A system for developing a computer implemented service, for deployment on computing infrastructure, generates variants of the model by automatically choosing values for a limited set of design variables, and evaluates the variants in operation. A model manager (187) stores in a model repository (107) a current variant (57) and at least some previous variants, and their evaluation results and derivation trails, the generating part being arranged to use the evaluation results and the derivation trails to generate a next current variant. Such use of the repository can help make more efficient the search by the model manager for variants that work well. In particular the derivation trails and evaluations can make it easier to determine when to revert to a preceding variant or determine what new design choices to try next.