The Once-Only Principle states that citizens and businesses provide data only once in contact with public administrations. So far, many European countries have started to implement the Once-Only Principle at national level, but its cross-border implementation is still fragmented and limited. This paper presents the development of a Reference Architecture for the Once-Only Principle in Europe. The case study, stemming from the EU-funded Once-Only Principle project (TOOP) highlights the challenges faced by the architecture team when developing the Reference Architecture that tackles the Once-Only Principle across different countries and policy domains. The architecture is not built from scratch, but re-uses and enhances already available building blocks in order to seamlessly preserve interoperability and to comply with regulations and existing technical standards, leaving at the same time enough space for vendors and open source developers to propose their compliant solutions, whatever is their business model.

The Once-Only Principle requires the public administrations to ensure that citizens and businesses supply the same information only once to the Public Administration as a whole. Widespread use of the Once-Only Principle has the potential to simplify citizens’ life, make businesses more efficient, and reduce administrative burden in the European Union. The Once-Only Principle project (TOOP) is an initiative, financed by the EU Program Horizon 2020, to explore the possibility to enable the cross-border application of the Once-Only Principle by demonstrating it in practice, through the development of selected piloting applications for specific real-world use cases, enabling the connection of different registries and architectures in different countries for better exchange of information across public administrations. These piloting ICT systems are designed as a result of a pan-European collaboration and they adopt a federated model, to allow for a high degree of independence between the participating parties in the development of their own solutions. The main challenge in the implementation of an OOP solution is the diversity of organizations, procedures, data, and services on all four main levels of interoperability: legal, organizational, semantic, and technical. To address this challenge, TOOP is developing and testing the TOOP Reference Architecture (TOOPRA) to assist organizations in the cross-border implementation of the OOP. The paper outlines the TOOPRA users, principles, and requirements, presents an overview of the architecture development, describes the main views of TOOPRA, discusses architecture profiling, and analyses the TOOPRA sustainability issues.