public boolean isHealthy(String upstreamName) {
        List<Server> upstreamServers = servers.getOrDefault(upstreamName, List.of());
        long upCount = upstreamServers.stream()
                                      .filter(Server::up)
                                      .count();
        return upCount > upstreamServers.size() * requiredUpFraction;
    }
Returns whether given upstream is healthy
routing-generator/src/main/java/com/yahoo/vespa/hosted/routing/status/ServerGroup.java