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.
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.
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