Basic XML concepts

Figure 10.1: An XML document.

An XML document is an ordered, labeled tree. Each node of the tree is an XML element and is written with an opening and closing tag . An element can have one or more XML attributes . In the XML document in Figure 10.1 , the scene element is enclosed by the two tags <scene ...> and </scene>. It has an attribute number with value vii and two child elements, title and verse.

Figure 10.2: The XML document in Figure 10.1 as a simplified DOM object.

Figure 10.2 shows Figure 10.1 as a tree. The leaf nodes of the tree consist of text, e.g., Shakespeare, Macbeth, and Macbeth's castle. The tree's internal nodes encode either the structure of the document (title, act, and scene) or metadata functions (author).

The standard for accessing and processing XML documents is the XML Document Object Model or DOM . The DOM represents elements, attributes and text within elements as nodes in a tree. Figure 10.2 is a simplified DOM representation of the XML document in Figure 10.1 .With a DOM API, we can process an XML document by starting at the root element and then descending down the tree from parents to children.

XPath is a standard for enumerating paths in an XML document collection. We will also refer to paths as XML contexts or simply contexts in this chapter. Only a small subset of XPath is needed for our purposes. The XPath expression node selects all nodes of that name. Successive elements of a path are separated by slashes, so act/scene selects all scene elements whose parent is an act element. Double slashes indicate that an arbitrary number of elements can intervene on a path: play//scene selects all scene elements occurring in a play element. In Figure 10.2 this set consists of a single scene element, which is accessible via the path play, act, scene from the top. An initial slash starts the path at the root element. /play/title selects the play's title in Figure 10.1 , /play//title selects a set with two members (the play's title and the scene's title), and /scene/title selects no elements. For notational convenience, we allow the final element of a path to be a vocabulary term and separate it from the element path by the symbol #, even though this does not conform to the XPath standard. For example, title#"Macbeth" selects all titles containing the term Macbeth.

We also need the concept of schema in this chapter. A schema puts constraints on the structure of allowable XML documents for a particular application. A schema for Shakespeare's plays may stipulate that scenes can only occur as children of acts and that only acts and scenes have the number attribute. Two standards for schemas for XML documents are XML DTD (document type definition) and XML Schema . Users can only write structured queries for an XML retrieval system if they have some minimal knowledge about the schema of the collection.

Figure 10.3: An XML query in NEXI format and its partial representation as a tree.

A common format for XML queries is NEXI (Narrowed Extended XPath I). We give an example in Figure 10.3 . We display the query on four lines for typographical convenience, but it is intended to be read as one unit without line breaks. In particular, //section is embedded under //article.

The query in Figure 10.3 specifies a search for sections about the summer holidays that are part of articles from 2001 or 2002. As in XPath double slashes indicate that an arbitrary number of elements can intervene on a path. The dot in a clause in square brackets refers to the element the clause modifies. The clause [.//yr = 2001 or .//yr = 2002] modifies //article. Thus, the dot refers to //article in this case. Similarly, the dot in [about(., summer holidays)] refers to the section that the clause modifies.

The two yr conditions are relational attribute constraints. Only articles whose yr attribute is 2001 or 2002 (or that contain an element whose yr attribute is 2001 or 2002) are to be considered. The about clause is a ranking constraint: Sections that occur in the right type of article are to be ranked according to how relevant they are to the topic summer holidays.

Figure 10.4: Tree representation of XML documents and queries.

We usually handle relational attribute constraints by prefiltering or postfiltering: We simply exclude all elements from the result set that do not meet the relational attribute constraints. In this chapter, we will not address how to do this efficiently and instead focus on the core information retrieval problem in XML retrieval, namely how to rank documents according to the relevance criteria expressed in the about conditions of the NEXI query.

If we discard relational attributes, we can represent documents as trees with only one type of node: element nodes. In other words, we remove all attribute nodes from the XML document, such as the number attribute in Figure 10.1 . Figure 10.4 shows a subtree of the document in Figure 10.1 as an element-node tree (labeled $d_1$).

We can represent queries as trees in the same way. This is a query-by-example approach to query language design because users pose queries by creating objects that satisfy the same formal description as documents. In Figure 10.4 , $q_1$ is a search for books whose titles score highly for the keywords Julius Caesar. $q_2$ is a search for books whose author elements score highly for Julius Caesar and whose title elements score highly for Gallic war.