Friday, October 30, 2009

Fixing Constraint Violations with spin:fix

Constraint checking is a popular feature of many Semantic Web tools to ensure that instance data meets the design objectives attached to classes and properties in an ontology. Data entry tools like TopBraid Composer and Ensemble use the SPARQL-based constraint checking language SPIN to make sure that users get warnings if the data they are entering is violating constraints. If a violation is reported, the user would read through the violation message and then change the data on the form.

One of the new features of SPIN 1.1 is the property spin:fix which can be used to let the system suggest operations that would repair a constraint violation automatically. The basic idea is that spin:fix can be attached to the spin:ConstraintViolation produced by a spin:constraint query to link the violation with one or more SPARQL Update requests. Those SPARQL Update requests may INSERT or DELETE triples to create a state in which the constraint is no longer violated. If spin:fixes are created, then the user interface may suggest them to the user with a single click. In the TopBraid Composer 3.2 screenshot below, the resource InvalidSquare violates the constraint that instances of Square must have equal width and height:

The following spin:constraint (attached to the Square class) implements the suggestions above:

You can see that the constraint creates a spin:ConstraintViolation that is attached to two instances of :SetObject via spin:fix. The class :SetObject is a SPIN template with the following definition:
The SPIN templates suggested as fixes could come from a library of re-usable building blocks. I assume that most constraint violations can be repaired by replacing, adding or deleting certain triples, and these cases can be generalized easily. Furthermore, the spin:constraints themselves can be generalized into templates, so that the ontology designer in the end just needs to pick the correct template to get a really powerful and convenient constraint checking mechanism.

Also note that the constraint fixing mechanism above could be useful beyond user interfaces, for example to automatically repair incoming data streams in web service calls.


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