I underlined some of the terms above. Because,
those terms are extremely XPath specific. This MUST-HAVE
clarification make this feature become completely XPath-specific.
If one wants to standardize the "create" feature in BPEL spec, ONE
MUST give the equivalent, crispy clear and portable terms of
underlined text for other expression languages and data-modeling
technologies. E.g. XQuery 1.0, XPath 2.0, Java (with or without JAXB
... and with or without SDO), C#, Javascript (with or without XML
extension), SQL, SQL/XML ... (I consider that mission-impossible).
"Fuzziness" of XPath Expression Subset Definition
Also, its current attempt to define the subset of XPath expressions
are supported by this feature is no where clear enough to be put on
the BPEL spec. As of now, it just tries to excludes 3 cases of XPath:
(1) must use abbreviation form (2) must not use "//" (3) must use
position predicate
For (1), the proposal does not provide a real good reason on why
restricting to abbreviation form only.
For (2), why we cannot support XPath "/abc//def/ghi[3]"? As long as
"/abc//def" returns one elment node?
For (3), is this Xpath "/abc/def/ghi[@abc='fff'][3]" supported? How
about "/abc/def[@abc='fff']/ghi[3]"? The proposal does NOT say it
clearly.
Loosely (or arbitrarily) clipping off some XPath features off the
support domain does NOT create a portable behavior of this "create"
feature.
I strongly urge that whoever wants to standardize this feature
(outside of this BPEL TC) should come up with a restricted form of
EBNF Grammar for XPath expression subset. Then, that grammar will
clearly state what XPath subset are supported and what are NOT. That
restricted EBNF Grammar needs to be based on the non-terminal in EBNF
Grammar of XPath 1.0
(e.g. http://www.w3.org/TR/xpath#predicates )
Implementation experiences say it even louder ...
As I mentioned before, we (Oracle) have a similar implementation
extension feature. We NEVER have the intention to standardize this
XPath-only extension feature in BPEL spec. Because BPEL is supposed
to expression language independent and BPEL spec is NEVER the right
place to standardize a feature which fudges and breaches the clean
border line between BPEL and expression language.
That is: The semantics of a BPEL construct (<copy create="...">)
relies on certain features and behavior of underlying expression
language. And, we cannot find a proper portable description of those
features and behaviors across a wide spectrum of expression languages
that can be put into the BPEL spec.
Also, in our implementation experience of similar features, we need
to play around with XPath parser implementation to achieve that
functionality. And, that implementation will be NEVER portable to
other expression languages. That again further illustrates that this
"create" feature is by definition XPath-concept dependent.
As far as I know some other vendors have similar extension features
as well, they have no plan to standardize this XPath-only feature
either. Hence, I am not alone in that crowd.
Thanks!
Regards,
Alex YIu
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