Spring Cloud：负载均衡 - Spring Cloud Loadbalancer原理

Spring Cloud 2020版本以后，默认移除了对Netflix的依赖，其中就包括Ribbon，官方默认推荐使用Spring Cloud Loadbalancer正式替换Ribbon，并成为了Spring Cloud负载均衡器的唯一实现。

今天我们深入分析一下Spring Cloud Loadbalancer的具体实现：

使用

1、公共依赖Spring Cloud，例如版本2020.0.2

        org.springframework.cloud
        spring-cloud-dependencies
        2020.0.2
        pom
        import

注意：

如果是Hoxton之前的版本，默认负载均衡器为Ribbon，需要移除Ribbon引用和增加配置spring.cloud.loadbalancer.ribbon.enabled: false

2、引入loadbalancer依赖

    org.springframework.cloud
    spring-cloud-starter-loadbalancer





    com.alibaba.cloud
    spring-cloud-starter-alibaba-nacos-discovery

注意：

Nacos使用请参考官网：https://nacos.io/zh-cn/index.html

3、使用RestTemplate实现Demo

引入web依赖：

    
    org.springframework.boot
    spring-boot-starter-web

编写DemoController：

@RestController
public class TestController {
    @Autowired
    private RestTemplate restTemplate;

    // 新增restTemplate对象注入方法，注意，此处LoadBalanced注解一定要加上，否则无法远程调用
    @Bean
    @LoadBalanced
    public RestTemplate restTemplate() {
        return new RestTemplate();
    }

    @GetMapping("/load")
    public String load() {
        return restTemplate.getForObject("http://demo-server/hello/", String.class);
    }

    @GetMapping(value = "/hello")
    public String hello() {
        return "Hello World";
    }
}

4、启动Nacos，调用接口http://localhost:8080/load

原理

上面是RestTemplate负载均衡的简单实现，除此之外，Spring Cloud LoadBalancer还支持Spring Web Flux响应式编程，这里我们不展开，两者的实现原理思想相同，都是通过客户端添加拦截器，在拦截器中实现负载均衡。

1、#RestTemplate，提供了一个方法setInterceptors，用于设置拦截器，拦截器需要实现ClientHttpRequestInterceptor接口即可，在实际远程去请求服务端接口之前会先调用拦截器的intercept方法逻辑。这里的拦截器相当于Servlet技术中的Filter功能。

// 代码实现在抽象父类InterceptingHttpAccessor里
// RestTemplate.InterceptingHttpAccessor#setInterceptors
public void setInterceptors(List interceptors) {
   // Take getInterceptors() List as-is when passed in here
   if (this.interceptors != interceptors) {
      this.interceptors.clear();
      this.interceptors.addAll(interceptors);
      AnnotationAwareOrderComparator.sort(this.interceptors);
   }
}

2、#LoadBalancerAutoConfiguration，由于@LoadBalanced注解由spring-cloud-commons实现，查看实现逻辑我们发现spring-cloud-commons存在自动配置类LoadBalancerAutoConfiguration，当满足条件时，将自动创建LoadBalancerInterceptor并注入到RestTemplate中。

    @Configuration(
        proxyBeanMethods = false
    )
    @Conditional({LoadBalancerAutoConfiguration.RetryMissingOrDisabledCondition.class})
    static class LoadBalancerInterceptorConfig {
        LoadBalancerInterceptorConfig() {
        }

        @Bean
        public LoadBalancerInterceptor loadBalancerInterceptor(LoadBalancerClient loadBalancerClient, LoadBalancerRequestFactory requestFactory) {
            return new LoadBalancerInterceptor(loadBalancerClient, requestFactory);
        }

        @Bean
        @ConditionalOnMissingBean
        public RestTemplateCustomizer restTemplateCustomizer(final LoadBalancerInterceptor loadBalancerInterceptor) {
            return (restTemplate) -> {
                List list = new ArrayList(restTemplate.getInterceptors());
                list.add(loadBalancerInterceptor);
                restTemplate.setInterceptors(list);
            };
        }
    }

3、#LoadRalancerLnterceptor，LoadBalancerInterceptor实现了ClientHttpRequestInterceptor接口，实现intercept方法，用于实现负载均衡的拦截处理。

public class LoadBalancerInterceptor implements ClientHttpRequestInterceptor {
    private LoadBalancerClient loadBalancer;
    private LoadBalancerRequestFactory requestFactory;

    public LoadBalancerInterceptor(LoadBalancerClient loadBalancer, LoadBalancerRequestFactory requestFactory) {
        this.loadBalancer = loadBalancer;
        this.requestFactory = requestFactory;
    }

    public LoadBalancerInterceptor(LoadBalancerClient loadBalancer) {
        this(loadBalancer, new LoadBalancerRequestFactory(loadBalancer));
    }

    public ClientHttpResponse intercept(final HttpRequest request, final byte[] body, final ClientHttpRequestExecution execution) throws IOException {
        URI originalUri = request.getURI();
        String serviceName = originalUri.getHost();
        Assert.state(serviceName != null, "Request URI does not contain a valid hostname: " + originalUri);
        return (ClientHttpResponse)this.loadBalancer.execute(serviceName, this.requestFactory.createRequest(request, body, execution));
    }
}

4、#LoadBalancerClient，负载均衡客户端，用于进行负载均衡逻辑，从服务列表中选择出一个服务地址进行调用。Spring Cloud LoadBalancer的默认实现为BlockingLoadBalancerClient，

    public  T execute(String serviceId, LoadBalancerRequest request) throws IOException {
        String hint = this.getHint(serviceId);
        LoadBalancerRequestAdapter lbRequest = new LoadBalancerRequestAdapter(request, new DefaultRequestContext(request, hint));
        Set supportedLifecycleProcessors = this.getSupportedLifecycleProcessors(serviceId);
        supportedLifecycleProcessors.forEach((lifecycle) -> {
            lifecycle.onStart(lbRequest);
        });
      //选择服务
        ServiceInstance serviceInstance = this.choose(serviceId, lbRequest);
        if (serviceInstance == null) {
            supportedLifecycleProcessors.forEach((lifecycle) -> {
                lifecycle.onComplete(new CompletionContext(Status.DISCARD, lbRequest, new EmptyResponse()));
            });
            throw new IllegalStateException("No instances available for " + serviceId);
        } else {
            return this.execute(serviceId, serviceInstance, lbRequest);
        }
    }

    public  T execute(String serviceId, ServiceInstance serviceInstance, LoadBalancerRequest request) throws IOException {
        DefaultResponse defaultResponse = new DefaultResponse(serviceInstance);
        Set supportedLifecycleProcessors = this.getSupportedLifecycleProcessors(serviceId);
        Request lbRequest = request instanceof Request ? (Request)request : new DefaultRequest();
        supportedLifecycleProcessors.forEach((lifecycle) -> {
            lifecycle.onStartRequest(lbRequest, new DefaultResponse(serviceInstance));
        });

        try {
            T response = request.apply(serviceInstance);
            Object clientResponse = this.getClientResponse(response);
            supportedLifecycleProcessors.forEach((lifecycle) -> {
                lifecycle.onComplete(new CompletionContext(Status.SUCCESS, lbRequest, defaultResponse, clientResponse));
            });
            return response;
        } catch (IOException var9) {
            supportedLifecycleProcessors.forEach((lifecycle) -> {
                lifecycle.onComplete(new CompletionContext(Status.FAILED, var9, lbRequest, defaultResponse));
            });
            throw var9;
        } catch (Exception var10) {
            supportedLifecycleProcessors.forEach((lifecycle) -> {
                lifecycle.onComplete(new CompletionContext(Status.FAILED, var10, lbRequest, defaultResponse));
            });
            ReflectionUtils.rethrowRuntimeException(var10);
            return null;
        }
    }

....  

    public ServiceInstance choose(String serviceId) {
        return this.choose(serviceId, ReactiveLoadBalancer.REQUEST);
    }

 //通过不同的负载均衡客户端实现选择不同的服务
    public  ServiceInstance choose(String serviceId, Request request) {
        ReactiveLoadBalancer loadBalancer = this.loadBalancerClientFactory.getInstance(serviceId);
        if (loadBalancer == null) {
            return null;
        } else {
            Response loadBalancerResponse = (Response)Mono.from(loadBalancer.choose(request)).block();
            return loadBalancerResponse == null ? null : (ServiceInstance)loadBalancerResponse.getServer();
        }
    }

5、#LoadBalancerClientFactory，BlockingLoadBalancerClient中持有LoadBalancerClientFactory通过调用其getInstance方法获取具体的负载均衡客户端。客户端实现了不同的负载均衡算法，比如轮询、随机等。LoadBalancerClientFactory继承了NamedContextFactory，NamedContextFactory继承ApplicationContextAware，实现Spring ApplicationContext容器操作。

public class LoadBalancerClientFactory extends NamedContextFactory implements Factory {
    public static final String NAMESPACE = "loadbalancer";
    public static final String PROPERTY_NAME = "loadbalancer.client.name";

    public LoadBalancerClientFactory() {
        super(LoadBalancerClientConfiguration.class, "loadbalancer", "loadbalancer.client.name");
    }

    public String getName(Environment environment) {
        return environment.getProperty("loadbalancer.client.name");
    }

    public ReactiveLoadBalancer getInstance(String serviceId) {
        return (ReactiveLoadBalancer)this.getInstance(serviceId, ReactorServiceInstanceLoadBalancer.class);
    }
}

在spring-cloud-loadbalabcer中的LoadBalancerAutoConfiguration实现了LoadBalancerClientFactory缺省值：

    @ConditionalOnMissingBean
    @Bean
    public LoadBalancerClientFactory loadBalancerClientFactory() {
        LoadBalancerClientFactory clientFactory = new LoadBalancerClientFactory();
        clientFactory.setConfigurations((List)this.configurations.getIfAvailable(Collections::emptyList));
        return clientFactory;
    }

6、#ReactiveLoadBalancer，负载均衡器，实现服务选择。Spring Cloud Balancer中实现了轮询RoundRobinLoadBalancer和随机数RandomLoadBalancer两种负载均衡算法。

如果没有显式指定负载均衡算法，默认缺省值为RoundRobinLoadBalancer。

LoadBalancerClientConfiguration#LoadBalancerClientConfiguration

    @Bean
    @ConditionalOnMissingBean
    public ReactorLoadBalancer reactorServiceInstanceLoadBalancer(Environment environment, LoadBalancerClientFactory loadBalancerClientFactory) {
        String name = environment.getProperty("loadbalancer.client.name");
        return new RoundRobinLoadBalancer(loadBalancerClientFactory.getLazyProvider(name, ServiceInstanceListSupplier.class), name);
    }

7、#LoadBalancerRequestFactory，LoadBalancerRequest工厂类，用于创建LoadBalancerRequest，调用createRequest方法。在内部持有LoadBalancerClient属性对象，即BlockingLoadBalancerClient。

public class LoadBalancerRequestFactory {
    private LoadBalancerClient 
;
    private List transformers;

    public LoadBalancerRequestFactory(LoadBalancerClient loadBalancer, List transformers) {
        this.loadBalancer = loadBalancer;
        this.transformers = transformers;
    }

    public LoadBalancerRequestFactory(LoadBalancerClient loadBalancer) {
        this.loadBalancer = loadBalancer;
    }

    public LoadBalancerRequest createRequest(final HttpRequest request, final byte[] body, final ClientHttpRequestExecution execution) {
        return (instance) -> {
            HttpRequest serviceRequest = new ServiceRequestWrapper(request, instance, this.loadBalancer);
            LoadBalancerRequestTransformer transformer;
            if (this.transformers != null) {
                for(Iterator var6 = this.transformers.iterator(); var6.hasNext(); serviceRequest = transformer.transformRequest((HttpRequest)serviceRequest, instance)) {
                    transformer = (LoadBalancerRequestTransformer)var6.next();
                }
            }

            return execution.execute((HttpRequest)serviceRequest, body);
        };
    }
}

 

整合Feign

在日常项目中，一般负载均衡都是结合Feign使用，下面我们讨论下结合Fegin的使用情况

1、引入依赖

        
        
            org.springframework.cloud
            spring-cloud-starter-openfeign
        

       

2、定义Feign接口

@FeignClient(name = "my-service")
public interface RemoteLogService {

    @PostMapping("/sys/log")
    R saveLog(@RequestBody SysLog sysLog);
}

3、调用Feign接口调试（正常负载均衡已经可以使用，无需做其他配置）

4、原理分析

下图是Feign的实现原理，详情可参考博文：Feign原理 （图解）

4.1、查看Feign的loadbalancer的自动配置：FeignLoadBalancerAutoConfiguration，存在

LoadBalancerClient和LoadBalancerClientFactory的bean时，配置生效，默认使用DefaultFeignLoadBalancerConfiguration。请注意，如果引用了OkHttp或HttpClient，将使用不同的configuration文件。

@ConditionalOnClass({Feign.class})
@ConditionalOnBean({LoadBalancerClient.class, LoadBalancerClientFactory.class})
@AutoConfigureBefore({FeignAutoConfiguration.class})
@AutoConfigureAfter({BlockingLoadBalancerClientAutoConfiguration.class, LoadBalancerAutoConfiguration.class})
@EnableConfigurationProperties({FeignHttpClientProperties.class})
@Configuration(
    proxyBeanMethods = false
)
@Import({HttpClientFeignLoadBalancerConfiguration.class, OkHttpFeignLoadBalancerConfiguration.class, HttpClient5FeignLoadBalancerConfiguration.class, DefaultFeignLoadBalancerConfiguration.class})
public class FeignLoadBalancerAutoConfiguration {
    public FeignLoadBalancerAutoConfiguration() {
    }
}

4.2、#DefaultFeignLoadBalancerConfiguration，将缺省创建FeignBlockingLoadBalancerClient并注入LoadBalancerClient和LoadBalancerClientFactory，这两个bean的创建请参考上文。

@Configuration(
    proxyBeanMethods = false
)
@EnableConfigurationProperties({LoadBalancerProperties.class})
class DefaultFeignLoadBalancerConfiguration {
    DefaultFeignLoadBalancerConfiguration() {
    }

    @Bean
    @ConditionalOnMissingBean
    @Conditional({OnRetryNotEnabledCondition.class})
    public Client feignClient(LoadBalancerClient loadBalancerClient, LoadBalancerProperties properties, LoadBalancerClientFactory loadBalancerClientFactory) {
        return new FeignBlockingLoadBalancerClient(new Default((SSLSocketFactory)null, (HostnameVerifier)null), loadBalancerClient, properties, loadBalancerClientFactory);
    }
	
	...
}

4.3、#FeignBlockingLoadBalancerClient，实现excute方法，实现Feign具体请求操作，通过loadBalancerClient.choose获取实例并执行请求，具体选择逻辑和RestTemplate一致。

public class FeignBlockingLoadBalancerClient implements Client {

    private final Client delegate;
    private final LoadBalancerClient loadBalancerClient;
    private final LoadBalancerProperties properties;
    private final LoadBalancerClientFactory loadBalancerClientFactory;

    public Response execute(Request request, Options options) throws IOException {
        URI originalUri = URI.create(request.url());
        String serviceId = originalUri.getHost();
        Assert.state(serviceId != null, "Request URI does not contain a valid hostname: " + originalUri);
        String hint = this.getHint(serviceId);
        DefaultRequest lbRequest = new DefaultRequest(new RequestDataContext(LoadBalancerUtils.buildRequestData(request), hint));
        Set supportedLifecycleProcessors = LoadBalancerLifecycleValidator.getSupportedLifecycleProcessors(this.loadBalancerClientFactory.getInstances(serviceId, LoadBalancerLifecycle.class), RequestDataContext.class, ResponseData.class, ServiceInstance.class);
        supportedLifecycleProcessors.forEach((lifecycle) -> {
            lifecycle.onStart(lbRequest);
        });
        ServiceInstance instance = this.loadBalancerClient.choose(serviceId, lbRequest);
        org.springframework.cloud.client.loadbalancer.Response lbResponse = new DefaultResponse(instance);
        String message;
        if (instance == null) {
            message = "Load balancer does not contain an instance for the service " + serviceId;
            if (LOG.isWarnEnabled()) {
                LOG.warn(message);
            }

            supportedLifecycleProcessors.forEach((lifecycle) -> {
                lifecycle.onComplete(new CompletionContext(Status.DISCARD, lbRequest, lbResponse));
            });
            return Response.builder().request(request).status(HttpStatus.SERVICE_UNAVAILABLE.value()).body(message, StandardCharsets.UTF_8).build();
        } else {
            message = this.loadBalancerClient.reconstructURI(instance, originalUri).toString();
            Request newRequest = this.buildRequest(request, message);
            return LoadBalancerUtils.executeWithLoadBalancerLifecycleProcessing(this.delegate, options, newRequest, lbRequest, lbResponse, supportedLifecycleProcessors);
        }
    }

}