Ultimately the behavior that matters is the effective behaviour of the system. But a system is built by many components. You can build a reliable system out o

Ultimately the behavior that matters is the effective behaviour of the system. But a system is built by many components. You can build a reliable system out of unreliable components. You can build a transactional system out of non-transactional components. However you pay a price for that, because you have to bear the consequences of this substrate in other layers, often in your business logic too.