Ribbon原理分析

Ribbon是使用拦截器来实现服务的远程调用的，源码如下：

public class LoadBalancerAutoConfiguration {
    

	@LoadBalanced
	@Autowired(required = false)
	private List<RestTemplate> restTemplates = Collections.emptyList();

	@Autowired(required = false)
	private List<LoadBalancerRequestTransformer> transformers = Collections.emptyList();

	@Bean
	// 循环全部增强类去给restTemplate增强，也是在这里给它添加拦截器的
	public SmartInitializingSingleton loadBalancedRestTemplateInitializerDeprecated(
			final ObjectProvider<List<RestTemplateCustomizer>> restTemplateCustomizers) {
    
		return () -> restTemplateCustomizers.ifAvailable(customizers -> {
    
			for (RestTemplate restTemplate : LoadBalancerAutoConfiguration.this.restTemplates) {
    
				for (RestTemplateCustomizer customizer : customizers) {
    
					customizer.customize(restTemplate);
				}
			}
		});
	}

	@Bean
	@ConditionalOnMissingBean
	public LoadBalancerRequestFactory loadBalancerRequestFactory(
			LoadBalancerClient loadBalancerClient) {
    
		return new LoadBalancerRequestFactory(loadBalancerClient, this.transformers);
	}

	@Configuration(proxyBeanMethods = false)
	@ConditionalOnMissingClass("org.springframework.retry.support.RetryTemplate")
	static class LoadBalancerInterceptorConfig {
    

		@Bean
		// 注册负载均衡拦截器，LoadBalancerClient 是执行正在服务在均衡的功能类
		public LoadBalancerInterceptor ribbonInterceptor(
				LoadBalancerClient loadBalancerClient,
				LoadBalancerRequestFactory requestFactory) {
    
			return new LoadBalancerInterceptor(loadBalancerClient, requestFactory);
		}

		@Bean
		@ConditionalOnMissingBean
		// 注册RestTemplate的增强类
		public RestTemplateCustomizer restTemplateCustomizer(
				final LoadBalancerInterceptor loadBalancerInterceptor) {
    
			return restTemplate -> {
    
				List<ClientHttpRequestInterceptor> list = new ArrayList<>(
						restTemplate.getInterceptors());
				// 添加拦截器
				list.add(loadBalancerInterceptor);
				restTemplate.setInterceptors(list);
			};
		}

	}
  // 略......

进入LoadBalancerInterceptor的intercept可以看到，它是使用LoadBalancerClient去执行真正的流程的，LoadBalancerClient是负责调用流程的，具体的执行则是由ILoadBalancer负载均衡器来执行的

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

接着进入RibbonLoadBalancerClient的execute方法查看。

	public <T> T execute(String serviceId, LoadBalancerRequest<T> request, Object hint)
			throws IOException {
    
		// 1、从容器中获取负载均衡器
		ILoadBalancer loadBalancer = getLoadBalancer(serviceId);
		// 2、使用ILoadBalancer获取一个服务
		Server server = getServer(loadBalancer, hint);
		if (server == null) {
    
			throw new IllegalStateException("No instances available for " + serviceId);
		}
		// 组装成一个服务实例信息
		RibbonServer ribbonServer = new RibbonServer(serviceId, server,
				isSecure(server, serviceId),
				serverIntrospector(serviceId).getMetadata(server));
		// 3、调用这个服务
		return execute(serviceId, ribbonServer, request);
	}

服务的发现与负载均衡都发生在这3步中，我们着重分析这3步都干了什么

1、getLoadBalancer

从容器中获取负载均衡器，在RibbonClientConfiguration给容器注册了ZoneAwareLoadBalancer，所以实际获取的就是它。

接着看一个ZoneAwareLoadBalancer的构建过程做了什么，点进去会来到它的父类的DynamicServerListLoadBalancer构造方法，源码如下

 public DynamicServerListLoadBalancer(IClientConfig clientConfig, IRule rule, IPing ping,
                                         ServerList<T> serverList, ServerListFilter<T> filter,
                                         ServerListUpdater serverListUpdater) {
    
        // 调用上级构造，主要初始化一些配置信息
        super(clientConfig, rule, ping);
        // serverList服务列表，用于发现服务的
        this.serverListImpl = serverList;
        // 服务的过滤器
        this.filter = filter;
        // 服务列表的更新器
        this.serverListUpdater = serverListUpdater;
        if (filter instanceof AbstractServerListFilter) {
    
            ((AbstractServerListFilter) filter).setLoadBalancerStats(getLoadBalancerStats());
        }
        // 初始化，拉取服务和开启服务更新
        restOfInit(clientConfig);
    }

restOfInit方法源码

    void restOfInit(IClientConfig clientConfig) {
    
        boolean primeConnection = this.isEnablePrimingConnections();
        // turn this off to avoid duplicated asynchronous priming done in BaseLoadBalancer.setServerList()
        this.setEnablePrimingConnections(false);
        // 开启服务列表的监听
        enableAndInitLearnNewServersFeature();
		// 拉取服务
        updateListOfServers();
        if (primeConnection && this.getPrimeConnections() != null) {
    
            this.getPrimeConnections()
                    .primeConnections(getReachableServers());
        }
        this.setEnablePrimingConnections(primeConnection);
        LOGGER.info("DynamicServerListLoadBalancer for client {} initialized: {}", clientConfig.getClientName(), this.toString());
    }

enableAndInitLearnNewServersFeature是开启一个定时任务拉取服务，调用的就是updateListOfServers，主要看updateListOfServers是如何拉取服务的，方法源码如下：

    public void updateListOfServers() {
    
        List<T> servers = new ArrayList<T>();
        if (serverListImpl != null) {
    
        	// 调用服务列表服务拉取并更新服务
            servers = serverListImpl.getUpdatedListOfServers();
            LOGGER.debug("List of Servers for {} obtained from Discovery client: {}",
                    getIdentifier(), servers);

            if (filter != null) {
    
                servers = filter.getFilteredListOfServers(servers);
                LOGGER.debug("Filtered List of Servers for {} obtained from Discovery client: {}",
                        getIdentifier(), servers);
            }
        }
        // 更新内存的服务列表信息
        updateAllServerList(servers);
    }

serverListImpl是负载均衡的规范接口，用于获取服务的，Nacos也是实现的这个接口给Ribbon获取的服务列表的

public interface ServerList<T extends Server> {
    

    public List<T> getInitialListOfServers();
    
    /** * Return updated list of servers. This is called say every 30 secs * (configurable) by the Loadbalancer's Ping cycle * */
    public List<T> getUpdatedListOfServers();   

}

NacosServerList的部分源码：

	@Override
	public List<NacosServer> getUpdatedListOfServers() {
    
		return getServers();
	}

	private List<NacosServer> getServers() {
    
		try {
    
			String group = discoveryProperties.getGroup();
			// 使用Nacos的NameingService发起Api调用获取服务的实例列表
			List<Instance> instances = discoveryProperties.namingServiceInstance()
					.selectInstances(serviceId, group, true);
			return instancesToServerList(instances);
		}
		catch (Exception e) {
    
			throw new IllegalStateException(
					"Can not get service instances from nacos, serviceId=" + serviceId,
					e);
		}
	}

到这也就知道服务的发现是怎样的了，剩下的就是根据负载策略选择一个服务去调用了。接着看下一步getServer

2、getServer

重新回到RibbonLoadBalancerClient的第二步getServer，这一步是使用负载均衡器选择一个服务：

protected Server getServer(ILoadBalancer loadBalancer, Object hint) {
    
		if (loadBalancer == null) {
    
			return null;
		}
		// Use 'default' on a null hint, or just pass it on?
		return loadBalancer.chooseServer(hint != null ? hint : "default");
	}

因为默认的话是不会使用zone的所以会直接进入到BaseLoadBalancer的chooseServer方法，源码如下：

    public Server chooseServer(Object key) {
    
    	// 创建计数器或者+1操作
        if (counter == null) {
    
            counter = createCounter();
        }
        counter.increment();
        if (rule == null) {
    
            return null;
        } else {
    
            try {
    
            	// 使用负载策略选择一个服务
                return rule.choose(key);
            } catch (Exception e) {
    
                logger.warn("LoadBalancer [{}]: Error choosing server for key {}", name, key, e);
                return null;
            }
        }
    }

这一步使用负载策略选择一个服务服务了，至于Rule策略，我会在下一步文章分析，接着看下一步

3、execute

这一步就是调用服务了

	public <T> T execute(String serviceId, ServiceInstance serviceInstance,
			LoadBalancerRequest<T> request) throws IOException {
    
		Server server = null;
		if (serviceInstance instanceof RibbonServer) {
    
			server = ((RibbonServer) serviceInstance).getServer();
		}
		if (server == null) {
    
			throw new IllegalStateException("No instances available for " + serviceId);
		}

		RibbonLoadBalancerContext context = this.clientFactory
				.getLoadBalancerContext(serviceId);
		RibbonStatsRecorder statsRecorder = new RibbonStatsRecorder(context, server);

		try {
    
			// 发送组装返回结果
			T returnVal = request.apply(serviceInstance);
			statsRecorder.recordStats(returnVal);
			return returnVal;
		}
		// catch IOException and rethrow so RestTemplate behaves correctly
		catch (IOException ex) {
    
			statsRecorder.recordStats(ex);
			throw ex;
		}
		catch (Exception ex) {
    
			statsRecorder.recordStats(ex);
			ReflectionUtils.rethrowRuntimeException(ex);
		}
		return null;
	}

Ribbon的负载与Nacos的服务发现流程到这就基本分析完了！！