The objective of this paper are twofold. On the one hand, it sims to show the advantages of Co-ordinated Atomic actions (CA actions) as a design concept for dependable distributed system development, and on the other hand, it explains how the formal language CO-OPN/2 can be used to express the semantics of CA action design. A fault-tolerant distributed application is developed according to a simple development life cycle: informal requirements, specification, design, implementation. The design phase is built according to the CA action concept. The CO-OPN/2 language is used to formally express the design phase. The implementation is made in Java based on a library of generic classes implementing the CA action concept. The paper is to seve as a basis for a more general approach aimed at defining CA action semantics.