Re: [wsbpel] Issue - 11 - Proposal (after off-line discussions)

[Ron Ten-Hove <Ronald.Ten-Hove@Sun.COM>]

Folks,

Following up on this morning's TC meeting, Danny pointed out that we need to address the issue of  how this proposal affects the "atomic assign" requirement of WS-BPEL. The problem reduces to the question: how and when can the BPEL processor determine which WS-BPEL variables are to be read for transfer to XSLT global parameters? This is an interesting question, touching on run-time semantics of the WS-BPEL data model.

The following approaches occur to me:

1. Say nothing explicit about how doXslTransform fits into this; let implementations figure out how to honour the "atomic assign" requirement. A processor can do one or more of the following
   
   • be pessimistic (lock all variables in scope),
     
   • analyse the style sheet at run-time, determining all process variable name matches,
     
   • if static analysis is possible, analyse the style sheet at compile or deployment time,
     
   • find some other means of ensuring the atomicity requirement.
     
     
2. Be explicit about how implementations shall behave:
   
   • We could include a list of process variables that can be mapped to XSLT parameters, eg:
     

     bpws:doXslTransform('urn:stylesheets:AddToPO.xsl', $PO, 'OutVar FooBar')
     

     which declares that only BPEL variables "OutVar" and "FooBar" can be read by the XSLT processor (via the global parameter mechanism).
     

I'm sure there are other approaches. Ideas? Thoughts?

Best regards,
-Ron

Ron Ten-Hove wrote:

Folks,

   Last week I sent out a message outlining three related variants to a solution for issue 11. This message details one of those variants (the XPath extension function), as further developed by a subgroup of our TC. This serves as an update to the proposal I made last week, amending Ugo's proposed solution for issue 11.

    This proposal has two parts: proposed changes to the specification, and a couple of illustrative examples.

    My thanks to all those who helped develop this proposal. (All the good stuff came from them; the typos are all mine. :-)

Best regards,
-Ron

P.S. A warning: this is best viewed using an HTML-enabled mail reader. Plain text will likely be a bit messy, and harder to read.

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Section 9.4: append the following:

The above copy mechanism, when combined with the default XPath 1.0 expression language, cannot perform complex XML transformations. To address this restriction in a portable fashion, a WS-BPEL processor MUST support the bpws:doXslTransform() XPath 1.0 extension function, as described in the following paragraphs.

Function signature: Object bpws:doXslTransform(String, node-set)

where:

• The first parameter (an XPath String) provides a URI naming the style sheet to be used by the WS-BPEL processor.
• The second parameter (an XPath node set) provides the source document for the transformation to be performed by the WS-BPEL processor. This set must contain a single EII (i.e. an element node in XPath 1.0 data model); if it does not, the WS-BPEL processor MUST throw a bpws:invalidXsltParams fault. The single EII as specified by this parameter MUST be treated as the single child of the root node of the source tree for XSLT processing.
• The result of the function MUST provide the result of the transformation. It will be one of the following infoset items, depending on the XSLT output method employed by the selected style sheet:
  
• A single TII (an XPath 1.0 text node), created by the "text" or "html" output methods, or
  
• A single EII (an XPath element node that is the single child of the root of the result tree), which is created by the "xml" output method.
  

The WS-BPEL processor MUST execute the bpws:doXslTransform method such that all of the following apply:

• The first parameter, naming the style sheet to be used, MUST be used to find the style sheet corresponding to the given URI. This is accomplished in an implementation-dependent fashion. If the style sheet corresponding to the given URI cannot be found, the WS-BPEL processor MUST throw a bpws:invalidXsltParams fault.
• The processor MUST perform an XSLT 1.0 transformation, as described in section 5.1 (Processing Model) of the XSLT 1.0 specification, using the named style sheet as primary sheet sheet, the provided root node as the source document, and the result tree as the result of the transformation.
• Global parameters (section 11.4 of [XSLT 1.0]) are used to pass BPEL variables to the XSLT processor. XSLT global parameters are matched to BPEL variables by name.¹ The WS-BPEL processor MUST map BPEL variables that are accessible in the scope of the activity containing the bpws:doXslTransform function call to the global parameters declared in the style sheet. A global parameter that does not have a matching BPEL variable name MUST retain its default value, as declared in the style sheet. A WS-BPEL processor MAY warn users of any top-level parameters in the named style sheet not matched by in-scope WS-BPEL variables.
  
• Any XSLT processing faults that occur during the transformation MUST result in a bpws:subLanguageExecutionFault being thrown.
  

Note that because XSLT is a side effect-free language, execution of the transformation cannot (by definition) cause any changes to BPEL variables referred to in the style sheet.

¹ Note that XSLT global parameters must use NCNames to order to match the NCNames used for BPEL variable names; QNames will not match.

Section A (Standard Faults): add the following faults to the table:

Fault Name	Reason
invalidXsltParams	An invalid parameter was provided for a bpws:doXslTransform function call

---

bpws:doXslTransform Example

Complex document transformation. A common pattern in BPEL processes involves a sequence of receiving an XML document from one service, converting it to a different schema to form a new request message, and sending the new request to another service. Such documentation conversion is easier accomplished using XSLT, using the doXslTransform function For example:

<variables>
    <variable name="A" type="foo:AType"/>
    <variable name="B" type="bar:BType"/>
</variables>

...
<sequence>
    <invoke ... inputVariable="..." outputVariable="A"/>
    <assign>
        <from>
            <expression>
                bpws:doXslTransform("urn:stylesheets:A2B.xsl", $A)
            </expression>
        </from>
        <to variable="B"/>
    </assign>
    <invoke ... inputVariable="B".../>
</sequence>

In the sequence, a service is invoked, and the result (of type foo:AType) copied to variable A. The assign activity is used to transform the contents of variable A to type bar:BType, and copy the result of that transformation to variable B. Variable B is used to invoke another service.

The style sheet A2B.xsl would contain the XSL rules for converting documents of schema foo:AType to schema bar:BType.

Iterative document construction and BPEL variable access. Suppose that we are constructing a document by repeatedly calling a service, and accumulating the result in an XML variable. The loop might look something like this:

<variables>
    <variable name="PO"     type="foo:POType"/>
    <variable name="OutVar" type="foo:ItemType"/>
</variables>

<!-- ... PO is initialized ... -->
<!-- Iteratively add more items to PO until complete -->
<while>
    <condition>...not done...</condition>
    <sequence>
        <!-- Fetch next chunk into OutVar -->
        <invoke ... inputVariable="..." outputVariable="OutVar"/>
        <assign>
            <from>
                <expression>
                    bpws:doXslTransform("urn:stylesheets:AddToPO.xsl", $PO)
                </expression>
            </from>
            <to variable="PO"/>
        </assign>
    </sequence>
</while>

To allow the XSLT style sheet access to the OutVar variable from the BPEL process, it must contain a global (top-level) parameter with the same name:

<xsl:transform version="1.0"
               xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
    <!-- OutVar variable from BPEL process; defaults to empty
         item
    -->
    <xsl:param name = "OutVar"><item/></xsl:param>

    ...
</xsl:transform>

The style sheet would contain a template similar to the following, responsible for appending OutVar (the new item) to the existing list of items in the PO variable.

<xsl:template match="item">             <!-- line 1 -->
  <xsl:copy-of select="."/>             <!-- line 2 -->
  <xsl:if test="position()=last()">     <!-- line 3 -->
      <xsl:copy-of select="$OutVar"/>   <!-- line 4 -->
  </xsl:if>                             <!-- line 5 -->
</xsl:template>                         <!-- line 6 -->

This template copies all existing items in the source document (lines 1 & 2), and appends the contents of the BPEL variable OutVar to the list of items (line 3 tests to see if the current node is at the end of the item list; line 4 copies the result-tree fragment from the BPEL variable OutVar to follow the the last item. Thus, if PO has a value of:

<po><item>item 1</item></po>

at the beginning of an iteration of the the above loop, and the <invoke> activity returns a value of <item>FooBar</item>, evaluation of the <from> expression will result in a value of:

<po><item>item 1</item><item>FooBar</item></po>

which, when the BPEL copy activity is complete, will become the new value of the PO variable.