Section: Scientific Foundations

XML Processing

Participants : Melisachew Chekol, Pierre Genevès, Nils Gesbert, Nicola Guido, Muhammad Junedi, Nabil Layaïda, Manh-Toan Nguyen, Vincent Quint.

Extensible Markup Language (XML) has gained considerable interest from industry, and plays now a central role in modern information system infrastructures. In particular, XML is the key technology for describing, storing, and exchanging a wide variety of data on the web. The essence of XML consists in organizing information in tree-tagged structures conforming to some constraints which are expressed using type languages such as DTDs, XML Schemas, and Relax NG.

There still exist important obstacles in XML programming, especially in the areas of performance and reliability. Programmers are given two options: domain-specific languages such as XSLT, or general-purpose languages augmented with XML application programming interfaces such as the Document Object Model (DOM ). Neither of these options is a satisfactory answer to performance and reliability issues, nor is there even a trade-off between the two. As a consequence, new paradigms are being proposed which all have the aim of incorporating XML data as first-class constructs in programming languages. The hope is to build a new generation of tools that are capable of taking reliability and performance into account at compile time.

One of the major challenges in this line of research is to develop automated and tractable techniques for ensuring static type safety and optimization of programs. To this end, there is a need to solve some basic reasoning tasks that involve very complex constructions such as XML types (regular tree types) and powerful navigational primitives (XPath expressions or CSS selectors). In particular, every future compiler of XML programs will have to routinely solve problems such as:

• XPath query emptiness in the presence of a schema: if one can decide at compile time that a query is not satisfiable, then subsequent bound computations can be avoided

• query equivalence, which is important for query reformulation and optimization

• path type-checking, for ensuring at compile time that invalid documents can never arise as the output of XML processing code.

All these problems are known to be computationally heavy (when decidable), and the related algorithms are often tricky.

We have developed an XML/XPath static analyzer based on a new logic of finite trees. This analyzer consists of:

• compilers that allow XML types, XPath queries, and CSS selectors to be translated into this logic

• an optimized logical solver for testing satisfiability of a formula of this logic.

The benefit of these compilers is that they allow one to reduce all the problems listed above, and many others too, to logical satisfiability. This approach has a couple of important practical advantages. First of all, one can use the satisfiability algorithm to solve all of these problems. More importantly, one could easily explore new variants of these problems, generated for example by the presence of different kinds of type or schema information, with no need to devise a new algorithm for each variant.