Ribbon源码解析

SpringCloud中的Ribbon开源项目，提供了客户端的负载均衡算法。这篇文章，我们来介绍下他是如何实现的。为了方便理解，我们以客户端调用的流程来介绍，其中会穿插介绍相关源代码。

简单回顾下Ribbon的使用，这里强调两点：

1、在启动类Application中，添加@LoadBalanced注解。

@Bean
	@LoadBalanced
	RestTemplate restTemplate() {
		return new RestTemplate();
	}

2、结合RestTemplate发起调用，调用时采用服务名称（如：COMPUTE-SERVICE）来实现。

        return restTemplate.getForEntity("http://COMPUTE-SERVICE/add?a=10&b=20", String.class).getBody();

先从拦截器LoadBalancerInterceptor开始介绍：

从请求中获取服务名称，即上文说到的COMPUTE-SERVICE，然后执行LoadBalancerClient的execute方法。

public class LoadBalancerInterceptor implements ClientHttpRequestInterceptor {
 
	private LoadBalancerClient loadBalancer;
 
	public LoadBalancerInterceptor(LoadBalancerClient loadBalancer) {
		this.loadBalancer = loadBalancer;
	}
 
	@Override
	public ClientHttpResponse intercept(final HttpRequest request, final byte[] body,
			final ClientHttpRequestExecution execution) throws IOException {
		final URI originalUri = request.getURI();
		String serviceName = originalUri.getHost();
		return this.loadBalancer.execute(serviceName,
				new LoadBalancerRequest<ClientHttpResponse>() {
					@Override
					public ClientHttpResponse apply(final ServiceInstance instance)
							throws Exception {
						HttpRequest serviceRequest = new ServiceRequestWrapper(request,
								instance, loadBalancer);
						return execution.execute(serviceRequest, body);
					}
 
				});
	}
}

这里，先简单介绍下这几个类。

1）顶层接口

实现该接口的类，会使用一个负载均衡器来选择一个server来转发请求。

public interface ServiceInstanceChooser {
 
    ServiceInstance choose(String serviceId);
}

2）继承接口

LoadBalancerClient

提供了两种不同的参数的execute()执行方法。

public interface LoadBalancerClient extends ServiceInstanceChooser {
 
	<T> T execute(String serviceId, LoadBalancerRequest<T> request) throws IOException;
 
	<T> T execute(String serviceId, ServiceInstance serviceInstance, LoadBalancerRequest<T> request) throws IOException;
 
	URI reconstructURI(ServiceInstance instance, URI original);
}

3）实现类



在RibbonLoadBalancerClient类中，可以看到具体的执行方法。

主要做了一下几件事：

根据serviceId获取负载均衡器；

根据负载均衡器获取server；

将请求转到具体的服务实例。

@Override
	public <T> T execute(String serviceId, LoadBalancerRequest<T> request) throws IOException {
		ILoadBalancer loadBalancer = getLoadBalancer(serviceId);
		Server server = getServer(loadBalancer);
		if (server == null) {
			throw new IllegalStateException("No instances available for " + serviceId);
		}
		RibbonServer ribbonServer = new RibbonServer(serviceId, server, isSecure(server,
				serviceId), serverIntrospector(serviceId).getMetadata(server));
 
		return execute(serviceId, ribbonServer, request);
	}
 
	@Override
	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 {
			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;
	}

其中，ServiceInstance——服务实例接口

	public interface ServiceInstance {
 
		String getServiceId();
                String getHost();
	        int getPort();
	        boolean isSecure();
 
		URI getUri();
 
		Map<String, String> getMetadata();
	}

RibbonServer——服务实例实现类

	protected static class RibbonServer implements ServiceInstance {
			private final String serviceId;
			private final Server server;
			private final boolean secure;
			private Map<String, String> metadata;
 
			protected RibbonServer(String serviceId, Server server) {
				this(serviceId, server, false, Collections.<String, String> emptyMap());
			}
 
			protected RibbonServer(String serviceId, Server server, boolean secure,
					Map<String, String> metadata) {
				this.serviceId = serviceId;
				this.server = server;
				this.secure = secure;
				this.metadata = metadata;
			}
 
		//省去getter和setter
			@Override
			public String toString() {
				final StringBuffer sb = new StringBuffer("RibbonServer{");
				sb.append("serviceId='").append(serviceId).append('\'');
				sb.append(", server=").append(server);
				sb.append(", secure=").append(secure);
				sb.append(", metadata=").append(metadata);
				sb.append('}');
				return sb.toString();
			}
		}

getServer()中，默认使用的ZoneAwareLoadBalancer负载均衡器。

部分代码r

public Server chooseServer(Object key) {		        Server server = null;
		        try {
		            //获取可用区域Zone
		            LoadBalancerStats lbStats = getLoadBalancerStats();
		            Map<String, ZoneSnapshot> zoneSnapshot = ZoneAvoidanceRule.createSnapshot(lbStats);
		                  Set<String> availableZones = ZoneAvoidanceRule.getAvailableZones(zoneSnapshot, triggeringLoad.get(), triggeringBlackoutPercentage.get());
		            logger.debug("Available zones: {}", availableZones);
		            if (availableZones != null &&  availableZones.size() < zoneSnapshot.keySet().size()) {
		                String zone = ZoneAvoidanceRule.randomChooseZone(zoneSnapshot, availableZones);
		                logger.debug("Zone chosen: {}", zone);
		                if (zone != null) {
		                    BaseLoadBalancer zoneLoadBalancer = getLoadBalancer(zone);
		                    server = zoneLoadBalancer.chooseServer(key);
		                }
		            }
		        } catch (Throwable e) {
		            logger.error("Unexpected exception when choosing server using zone aware logic", e);
		        }
		        if (server != null) {
		            return server;
		        }
}

具体的分配算法：

static String randomChooseZone(Map<String, ZoneSnapshot> snapshot,
            Set<String> chooseFrom) {
        if (chooseFrom == null || chooseFrom.size() == 0) {
            return null;
        }
        String selectedZone = chooseFrom.iterator().next();
        if (chooseFrom.size() == 1) {
            return selectedZone;
        }
        int totalServerCount = 0;
        for (String zone : chooseFrom) {
            totalServerCount += snapshot.get(zone).getInstanceCount();
        }
        int index = random.nextInt(totalServerCount) + 1;
        int sum = 0;
        for (String zone : chooseFrom) {
            sum += snapshot.get(zone).getInstanceCount();
            if (index <= sum) {
                selectedZone = zone;
                break;
            }
        }
        return selectedZone;
    }