public OptionalDouble allowedDownRatio() {
        return 0.0 <= allowedDownRatio && allowedDownRatio <= 1.0 ?
               OptionalDouble.of(allowedDownRatio) :
               OptionalDouble.empty();
    }
The ratio of services that are allowed to be down.
orchestrator/src/main/java/com/yahoo/vespa/orchestrator/policy/ClusterParams.java