Introduction
to XML Pipeline Language - XPL
The
XML Pipeline Language (XPL) is a vocabulary for describing the processing
model for the XML components. This vocabulary is in the draft stage and
has been submitted to the W3C for discussion.
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An XPL program is a well formed XML document conforming to the namespaces
in XML Recommendation. Moreover the XPL program should also conform to
the XPL specification.
An orchestrated
sequence of operations is performed on XML Information Sets known as Infosets
in an XPL program. Production, consumption, and transformation are the
operations that can be performed on the Infosets by the XPL program.
Unconditional
operations, conditions, loops and change of control are all supported
in an XPL program. XPL addresses the problem of interoperability of the
specifications from the point of view of the Infosets that are produced
by the different specifications which can operate on the XML documents.
Before we
get into the syntax of the XPL we should know that an XML processor is
a component of an XPL program and an XML processor is identified by its
QName. QName is shorter to write than the full URIs. It allows for logical
grouping of the XML processors and is consistent with the other specifications
like XSLT 2.0 Working Draft.
An XPL program
consists of input and output parameters and a sequence of statements.
Input parameters are those that provide XML Infoset to the XPL program
and the output parameters are those that produces XML Infoset. It is possible
for an XPL program to have no input or output.
The syntax
for XPL is always organized into modules. The basic framework of the XPL
consists of the Pipeline module and the Processor module. An XPL program
always starts with an p:pipeline element. It is the root element in the
XML document which has the XPL program. The p:pipeline element has a compulsory
version attribute. At present the version attribute should have a value
of “1.0”. The following code shows how the pipeline element
looks like,
<p:pipeline
version = “1.0”>
...
...
</p:pipeline>
The p:input
and the p:output elements define the inputs and the outputs to the XPL
program. An p:input element will look as given below.
<p:input
name = ncname
infoset? = ncname
schema-uri? = uri-reference
schema-href? = uri-reference />
An input element should have a name. It is optional to have the other
attributes like infoset, schema-uri, and schema-href. There can be any
number of p:input elements within the p:pipeline element as its children.
No two p:input elements can be the same name but an p:input and p:output
can have the same name. The p:input element should occur before any element
in the XPL program. The attributes schema-uri, and schema-href are used
to validate the XML infosets associated with the input.
The p:output
element defines one XPL program output. The name and the infosetref attributes
are mandatory in the p:output element. The output element will look as
given below. It can be within a p:pipeline element.
<p:output
name = ncname
infosetref = infoset-reference
schema-uri? = uri-reference
schema-href? = uri-reference />
The processor
element p:processor identifies the single XML processor instance and it
comes in the Processor Module. The processor element can have p:input
and p:output elements within it. The p:choose element is used to execute
different sequences of statements depending on the conditions. Branching
within the XPL program is done using the p:when and p:otherwise elements.
The statements
within the p:when element is executed if the expression in the test attribute
is true. The p:otherwise element is always a child element of the p:choose
element. To execute a sequence of statement multiple times you can use
a p:for-each element.
The processing
model for the XPL is unlike the other programming languages. In this processing
model the results to be produced are first determined and then the processing
order is chosen to determine what statements should be executed.
During the
initialization phase of the XML Processor execution if there is no declared
output the execution of the processor terminates with the initialization
phase. For more information on the processing model and the future improvements
that can be done to this XPL you can visit the link http://www.w3.org/Submission/xpl/
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