Microservices Engine:Instance weight changes not taking effect on MSE Nacos

Spring Cloud Alibaba

By default, Spring Cloud Alibaba treats Nacos weights as binary: zero means no traffic, non-zero means equal traffic. To enable weighted load balancing, activate the Nacos-aware load balancer.

Add the following to application.properties or application.yml:

spring.cloud.loadbalancer.nacos.enabled=true

If you use Spring Cloud LoadBalancer directly, enable the WeightedServiceInstanceListSupplier:

spring.cloud.loadbalancer.configurations=weighted

Or configure programmatically:

public class CustomLoadBalancerConfiguration {
    @Bean
    public ServiceInstanceListSupplier discoveryClientServiceInstanceListSupplier(
            ConfigurableApplicationContext context) {
        return ServiceInstanceListSupplier.builder()
                    .withDiscoveryClient()
                    .withCaching()
                    .withWeighted()
                    .build(context);
    }
}

Dubbo

Dubbo has its own load balancing configuration and does not read Nacos instance weights by default. Configure weight-based load balancing at the global, interface, or method level.

Global configuration (application.yml):

dubbo:
  consumer:
    loadbalance: roundrobin

Interface-level configuration (provider side):

@DubboService(loadbalance = "roundrobin")
public class DemoServiceImpl implements DemoService {}

Interface-level configuration (consumer side, takes priority):

@DubboReference(loadbalance = "roundrobin")
private DemoService demoService;

Dubbo supports the following load balancing algorithms:

Custom Nacos client implementations

If your application calls the Nacos API directly, confirm that your addressing logic reads the weight field.

Option 1: Use selectOneHealthyInstance (recommended)

The Nacos client selectOneHealthyInstance method performs weighted random selection based on instance weights:

NamingService namingService = NacosFactory.createNamingService(properties);
// Returns a single instance selected by weighted random
Instance instance = namingService.selectOneHealthyInstance("your-service-name");

Option 2: Implement custom weighted selection

If you retrieve the full instance list, apply weight-based selection manually:

NamingService namingService = NacosFactory.createNamingService(properties);
List<Instance> instances = namingService.getAllInstances("your-service-name");

// Filter healthy instances and apply weighted random selection
List<Instance> healthy = instances.stream()
    .filter(Instance::isHealthy)
    .filter(i -> i.getWeight() > 0)
    .collect(Collectors.toList());

// Calculate total weight
double totalWeight = healthy.stream()
    .mapToDouble(Instance::getWeight)
    .sum();

// Weighted random selection
double random = ThreadLocalRandom.current().nextDouble() * totalWeight;
double cumulative = 0;
for (Instance inst : healthy) {
    cumulative += inst.getWeight();
    if (random < cumulative) {
        // Use this instance
        break;
    }
}

For the full API reference, see Nacos Java SDK.