XCSL : XML Constraint Specification Language

http://www.di.uminho.pt/~jcr/PROJS/xcsl-www/

Authors:

Josť Carlos Ramalhojcr@di.uminho.pthttp://www.di.uminho.pt/~jcr
Giovani Rubert Librelottogrl@di.uminho.pthttp://www.di.uminho.pt/~grl
Marta Jacintomarta.jacinto@itij.mj.pthttp://www.martajacinto.com
Pedro Rangel Henriquesprh@di.uminho.pthttp://www.di.uminho.pt/~prh

XCSL, XML Constraint Specification Language, is a domain specific language conceived with the purpose of allowing XML designers to restrict the content of XML documents. It is a simple, and small language useful to write contextual constraints over the textual value of XML elements and attributes.

Basically, the language provides the constructors necessary to define the actions to be taken by a semantic validator. Those actions will triggered whenever a boolean expression, over the value of the attributes or the content of the elements, evaluates to false. It means that XCSL also designates a processor that traverses the documentís internal representation (the tree) and checks its correctness from a semantic point of view (we shall stress that the structural correctness is validated by the parser that builds the tree).

Moreover, XCSL is an XML language; so it becomes possible to add restrictions to XML documents using an XML dialect. That approach offers document designers a complete XML framework to couple with syntax and semantics. The benefits of such an approach are obvious.

Figure 1: XCSL workflow

The XCSL Language

The first version of the Constraint Specification Language is formally defined in [Ramalho, 2000]. This exercise of formalization helped us to find the core structure of the language.

A specification in XCSL is composed by one or more tuples. Each tuple has three parts [Ramalho and Henriques, 1998]:

In a more formal notation we can write:

        ConstraintSpec = Constraint+
        Constraint = (ContextSelector, ContextCondition, Action) 
We could use a grammar to define the language, but as we stated before, we decided to use XSLT to specify the constraints; in order to be coherent, we needed an XML wrapper for the XSLT expressions (like in XSL). So, each XCSL specification is defined as an XML instance and the XCSL language is defined by a DTD (or an XML-Schema); the present version of the DTD that specifies XCSL (named xcsl.dtd) is shown below.

<?xml version="1.0" encoding="UTF-8"?>
<!-- XCSL: XML Constraint Specification Language -->
<!ELEMENT CS (CONSTRAINT)+> 
<!ATTLIST CS
    DTD CDATA #IMPLIED
    DATE CDATA #IMPLIED
    VERSION CDATA #IMPLIED
> <!ELEMENT CONSTRAINT (SELECTOR, LET*, CC, ACTION)>
<!ELEMENT SELECTOR EMPTY> 
<!ATTLIST SELECTOR
    SELEXP CDATA #REQUIRED
> 
<!ELEMENT LET EMPTY> 
<!ATTLIST LET
    NAME CDATA #REQUIRED
    VALUE CDATA #REQUIRED
> <!ELEMENT CC (#PCDATA | VARIABLE)*>
<!ELEMENT VARIABLE EMPTY> 
<!ATTLIST VARIABLE
    SELEXP CDATA #REQUIRED
> 
<!ELEMENT ACTION (MESSAGE*)> 
<!ELEMENT MESSAGE (#PCDATA | VALUE)*> 
<!ELEMENT VALUE EMPTY> 
<!ATTLIST VALUE
    SELEXP CDATA #REQUIRED
> 

Nowadays, XML-Schema has overcome DTDs as a definition of classes for XML instances according to what was said before. We also made that upgrade, however, as XML-Schema is much more verbose than the correspondent DTD, we decided to include here the DTD and just a diagrammatic description of the XCSL XML-Schema. That diagram is shown in Figure 2, as obtained with the XML Spy 4.1, from Altova.

Figure 2: XCSL version 1.0 XML-Schema diagram

Index