DE4A (Digital Europe for All) is a project funded by the European Commission that has developed a multi-pattern architecture for eGovernment interoperability with a focus on digital-by-default procedures for citizens and businesses and the full implementation of the Once-Only Principle. It provides guidance to the DE4A pilots and to projects seeking to implement cross-border once-only enabled eProcedures.

DE4A has produced a publicly available toolbox  in the form of an online wiki which contains relevant documentation such as interaction patterns, common components or pilots.

The activities of the project have covered three pilots that fall within the scope of the SDG online procedures: studying abroad, doing business abroad and moving abroad.