Tuesday, August 11, 2009

Adding Semantics to SOA

What can Semantic Web technologies such as RDF, OWL, SKOS, SWRL, and SPARQL bring to Web Services. One of the most difficult challenges of SOA is data model transformation. This problem occurs when services don't share a canonical XML schema. XML transformation languages such as XSLT and XQuery are typically used for data mediation in such circumstances.

While it is relatively easy to write these mappings, the real difficulty lies in mapping concepts across domains. This is particularly important in B2B scenarios involving multiple trading partners. In addition to proprietary data models, it is not uncommon to have multiple competing XML standards in the same vertical. In general, these data interoperability issues can be syntactic, structural, or semantic in nature. Many SOA projects can trace their failure to those data integration issues.

This is where semantic web technologies can add significant value to SOA. The Semantic Annotations for WSDL and XML Schema (SAWSDL) is a W3C recommendation which defines the following extension attributes that can be added to WSDL and XML Schema components:

  • The modelReference extension attribute associates a WSDL or XML Schema component to a concept in a semantic model such as OWL. The semantic representation is not restricted to OWL (for example it could be an SKOS concept). The modelReference extension attribute is used to annotate XML Schema type definitions, element and attribute declarations as well as WSDL interfaces, operations, and faults.
  • The liftingSchemaMapping and loweringSchemaMapping extension attributes typically point to an XSLT or XQuery mapping file for transforming between XML instances and ontology instances.

A typical example of how SAWSDL might be used is in an electronic commerce network where trading partners use various standards such as EDI, UBL, ebXML, and RosettaNet. In this case, the modelReference extension attribute can be used to map a WSDL or XML Schema component to a concept in a common foundational ontology such as one based on the Suggested Upper Merged Ontology (SUMO). In addition, lifting and lowering XSLT transforms are attached to XML Schema components in the SAWSDL with liftingSchemaMapping and loweringSchemaMapping extension attributes respectively. Note that any number of those transforms can be associated with a given XML schema component.

Traditionally, when dealing with multiple services (often across organizational boundaries), an Enterprise Services Bus (ESB) provides mediation services such as business process orchestration, business rules processing, data format and data model transformation, message routing, and protocol bridging. Semantic mediation services can be added as a new type of ESB service. The SAWSDL4J API defines an object model that allows SOA developers to access and manipulate SAWSDL annotations.

Ontologies have been developed for some existing e-commerce standards such as EDI X12, RosettaNet, and ebXML. When required, ontology alignment can be achieved with OWL constructs such as subClassOf , equivalentClass , and equivalentProperty.

Semantic annotations provided by SAWSDL can also be leveraged in orchestrating business processes using the business process execution language (BPEL). To facilitate service discovery in SOA Registries and Repositories, interface definitions in WSDL documents can be associated with a service taxonomy defined in SKOS. In addition, once an XML message is lifted to an ontology instance, the data in the message becomes available to Semantic Web tools like OWL and SWRL reasoners and SPARQL query engines.