private static OptionalInt defaultRackAwareTrafficCost(final OptionalInt rackAwareTrafficCost) {
        if (rackAwareTrafficCost == null) {
            return OptionalInt.empty();
        }
        return rackAwareTrafficCost;
    }
The rack-aware assignment strategy as configured via
{@link StreamsConfig#RACK_AWARE_ASSIGNMENT_STRATEGY_CONFIG}
streams/src/main/java/org/apache/kafka/streams/processor/assignment/AssignmentConfigs.java