Two safety patterns

Safety is the avoidance of unacceptable hazards, including threats to human lives, the environment, or to costly facilities. Safety constraints are expressed using assertions that define system states that should not occur because they may lead to mishaps. We present here two safety patterns. The Safety Assertion pattern describes the contents of an assertion that indicates a state of the system that must not happen. The Safety Assertion Enforcer pattern evaluates safety assertions when there is an incoming event that can change the state of the system and prevents the change if it violates an assertion. These patterns can be useful for designing or certifying safe systems.