XML
in, XML out, sounds easy. But the call for a universal interface between
languages based on a standard IDL is an old one. I would respectfully suggest
that re-inventing Corba is out of scope for the BPEL TC.
Hi, Yaron,
Thanks for renaming the
subject for a better organization of email threads.
Just want to add a
couple of more points for clarification:
- Issue 13 is an issue separated from bringing XQuery to BPEL. However,
the syntax change is kind of triggered by the concern on how to inline a
language (similar to XQuery) directly into BPEL.
- I guess when we evaluate whether we need to make a syntax change as
suggested in Issue 13, we may want to use XQuery as our key language example
in the context of brainstorming and verifying our design and the benefits /
complications of new syntax. Because, if we want to embed other relatively
full-blown programming languages (e.g. ECMAScript) similar to XQuery, it
will share similar implications.
- If consensus in BPEL TC leans towards leveraging function-call approach
in XPath (suggested in my previous email), I would say we may not want to
change the syntax for query/expression from attribute to element *for this
BPEL release*, as there may not be any urgent need for that.
Yes, I think we should have separate email threads for two
discussions. But, I see these two discussions are inter-related in some ways.
Also, the function-call approach would not create direct dependency on
XQuery either. Note that the same function-call approach would work for other
programming languages, as long as they are XML-capable in a sense.
Looking forward to further feedback and discussion on Issue 13 and
this email thread regarding to how to bring XQuery into BPEL.
Thanks!
Regards, Alex Yiu
Yaron Goland
wrote:
I
would ask that anyone responding to Alex's e-mail not use Issue 13 in
the subject line.
Issue 13 does not propose the introduction of XQUERY into BPEL. Issue
13 only proposes to change the query/expression syntax from attributes to
elements so that at some point in the future other languages, such as
XQUERY, can be added in.
The TC has previously discussed the introduction of XQUERY into BPEL
and my understanding of the group's consensus was that the group felt it was
inappropriate to put any references to XQUERY into BPEL because XQUERY will
not be a W3C recommendation by the time BPEL is expected to
ship.
Yaron
Hi, all,
I hope I am not too
late to join this discussion. :-)
In general, I am all for
bringing the capability of XQuery to the BPEL world. It can
potentially do some simple data transformation and merging multiple data
documents into one document.
I hope this email may bring a new
prospective on how to leverage power of XQuery or other similar
languages in BPEL, other than XPath 1.0 or 2.0.
I see the current
syntax changes is the first step to allow more generic syntax for Query
and Expression Language (from attribute to element). However,
this is just the first step from my viewpoint. There are a lot of
not-resolved-yet issues in the semantics space with this inline-oriented
proposal. Using this syntax change to introduce XQuery power to BPEL may
create more issues. IMHO, we may NOT be ready for it yet.
At the
end of this email, I would suggest a potentially cleaner way to introduce
XQuery into BPEL.
Unresolved Issues include:
- Language Binding Issue between BPEL and the
target-Query/Expr-language:
- Variable Binding Issue: how variables in BPEL are exposed into the
XQuery world? What is the name of variables? Are variable names shared
between BPEL world and XQuery world? Can XQuery handle WSDL 1.1
MessageType directly?
- Namespace Sharing Issue: the XQuery language has two styles:
human-readable XQuery language and not-so-human readable XQueryX (XML
version of XQuery). It would be treated as Text or CData Node in XML
for plain XQuery, while XQueryX will wrap around target data element
and attribute with <xqx:tagName> and <xqx:attributeName>
(unlike the literal style of <from> element in BPEL). One way or
the other, there is no easy way to effectivly share XMLNS declaration
between BPEL and XQuery world. That sort of defeats the purpose
of embedding XQuery syntax directly into BPEL.
Some of the true power of XQuery
actually are classified into two following aspects:
- Result Data Model: The primary Data Model of XQuery is a data
sequence. This data sequence length can be unbounded / stream and data
items in that sequence can be of same types or different types. BPEL
Data Model is much restrictive than that. Hence, there is a mismatch in
Data Models. We need to enrich our BPEL Data Model to fully leverage
XQuery.
- Transform Multiple XML Data Sequence into one single result XML Data
Sequence: by just allowing embedding XQuery syntax as element content,
we do not define / solve how to refer multiple variables and variable
binding issue listed above.
Basically, before we resolve
the above issues, I don't think we are ready to embed a language, which
can be used in a relatively complicated way like XQuery, directly into
BPEL. I don't want gives people the illusion all the XQuery binding issues
is resolved by just introducing these syntax changes. Also, I don't
want open up this syntax gateway which individual vendors tried to solve
the above issues in their proprietary ways. That is a big portability
issues.
However, we can still leverage XQuery in BPEL
without placing the XQuery or XQueryX syntax directly into a
BPEL document. Because, XQuery already allows developers to define a
XQuery-based function. That is a more well-defined interface for
separating:
- Syntax
- Datatypes
- XML namespace
- variable namespace
It also encourages better
encapsulation and components reuse. The same XQuery function can be
used within BPEL or other XML-related components within same application.
If the XQuery logic needs to be used multiple time within the same BPEL or
multiple BPEL, they can now be re-used with this function-oriented
approach. (On the other hand, the inline-approach would encourage users to
copy XQuery fragments everywhere in BPEL)
http://www.w3.org/TR/xquery/#FunctionDeclns
E.g.: In
an XQuery file: --------------------------------------------
declare function local:summary($emps as element(employee)*)
as element(dept)*
{
for $d in fn:distinct-values($emps/deptno)
let $e := $emps[deptno = $d]
return
<dept>
<deptno>{$d}</deptno>
<headcount> {fn:count($e)} </headcount>
<payroll> {fn:sum($e/salary)} </payroll>
</dept>
}; -------------------------------------------- And, this XQuery
file / component is deployed together with the BPEL components in the same
application.
In BPEL, we can do the following:
<assign> <from
expression="local:summary(myBPELVar)" /> <to ...
/> </assign> <switch> <case
condition="local:total(PO,Discount) > 500">
...
</case> </switch>
Note:
- We use "expression" as opposed to "query" here.
- The expression / query language does not need to change to XQuery.
It can stay with XPath 1.0 or XPath 2.0
- This function call business is nothing in XML-world. And, we don't
need to reinvent the wheel and invent anything brand-new (e.g. variable
binding semantics). We can borrow a lot of definition from other
specificiations. E.g.:
Function Call in XPath 1.0: http://www.w3.org/TR/xpath#section-Function-Calls Function
Call in XPath 2.0: http://www.w3.org/TR/xpath20/#id-function-calls Function
Call in XQuery 1.0: http://www.w3.org/TR/xquery/#id-function-calls
Function
Declaration in XQuery 1.0: http://www.w3.org/TR/xquery/#FunctionDeclns
- I would even say, this approach is ALREADY allowed by BPEL
implicitly as of today. Because, BPEL allows XPath expression,
which in turn allows this kind of function-call already. We just need to
make the expression available to assign statement in BPEL.
- This function-call approach will allow users to switch the
implementation choice of language with minimal impact BPEL document
itself. E.g. changing from XQuery 1.0 to XQuery 2.0
- The development overhead of this approach is very minimal. One
seperate XQuery file for ALL XQuery logic and one extra line of function
signature declaration for each XQuery logic.
I
hope this proposal make senses to you guys.
Thanks!
Regards, Alex
Yiu
Yaron Goland wrote:
The question is what will cause the least amount of effort for users? To
take an existing language that they are familiar with and routinely use and
then create a sub-set of it for use in join-conditions or to create an
entirely new language specifically for join conditions and now force users
to learn two languages with potentially unrelated and inconsistent syntaxes?
I personally favor the former. I also think that if we are to finish this
spec in a reasonable amount of time we should avoid creating new query
languages.
Just my two cents,
Yaron
-----Original Message-----
From: Maciej Szefler [mailto:mbs@fivesight.com]
Sent: Wednesday, January 07, 2004 9:55 AM
To: ygoland@bea.com; 'Assaf Arkin'
Cc: 'rkhalaf'; wsbpel@lists.oasis-open.org
Subject: RE: [wsbpel] Issue 13 - Updated Proposed Resolution]
Consistency should be reserved for areas where there is some
common domain.
As Assaf points out join conditions have nothing to do with XML node
selection, so why would we try to adopt an expression language aimed
squarely at XML node selection crippling it so that it no
longer does what
it was intended to do? It makes no sense to me. If anything
it makes things
confusing.
Would a structured XML representation such as "<and><link
name="foo"/<link
name="bar"></and>" satisfy your objection to creating multiple sets of
grammars?
-maciej
-----Original Message-----
From: Yaron Goland [mailto:ygoland@bea.com]
Sent: Wednesday, January 07, 2004 10:08 AM
To: 'Assaf Arkin'
Cc: 'rkhalaf'; wsbpel@lists.oasis-open.org
Subject: RE: [wsbpel] Issue 13 - Updated Proposed Resolution]
This has nothing to do with XML manipulation, this has to do
with the need
to have a consistent expression/query language used
throughout BPEL. If
someone is moving to a new expression/query language for all other
expressions in BPEL they should not be forced to use a different
expression/query language just for join conditions. That is why join
condition must have the same syntax flexibility that is
available to all
other expressions/queries.
-----Original Message-----
From: Assaf Arkin [mailto:arkin@intalio.com]
Sent: Monday, January 05, 2004 7:00 PM
To: ygoland@bea.com
Cc: 'rkhalaf'; wsbpel@lists.oasis-open.org
Subject: Re: [wsbpel] Issue 13 - Updated Proposed Resolution]
Yaron Goland wrote:
Imagine a prefix style XML manipulation language is
introduced that does
things like "and(foo,bar)". No tool in its right mind is
going to say to the
user 'well you can use the prefix style everywhere in BPEL
but this one
single place, join conditions, where you have to use an
infix "foo and bar"
style.'
You're right.
I've read the spec over and over and over and I still don't
understand
what XML manipulation has to do with join conditions. I don't
see node
selection, there's no context node or any variable/function
you can use
to operate on nodes. No operators are allowed unless they deal with
binary values. If nodes are non-existent, then where does XML
manipulation come into play?
So while I agree with the logic you presented, I still fail
to see how
it applies to join conditions.
arkin
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