Summary: We outline ensemble, a design and implementation methodology for composing Message Passing (MP) applications from program components directed by scripts. We define specification components corresponding to program components and we compose them, directed by the same scripts, obtaining formal specifications of the composed applications. We use the Petri net formalism to express component and application specifications. Petri net composition is modeled by appropriate coupling of inscriptions on interface input and output arcs to and from a unified environment place. We elaborate on testing and debugging of MP applications based on the synergy of tools for Petri-net simulations with tools for monitoring program executions.