INTER Platform Semantic Mediator
Semantic interoperability is the ability of different applications and business partners to exchange data with unambiguous, shared meaning. As a result, data analysis and knowledge discovery can be done on a federation of systems.
Inter Platform Semantic Mediator (IPSM) is an independent component (developed within INTER-IoT project) that enables alignments-based semantic translation of RDF data (messages).
Semantic translation performed by IPSM component is based on alignments and uses a deployment-specific modularized central ontology. For IoT domain, the core modules describing, e.g. devices, observations are (usually) based on GOIoTP (Generic Ontology of IoT Platforms) that is a meta-data reference data model. Additionally, any domain specific module can be included e.g. medical ontology, logistic ontology. However, central ontology can be any ontology that can serve as a central data model. It is not directly used configure IPSM but should be considered when defining alignments that are used for IPSM configuration. Additionally, any domain specific module can be included e.g. medical ontology, logistic ontology.
The alignment (in IPSM-AF format) is a set of correspondences between simple entities or complex structure from source and target ontologies. It contains rules for transformation between input and output RDF graphs. Specifically, IPSM translates RDF graph named payload (that is part of the message send to IPSM). Alignments are parts of IPSM instance configuration, and are used directly to execute translation. In basic INTER-IoT use case, semantic translation always constitutes application of two alignments - one from source ontology to central ontology, the other from central ontology to target ontology.
The following figure shows a sample situation with four IoT artifacts (P1-4). Each with it’s own ontology (O1-4). The central ontology contains modules g1...gn.
The two-way communication requires preparation of two alignments: (i) from artifact’s ontology to central ontology e.g. A1G, (ii) from central ontology to artifact’s ontology e.g. AG1.
Each alignments contains correspondences between ontology modules that are required for this part of communication.
Overview of translation with central modularized ontology
To achieve semantic interoperability between two artifacts:
- Instantiate modular central ontology
- Select/define artifact's ontologies: create from scratch or use one of existing tools
- Align semantics between ontology of each artifact and central ontology (set of alignments)
- Implement syntactic translators
- Configure IPSM - upload alignments, create translation channels
The following figure shows a process of sending a message from source to target artifact that needs to be semantically and syntactically translated.
Process overview
The message originates at source artifact in it's format and semantics e.g. XML message with respect to XML Schema. To use IPSM the message needs to be transformed to RDF with source artifact's semantics. This translation is called syntactic translation and in INTER-IoT is implemented with INTER-MW Bridges or INTER-AS2AS Converter Nodes. In fact, when IPSM is to be used in a standalone mode (without other INTER-IoT layers), syntactic translation can be implemented in any arbitrary component that will "prepare" input for semantic translation. Note that, conversion to RDF may not be necessary, when artifacts already has communication based on RDF. When a source artifact does not support semantics, an RDF represantation of data exchanged with the ecosystem needs to be proposed - see FAQ section.
When the message arrives at IPSM, the RDF named graph payload is translated with respect to configuration of the semantic translation channel (see Architecture). Usually two alignments are applied, however IPSM can be configured with special predefined IDENTITY alignment that does not change the graph. Another remark here, is that IPSM follows the rule: translate only what can be translated, leave the rest as it was. The resultant message is expressed in RDF with semantics corresponding to target semantic of last applied alignment.
This message can be feed into another syntactic translator that will transform it's format to e.g. JSON cosidering target artifact's semantics. Another possible scenario is that there are applications consuming data in RDF and central semantics. In such case, second syntactic translation is not necessary.