<to variable=”x” part=”y”>
<query create=”yes” queryLanguage=”jscript”>
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
Thereby the “create” could be
applied to a non-xpath language if the binding makes it so.
From: Alex Yiu [mailto:firstname.lastname@example.org]
Sent: Monday, May 09, 2005 9:46 PM
Cc: Alex Yiu
Subject: Re: [wsbpel] Issue 11 -
why <copy create="yes"> is instrincally interwined with XPath
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
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
- <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
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:
<def id="d1"> <ghi id="g1">
<def id="d2"> <ghi id="g2">
/abc/def/ghi points to "g1"
/abc/def/ghi points to "g2"
Now, if someone tries to use this "create" feature to copy to
"/abc/def/ghi", which <def> is the parent? "d1" or
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" => "/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#,
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
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"? As long as
"/abc//def" returns one elment node?
For (3), is this Xpath "/abc/def/ghi[@abc='fff']" supported? How
about "/abc/def[@abc='fff']/ghi"? The proposal does NOT say it
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
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
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.