XCSL : XML Constraint Specification Language

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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 document’s 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

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

• Context Selector: As the name suggests, is the expression that selects the context where we want to enforce the constraint.
• Context Condition: The condition we want to enforce.
• Action: The action we want to trigger every time the condition does not hold.

In a more formal notation we can write:

        ConstraintSpec = Constraint+
        Constraint = (ContextSelector, ContextCondition, Action) 

<?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

• XCSL Elements
• XCSL Processor
• Case Studies
  • The Reservations Example
  • Certidão Fiscal
  • Base de Dados