Possibly the most important European strategic objective in eHealth is to provide interoperability among healthcare information systems. The CEN/ISSS eHealth Standardisation Focus Group has identified five prominent strategic aims of healthcare informatics in Europe to be (CEN/ISSS 2004):

• Improving access to clinical records 
• Enabling patient mobility and cross border access to healthcare 
• Reducing clinical errors and improving safety
• Improving access to quality information on health for patients and professionals 
• Improving efficiency of healthcare processes

The interoperability of healthcare information systems is the key component in meeting these challenges and a number of standardization efforts are addressing the issue.

Semantic interoperability

One of the most promising routes is based on semantic interoperability to support exchange of meaningful clinical information among healthcare systems. Since it is not realistic to expect all the healthcare institutes to conform to a single standard, there is a need to address the interoperability at the semantic level.
Semantic interoperability is the ability for information shared by systems to be understood at the level of formally defined domain concepts so that the information can be processed by the receiving system (ISO2007).

Different layers of web technologies

The Semantic Web principles are implemented in different layers of web technologies and standards.

The Unicode and URI layers provide means for identifying medical knowledge objects on the Semantic Web. The XML layer with namespace and schema definitions makes sure that applications can integrate the Semantic Web definitions with other XML based standards in medical databases and knowledge repositories such as the “Cross Enterprise Document Sharing (XDS)" profile, which stores healthcare documents in an ebXML registry/repository architecture (The “Integrating the Healthcare Enterprise” initiative, http://www.ihe.net/).

With RDF and RDFSchema (Resource Description Framework Schema, http://www.w3.org/TR/rdf-schema/)  it is possible to make statements about objects with URI's and define vocabu¬laries that can be referred to by URI's. This is the layer where we can give types to resources and links.

The Ontology layer supports the evolution of vocabularies as it can define relations between the different concepts. An ontology based vocabulary of commonly used data exchange standards in healthcare such as Health Level 7 (HL7), which has categorized events in the healthcare domain by considering service functionality which also reflects the business logic, GEHR or CEN's ENV 13606 needs to be developed or adapted from projects. In the project eu-DOMAIN, HL7 was used to provide a domain model, which facilitated integration with legacy health systems.

The backend knowledge discovery processes in REACTION will be implemented using Semantic Web Services to prove an automated environment for delivering eHealth services and application.

The OWL Web Ontology Language is designed for use by applications that need to process the content of information instead of just presenting information to humans. OWL facilitates greater machine interpretability of web content than that supported by XML, RDF, and RDF Schema (RDF-S) by providing additional vocabulary along with formal semantics. In most healthcare domains, the exchanged message instances are EDI or XML, not messages conforming to an ontology.

Web service technology

Web Service technologies provide standard, simple and lightweight mechanisms for exchanging structured and typed information between services in a decentralised and distributed environment. Traditional Service Oriented Architectures (SOAs) are based on the client/server architecture where a server application, hosted by an always-on end system, provides services to many other client applications hosted by sometimes-on end systems. The server has a fixed, well-known IP address.

However, it is a usual phenomenon in a client/server application that a single server host is incapable of keeping up with all the requests from its clients. For this reason, clusters of hosts, sometimes referred as server farm, are often used to create a powerful virtual server in client/server architectures. In this context, providers typically publish service interfaces on index services which provide white/yellow pages functionalities.

REACTION applies a novel approach to large-scale SOA and envisions arbitrary pairs of peer application entities communicating and providing services directly to and with each other. In a peer-to-peer SOA, none of the participant hosts is required to be always on; moreover, a participating host may change its IP address each time it comes on. The design and implementation of the SOAP API is being carried out with the purpose to enable peer-to-peer sharing of XML-based Web Services that are programming language independent and can be developed in C# or Java.