I'd also add: 1. Formal specifications. Complex distributed algorithms are too difficult to understand without exhaustively checking a design for all possible

I'd also add: 1. Formal specifications. Complex distributed algorithms are too difficult to understand without exhaustively checking a design for all possible executions that end in dead-locks or violation of some invariant. Clouds require sometimes complex orchestration between various services. Unit tests aren't going to cover you if your design isn't correct (they'll ensure that some small portion of your system is correct and scale has a funny way of revealing everything you didn't test for).