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What is XSLT? (I)

August 16, 2000

The Context of XSL Transformations and the XML Path Language

Table of Contents

1. The context of XSL Transformations and the XML Path Language
 •1.1 The XML family of
   Recommendations
      ·1.1.1 Extensible Markup
       Language (XML)
      ·1.1.2 XML Path Language
       (XPath)
      ·1.1.3 Styling structured
       information
      ·1.1.4 Extensible Stylesheet
       Language (XSL)
      ·1.1.5 Extensible Stylesheet
        Language Transformations
       (XSLT)
      ·1.1.6 Namespaces
      ·1.1.7 Stylesheet association
   •1.2 Transformation data
    flows
      ·1.2.1 Transformation from
       XML to XML
      ·1.2.2 Transformation from
       XML to XSL formatting
       semantics
      ·1.2.3 Transformation from
       XML to non-XML
      ·1.2.4 Three-tiered
       architectures

This first chapter examines the context of two W3C Recommendations -- Extensible Stylesheet Language Transformations (XSLT) and XML Path Language (XPath) -- within the growing family of Recommendations related to the Extensible Markup Language (XML). Later we will look at detailed examples, but first let's focus on XSLT and XPath in the context of a few of the Recommendations in the XML family and examine how these two Recommendations work together to address separate and distinct functionality required when working with structured information technologies.

This chapter does not attempt to address all of the numerous XML-related Recommendations currently released or in development. Specifically, we will be looking at only the following as they relate to XSLT and XPath:

Extensible Markup Language (XML)

For years, applications and vendors have imposed their constraints on the way we can represent our information. Our data has been created, maintained, stored and archived according to the rules enforced by others. The advent of the Extensible Markup Language (XML) moves the control of our information out of the hands of others and into our own by providing two basic facilities.

XML describes rules for structuring our information using embedded markup of our own choice. We can take control of our information representation by creating and using a vocabulary we design of elements and attributes that makes sense for the way we do our business and use our data.

In addition, XML describes a language for formally declaring the vocabularies we use. This allows our tools to constrain the creation of an instance of our information, and allows our users to validate a properly created instance of information against our set of constraints.

Note 1:

An XML document is just an instance of well-formed XML. The two terms document and instance could be used interchangeably, but this reference material uses the term instance to help readers remember that XML isn't just for documents or documentation. With XML we describe a related set of information in a tree-like hierarchical fashion, and gain the benefits of having done so, whether the information captures an invoice-related transaction between computers, or the content of a user manual rendered on paper.

XML Path Language (XPath)

XPath is a string syntax for building addresses to the information found in an XML document. We use this language to specify the locations of document structures or data found in an XML document when processing that information using XSLT. XPath allows us from any location to address any other location or content.

Extensible Stylesheet Language Family (XSLT/XSL)

Two vocabularies specified in separate W3C Recommendations provide for the two distinct styling processes of transforming and rendering XML instances.

We can transform information using one vocabulary into an alternate form by using the Extensible Stylesheet Language Transformations (XSLT).

The Extensible Stylesheet Language (XSL) is a rendering vocabulary describing the semantics of formatting information for different media.

Namespaces

We use XML namespaces to distinguish information when mixing multiple vocabularies in a single instance. Without namespaces our processes would find the information ambiguous when identical names have been chosen by the designers of the vocabularies we use.

Stylesheet Association

We declare our choice of an associated stylesheet for an XML instance by embedding the construct described in the Stylesheet Association Recommendation. Recipients and applications can choose to respect or ignore this choice, but the declaration indicates that we have tied some process (typically rendering) to our data, which specifies how to consume or work with our information.

1.1  The XML family of Recommendations

Now let's look at the objectives of these selected Recommendations.

1.1.1  Extensible Markup Language (XML)

Historically, the ways we have expressed, created, stored and transmitted our electronic information have been constrained and controlled by the vendors we choose and the applications we run. Alternatively, we now can express our data in a structured fashion oriented around our perspective of the nature of the information itself rather than the nature of an application's choice of how to represent our information. With Extensible Markup Language (XML), we describe our information using embedded markup of elements, attributes and other constructs in a tree-like structure.

1.1.1.1  Structuring information

Contrasted to a file format where information identification relies on some proprietary hidden format, predetermined ordering, or some kind of explicit labeling, the tree-like hierarchical storage structure infers relationships by the scope of values encompassing the scopes of other values.

Though trees shape a number of areas of XML, both logically (markup) and physically (entities such as files or other resources), they are not the only means by which relationships are specified. For example, a quantum of information can arbitrarily point or refer to other information elsewhere through use of unique identifiers.

Two basic objectives of representing information hierarchically are satisfied by the XML Recommendation. It provides:

  • an unambiguous mechanism for constraining structure in a stream of information

    XML defines the concept of well-formedness. Well-formedness dictates the syntax used for markup languages within the content of an instance of information. This is the syntax of using angle brackets ("<" and ">") and the ampersand ("&") to demarcate and identify constituent components of information within a file, a resource or a bound data stream. Users of the Hypertext Markup Language (HTML) will recognize the use of these characters for marking the vocabulary described by the designers of the World Wide Web in their web documents.

  • a language for specifying how a system can constrain the allowed logical hierarchy of information structures

    XML defines the concept of validity with a syntax for a meta-markup language used to specify vocabularies. A Document Type Definition (DTD) describes the structural schema mandating the user-defined constraints on well-formed information. The designers of HTML have formalized their vocabulary through such a DTD, thus declaring the allowed or expected relationships between components of a hypertext document.

There is an implicit document model for an instance of well-formed XML defined by the mere presence of nested elements found in the information. There is no need to declare this model because the syntax rules governing well-formedness guarantee the information to be seen properly as a hierarchy. As with all hierarchies, there are family-tree-like relationships of parent, child, and sibling constructs relative to each construct found.

Consider the following well-formed XML instance purc.xml:

01  <?xml version="1.0"?>
02  <purchase id="p001">
03    <customer db="cust123"/>
04    <product db="prod345">
05      <amount>23.45</amount>
06    </product>
07  </purchase> 
Example 1-1: A well-formed XML purchase order instance.

Observe the content nesting (whitespace has been added only for illustrative purposes). The instance follows the lexical rules for XML markup and the hierarchical model is implicit by the nesting of elements. Pay particular attention to the markup on line 3 for the empty element named customer, with the attribute named db. It will be used later in examples throughout this chapter. The customer element is a child of the document element, which is named purchase.

Although the presence of an explicit formal document model is useful to an XML processor or to a system working with XML instances, that model has no impact on the implicit structural model and only minor influence on the interpretation of content found in the instance. This point holds true whether the model is expressed in a DTD or in some of the other Recommendations for structural and content schemata being developed.

Consider the following valid XML instance purcdtd.xml:

01  <?xml version="1.0"?>
02  <!DOCTYPE purchase [
03  <!ELEMENT purchase ( customer, product+ )>
04  <!ATTLIST purchase id ID #REQUIRED>
05  <!ELEMENT customer EMPTY>
06  <!ATTLIST customer db CDATA #REQUIRED>
07  <!ELEMENT product  ( amount )>
08  <!ATTLIST product  db CDATA #REQUIRED>
09  <!ELEMENT amount   ( #PCDATA )>
10  ]>
11  <purchase id="p001">
12    <customer db="cust123"/>
13    <product db="prod345">
14      <amount>23.45</amount>
15    </product>
16  </purchase>
Example 1-2: A valid XML purchase order instance

See how the information content is no different from the previous example, but in this case an explicit document model using XML 1.0 DTD syntax is included (it could have been included by reference to a separate resource). A processor can validate that the information content conforms not only to the lexical rules for XML (well-formedness) but also the syntax rules dictated by the supplied document model (validity).

Looking at the same customer element as before (now on line 12), the document model indicates on line 6 that the db attribute is, indeed, required: if the attribute is absent the XML processor can report syntactic model constraint violation even if the element is otherwise lexically well-formed. The document model can also provide additional information not evident without a document model (such as the information on line 4 that the id attribute for purchase is of XML type ID).

1.1.1.2  No built-in meanings or concepts

The area of semantics associated with XML instances is very gray. A document model is but one component used to help describe the semantics of the information found in an instance. While well-formed instances do not have a formal document model, often the names of the constructs used within the instances give hints to the associated semantics. Without a formalism yet available in our community to express semantics in a rigorous fashion, we users of XML do (or should!) capture the semantics of a given vocabulary in prose, whether or not the document model is formalized.

The XML 1.0 Recommendation only describes the behavior required of an XML processor acting on an XML stream, and how it must identify constituent data and provide that data to an application using the processor:

Since there are no formalized semantic description facilities in XML, any XML that is used is not tied to any one particular concept or application. There are no rendition or transformation rules or constructs defined in XML. The only purpose of XML is to unambiguously identify and deliver constituent components of data. There are no inherent meanings or semantics of any kind associated with element types defined in a document model. There are no defined controls for implying any rendering semantics.

Even the xml:space attribute allowing for the differentiation of whitespace found in a document is not an aspect of rendering but of information description. The author or modeler of an instance is indicating with this reserved attribute (termed "special" in XML 1.0) the nature of the information and how the whitespace found in the information is to be either preserved or handled by a processor in a default fashion.

Some new users of XML who have a background in a markup language such as HTML often assume a magical association of semantics with element types of the same names they have been exposed to in their prior work. In a web page, they can safely assume that the construct <p> will be interpreted as a paragraph or <em> as emphasized text. However, this interpretation is solely the purview of the designers of HTML and user agents attempting to conform to the World Wide Web Consortium (W3C)-published semantics. Nothing is imposed by any process when creating a new XML vocabulary that happens to use the same names. Applications using XML processors to access XML information must be instructed how to interpret and implement the desired semantics.

1.1.2  XML Path Language (XPath)

Assuming that we have structured our information using XML, how are we going to talk about (address) what is inside our documents? Locating information in an XML document is critical to both transforming it and to associating or relating it to other information. When we write stylesheets and use linking languages, we can address components of our information for a processor by our use of the XML Path Language, also called XPath:

1.1.2.1  Addressing structured information

The W3C working group responsible for stylesheets collaborated with the W3C working group responsible for the next generation of hyperlinking to produce XPath as a common base for addressing requirements shared by their respective Recommendations. Both groups extend the core XPath facilities to meet the needs they have in each of their domains: the stylesheet group uses XPath as the core of expressions in XSLT; the linking group uses XPath as the core of expressions in the XPointer Recommendation.

In order to address components you have to know the addressing scheme with which the components are arranged. The basis of addressing XML documents is an abstract data model of interlinked nodes arranged hierarchically echoing the tree-shape of the nested elements in an instance. Nodes of different types make up this hierarchy, each node representing the parsed result of a syntactic structure found in the bytes of the XML instance.

This abstraction insulates addressing from the multiple syntactic forms of given XML constructs, allowing us to focus on the information itself and not the syntax used to represent the information.

Note 2:

We see XML documents as a stream or string of bytes that follow the rules of the XML 1.0 Recommendation. Stylesheets do not regard instances in this fashion, and we have to change the way we think of our XML documents in order to successfully work with our information. This leap of understanding ranks high on the list of key aspects of stylesheet writing I needed to internalize before successfully using this technology.

We are given tools to work in the framework provided by the abstraction: a set of data types used to represent values found in the generalization, and a set of functions we use to manipulate and examine those values. The data types include strings, numbers, boolean values and sets of nodes of our information. The functions allow us to cast these values into other data type representations and to return massaged information according to our needs.

1.1.2.2  Addressing identifies a hierarchical position or positions

XPath defines common semantics and syntax for addressing XML-expressed information, and bases these primarily on the hierarchical position of components in the tree. This ordering is referred to as document order in XPath, while in other contexts this is often termed either parse order or depth-first order. Alternatively, we can access an arbitrary location in the tree based on points in the tree having unique identifiers.

We convey XPath addresses in a simple and compact non-XML syntax. This allows us to use an XPath expression as the value of an attribute in an XML vocabulary as in the following examples:

01  select="answer"
Example 1-3: A simple XPath expression in a select attribute

The above attribute value expresses all children named "answer" of the current focus element.

01  match="question|answer"
Example 1-4: An XPath expression in a match attribute

The above attribute value expresses a test of an element being in the union of the element types named "question" and "answer".

The XPath syntax looks a lot like addressing subdirectories in a file system or as part of a Universal Resource Identifier (URI). Multiple steps in a location path are separated by either one or two oblique "/" characters. Filters can be specified to further refine the nature of the components of our information being addressed.

01  select="question[3]/answer[1]"
Example 1-5: A multiple step XPath expression in a select attribute

The above example selects only the first "answer" child of the third "question" child of the focus element.

01  select="id('start')//question[@answer='y']"
Example 1-6: A more complex XPath expression in a select attribute

The above example uses an XPath address identifying some descendants of the element in the instance that has the unique identifier with the value "start". Those identified are the question elements whose answer attribute is equal to the string equal to the lower-case letter 'y'. The value returned is the set of nodes representing the elements meeting the conditions expressed by the address. The address is used in a select attribute, thus the XSLT processor is selecting all of the addressed elements for some kind of processing.

1.1.2.3  XPath is not a query language

It is important to remember that addressing information is only one aspect of querying information. Other aspects include query operators that massage intermediate results into a final result. While a few operators and functions are available in XSLT to use values identified in documents, these are oriented to string processing, not to complex operations required by some applications.

Note 3:

When query Recommendations are developed, I would hope that the addressing portion is based on XPath as a core, just as with XSLT.

This is a prose version of an excerpt from the book "Practical Transformation Using XSLT and XPath" (Eighth Edition ISBN 1-894049-05-5 at the time of this writing) published by Crane Softwrights Ltd., written by G. Ken Holman; this excerpt was edited by Stan Swaren, and reviewed by Dave Pawson.

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