Streamlining SHACL: Introducing `rdf-construct Shacl-gen`

by Alex Johnson 58 views

Are you tired of the tedious task of manually writing SHACL shapes? Do you wish there was a way to automate the process and ensure your shapes stay aligned with your ontology? Well, get ready for some exciting news! This article delves into a proposal to add a new command, rdf-construct shacl-gen, to streamline SHACL shape generation. This new feature promises to save you time, reduce errors, and make data validation a whole lot easier.

The Motivation Behind shacl-gen

Let's face it: writing SHACL shapes from scratch can be a real pain. It's a laborious process that's prone to errors, especially when dealing with complex ontologies. The good news is that much of the information needed for validation already exists within the ontology itself. Think about it:

  • rdfs:domain tells us which class should have which properties.
  • rdfs:range specifies the expected values for those properties.
  • owl:cardinality restrictions define the minimum and maximum number of values allowed.
  • owl:FunctionalProperty indicates that a property can have at most one value.

The core idea behind shacl-gen is to leverage this existing information to automatically generate SHACL shapes. A generator that converts these RDF/OWL patterns to SHACL can save you hours of writing boilerplate code and ensures that your shapes remain consistent with your ontology. Imagine the time and effort you'll save!

Introducing the Proposed Solution

The proposed solution is to introduce a new command-line tool, rdf-construct shacl-gen, that can generate SHACL shapes from RDF/OWL ontology definitions. This tool will provide a valuable starting point for data validation, significantly reducing the manual effort required.

The CLI Interface: A User-Friendly Approach

The proposed command-line interface (CLI) is designed to be flexible and user-friendly. Here are some examples of how you can use the rdf-construct shacl-gen command:

  • Basic Generation:

rdf-construct shacl-gen ontology.ttl -o shapes.ttl ```

This command generates SHACL shapes from the `ontology.ttl` file and saves them to `shapes.ttl`.

  • Output Format:

rdf-construct shacl-gen ontology.ttl --format turtle|jsonld ```

You can specify the output format as either Turtle or JSON-LD.

  • Strictness Level:

rdf-construct shacl-gen ontology.ttl --level minimal|standard|strict ```

The `--level` option allows you to control the strictness of the generated shapes. We'll discuss the different strictness levels in more detail later.

  • Focus on Specific Classes:

rdf-construct shacl-gen ontology.ttl --classes ex:Building,ex:Floor ```

You can generate shapes for specific classes by using the `--classes` option.

  • Configuration File:

rdf-construct shacl-gen ontology.ttl --config shacl-config.yml ```

A configuration file allows you to customize the generation process further.

  • Include Closed Shapes:

rdf-construct shacl-gen ontology.ttl --closed ```

The `--closed` option generates closed shapes, which means that no extra properties are allowed.

  • Generate with Severity Levels:

rdf-construct shacl-gen ontology.ttl --default_severity warning ```

You can set the default severity level for validation violations using the `--default_severity` option.

Conversion Rules: Translating OWL to SHACL

The heart of shacl-gen lies in its ability to convert OWL (Web Ontology Language) constructs into SHACL shapes. Let's take a look at some of the key conversion rules:

Domain → sh:targetClass + sh:property

The rdfs:domain property tells us which class a particular property applies to. shacl-gen translates this into sh:targetClass and sh:property in SHACL.

# OWL
ex:hasFloor rdfs:domain ex:Building .

# SHACL
ex:BuildingShape a sh:NodeShape ;
    sh:targetClass ex:Building ;
    sh:property [
        sh:path ex:hasFloor ;
    ] .

In this example, the OWL statement ex:hasFloor rdfs:domain ex:Building indicates that the ex:hasFloor property applies to the ex:Building class. The shacl-gen command converts this into a SHACL shape that targets the ex:Building class and includes a property shape for ex:hasFloor.

Range → sh:class or sh:datatype

The rdfs:range property specifies the expected type of values for a property. This can be either a class (sh:class) for object properties or a datatype (sh:datatype) for datatype properties.

# OWL (object property)
ex:hasFloor rdfs:range ex:Floor .

# SHACL
sh:property [
    sh:path ex:hasFloor ;
    sh:class ex:Floor ;
] .

# OWL (datatype property)
ex:floorArea rdfs:range xsd:decimal .

# SHACL
sh:property [
    sh:path ex:floorArea ;
    sh:datatype xsd:decimal ;
] .

For object properties, shacl-gen generates a property shape with the sh:class constraint. For datatype properties, it uses the sh:datatype constraint.

Cardinality Restrictions

OWL allows you to specify cardinality restrictions using properties like owl:cardinality, owl:minCardinality, and owl:maxCardinality. shacl-gen converts these restrictions into sh:minCount and sh:maxCount constraints in SHACL.

# OWL
ex:Building rdfs:subClassOf [
    a owl:Restriction ;
    owl:onProperty ex:hasAddress ;
    owl:cardinality 1
] .

# SHACL
sh:property [
    sh:path ex:hasAddress ;
    sh:minCount 1 ;
    sh:maxCount 1 ;
] .

# owl:minCardinality → sh:minCount
# owl:maxCardinality → sh:maxCount
# owl:someValuesFrom → sh:minCount 1

In this example, the OWL restriction specifies that a building must have exactly one address (owl:cardinality 1). This is translated into a SHACL property shape with both sh:minCount and sh:maxCount set to 1.

Functional Properties

An owl:FunctionalProperty indicates that a property can have at most one value. shacl-gen converts this into a sh:maxCount 1 constraint in SHACL.

# OWL
ex:hasMainEntrance a owl:FunctionalProperty .

# SHACL
sh:property [
    sh:path ex:hasMainEntrance ;
    sh:maxCount 1 ;
] .

Inverse Functional Properties

An owl:InverseFunctionalProperty indicates that a property uniquely identifies an individual. shacl-gen generates a sh:maxCount 1 constraint on the property shape to ensure that each subject has at most one value for this property.

# OWL
ex:hasSerialNumber a owl:InverseFunctionalProperty .

# SHACL (on property shape)
sh:property [
    sh:path ex:hasSerialNumber ;
    sh:maxCount 1 ;  # At most one per subject
] .
# Note: True inverse-functional requires SPARQL constraint

It's important to note that true inverse-functional validation requires a SPARQL constraint, which is not automatically generated by shacl-gen.

Value Constraints

OWL allows you to define value constraints using owl:oneOf, which specifies an enumeration of allowed values. shacl-gen converts this into a sh:in constraint in SHACL.

# OWL (enumeration)
ex:Status owl:oneOf (ex:Active ex:Inactive ex:Pending) .

# SHACL
sh:property [
    sh:path ex:hasStatus ;
    sh:in (ex:Active ex:Inactive ex:Pending) ;
] .

Strictness Levels: Tailoring the Generation Process

The --level option in shacl-gen allows you to control the strictness of the generated shapes. This is a powerful feature that lets you customize the level of validation you want to enforce.

  • Minimal: This level generates shapes with only explicit constraints, such as cardinality restrictions, explicit range declarations, and functional property constraints. It's a good starting point for basic validation.
  • Standard (default): The standard level includes all the minimal rules, plus inferred constraints. This includes domain-based property assignment and inherited constraints from superclasses. It provides a good balance between strictness and usability.
  • Strict: The strict level aims for maximum validation. It includes all the standard rules, plus sh:closed true for all shapes (meaning no extra properties are allowed), required labels (sh:pattern for naming), and type enforcement (sh:class for all object properties).

Configuration File: Fine-Grained Control

For even more control over the generation process, shacl-gen supports a configuration file. This file allows you to specify various options, such as the namespace for generated shapes, the default severity for violations, classes to include or exclude, properties to ignore, and more.

Here's an example of a shacl-config.yml file:

# shacl-config.yml
level: standard

# Namespace for generated shapes
shape_namespace: "http://example.org/shapes#"
shape_prefix: "shape"

# Default severity for violations
default_severity: violation  # warning | info | violation

# Classes to include (empty = all)
include_classes: []

# Classes to exclude
exclude_classes:
  - owl:Thing
  - rdfs:Resource

# Properties to ignore
ignore_properties:
  - rdfs:label
  - rdfs:comment
  - owl:deprecated

# Generate closed shapes
closed: false

# Property shape options
property_shapes:
  # Require all properties to have values
  require_domain_properties: false
  
  # Generate sh:name from rdfs:label
  include_names: true
  
  # Generate sh:description from rdfs:comment
  include_descriptions: true

# Inheritance handling
inheritance:
  # Include inherited property constraints
  include_inherited: true
  
  # Generate shapes for abstract classes
  include_abstract: false

Output Example: Putting it All Together

Let's take a look at an example of the output generated by shacl-gen:

@prefix sh: <http://www.w3.org/ns/shacl#> .
@prefix ex: <http://example.org/> .
@prefix shape: <http://example.org/shapes#> .

# Generated by rdf-construct shacl-gen
# Source: ontology.ttl
# Level: standard

shape:BuildingShape a sh:NodeShape ;
    sh:targetClass ex:Building ;
    sh:name "Building Shape" ;
    sh:description "Validates instances of ex:Building" ;
    
    # From rdfs:domain declarations
    sh:property [
        sh:path ex:hasFloor ;
        sh:class ex:Floor ;
        sh:name "has floor" ;
    ] ;
    
    sh:property [
        sh:path ex:hasAddress ;
        sh:minCount 1 ;
        sh:maxCount 1 ;
        sh:datatype xsd:string ;
        sh:name "has address" ;
    ] ;
    
    sh:property [
        sh:path ex:floorArea ;
        sh:datatype xsd:decimal ;
        sh:name "floor area" ;
    ] .

shape:FloorShape a sh:NodeShape ;
    sh:targetClass ex:Floor ;
    # ... properties ...
    .

This example shows a SHACL shape generated for the ex:Building class. It includes constraints derived from rdfs:domain declarations and cardinality restrictions. The shape also includes human-readable names and descriptions, making it easier to understand and maintain.

Implementation Notes: Under the Hood

To give you a glimpse into the implementation, let's discuss the proposed module structure and conversion architecture.

Module Structure: Organizing the Code

The proposed module structure for shacl-gen is as follows:

src/rdf_construct/
├── shacl/
│   ├── __init__.py
│   ├── generator.py       # Main generation logic
│   ├── converters.py      # OWL → SHACL conversion rules
│   ├── config.py          # Configuration handling
│   └── templates.py       # Shape construction helpers

This structure helps to organize the code into logical components, making it easier to develop, maintain, and test.

Conversion Architecture: A Step-by-Step Process

The core generation logic is encapsulated in the ShapeGenerator class. Here's a simplified overview of the generation process:

class ShapeGenerator:
    def generate(self, graph: Graph, config: ShaclConfig) -> Graph:
        shapes = Graph()
        
        for cls in self.get_target_classes(graph, config):
            shape = self.create_node_shape(cls, graph, config)
            shapes += shape
        
        return shapes
    
    def create_node_shape(self, cls: URIRef, graph: Graph, config: ShaclConfig) -> Graph:
        shape_uri = self.shape_uri_for(cls, config)
        shape = Graph()
        
        shape.add((shape_uri, RDF.type, SH.NodeShape))
        shape.add((shape_uri, SH.targetClass, cls))
        
        for prop in self.get_domain_properties(cls, graph):
            prop_shape = self.create_property_shape(prop, graph, config)
            shape.add((shape_uri, SH.property, prop_shape))
        
        return shape

The generate method iterates through the target classes in the ontology and creates a node shape for each class. The create_node_shape method then adds the necessary triples to the shape, including the target class and property shapes.

Handling OWL Restrictions: A Focus on Common Patterns

OWL restrictions can be complex, so shacl-gen focuses on handling common patterns. Here's an example of how it extracts cardinality restrictions:

def extract_restrictions(cls: URIRef, graph: Graph) -> list[Restriction]:
    restrictions = []
    
    for superclass in graph.objects(cls, RDFS.subClassOf):
        if (superclass, RDF.type, OWL.Restriction) in graph:
            on_prop = graph.value(superclass, OWL.onProperty)
            
            # Check for cardinality
            if (card := graph.value(superclass, OWL.cardinality)):
                restrictions.append(CardinalityRestriction(on_prop, int(card), int(card)))
            elif (min_card := graph.value(superclass, OWL.minCardinality)):
                restrictions.append(CardinalityRestriction(on_prop, int(min_card), None))
            # ... etc
    
    return restrictions

This code iterates through the superclasses of a given class and extracts cardinality restrictions from OWL restrictions. It then creates CardinalityRestriction objects to represent these constraints.

Limitations: What shacl-gen Doesn't Do (Yet)

It's important to be aware of the limitations of shacl-gen. While it can handle many common OWL constructs, there are some things it doesn't convert (at least initially):

  1. Complex class expressions: Unions, intersections beyond simple cases.
  2. SPARQL-equivalent constraints: Some OWL axioms need SPARQL rules.
  3. Inverse functional uniqueness: Requires SPARQL to validate globally.
  4. Transitive closures: Cannot validate in pure SHACL.
  5. Qualified cardinality on expressions: Complex QCRs.

To address these limitations, shacl-gen generates TODO comments for unsupported patterns in the output SHACL, like this:

# TODO: owl:complementOf not converted - requires SPARQL constraint
# Original: ex:NonBuilding owl:complementOf ex:Building

This helps you to identify areas where manual intervention is needed.

Acceptance Criteria: Ensuring Quality and Functionality

To ensure the quality and functionality of shacl-gen, the following acceptance criteria have been defined:

  • [ ] rdf-construct shacl-gen command implemented
  • [ ] Converts domain/range to property shapes
  • [ ] Converts cardinality restrictions
  • [ ] Converts functional properties
  • [ ] Supports --level (minimal/standard/strict)
  • [ ] Configuration file support
  • [ ] Generates valid SHACL (validates with pySHACL)
  • [ ] Clear comments for unconverted patterns
  • [ ] Unit tests for each conversion rule
  • [ ] Integration tests with example ontologies
  • [ ] Documentation in docs/user_guides/SHACL_GUIDE.md

These criteria cover various aspects of the tool, from its core conversion capabilities to its usability and documentation.

Future Enhancements: Looking Ahead

While shacl-gen is a significant step forward, there are several potential enhancements that could be explored in the future. These include:

  • SPARQL constraint generation for complex patterns
  • SHACL → OWL reverse conversion
  • Validation result formatting
  • Shape inheritance optimisation
  • SHACL-AF (Advanced Features) support

These enhancements would further expand the capabilities of shacl-gen and make it an even more valuable tool for data validation.

Conclusion

The proposed rdf-construct shacl-gen command promises to be a game-changer for SHACL shape generation. By automating the conversion of OWL ontologies to SHACL shapes, it will save you time, reduce errors, and make data validation a more efficient process. With its flexible CLI, configurable options, and clear conversion rules, shacl-gen is poised to become an indispensable tool for anyone working with semantic data. This tool empowers users to generate SHACL shapes from existing RDF/OWL ontologies, reducing manual effort and ensuring consistency. The different strictness levels cater to diverse validation needs, while the configuration file offers fine-grained control over the generation process. By addressing the limitations and incorporating future enhancements, shacl-gen can evolve into a powerful asset for semantic data management. Stay tuned for its implementation and get ready to streamline your SHACL workflows!

For more information on SHACL and data validation, check out the W3C SHACL specification.