SHACL

SHACL, short for Shapes Constraint Language, is a W3C standard for validating RDF graphs against a set of constraints called shapes. It provides a formal, machine-readable way to express data quality requirements and to check data consistency across datasets and systems.

A SHACL schema (or shapes graph) defines shapes, which are either node shapes or property shapes. Node shapes impose constraints on a focus node; property shapes constrain particular properties of a focus node. Shapes are associated with a data graph via targets, such as sh:targetClass, sh:targetNode, sh:targetSubjectsOf, or sh:targetObjectsOf, which determine the set of focus nodes to validate.

Constraints are expressed with constraint components (e.g., sh:datatype, sh:minCount, sh:maxCount, sh:class, sh:nodeKind, sh:minLength, sh:pattern) and can be combined with logical operators (sh:and, sh:or, sh:not). Paths can be complex RDF property paths, allowing nested or repeated properties. SHACL also supports SHACL-SPARQL, which enables constraint validation via SPARQL queries for cases not covered by built-in components.

Validation proceeds by running a SHACL processor that validates the data graph against the shapes graph and produces a validation report. The report lists results for failing focus nodes, including the shape that was violated, the constraint component, a message, and severity. A graph is conformant if the report contains no violations.

SHACL is used for data governance, schema validation, data integration, and publishing interoperable Linked Data. It is supported by multiple RDF data platforms and tools and is available in RDF serializations such as Turtle and RDF/XML. The W3C SHACL specification evolved from SHACL Core and SHACL-SPARQL, with ongoing extensions and tooling.