深入理解@LoadBalanced注解的实现原理与客户端负载均衡
- 2019 年 11 月 14 日
- 筆記
前提
在阅读这篇博客之前,希望你对SpringCloud套件熟悉和理解,更希望关注下微服务开发平台
概述
在使用springcloud ribbon客户端负载均衡的时候，可以给RestTemplate bean 加一个@LoadBalanced注解，就能让这个RestTemplate在请求时拥有客户端负载均衡的能力,先前有细嚼过但是没有做过笔记,刚好处理此类问题记录下
@LoadBalanced
/** * 注释将RestTemplate bean标记为配置为使用LoadBalancerClient。 */ @Target({ ElementType.FIELD, ElementType.PARAMETER, ElementType.METHOD }) @Retention(RetentionPolicy.RUNTIME) @Documented @Inherited @Qualifier public @interface LoadBalanced { }通过源码可以发现这是一个LoadBalanced标记注解并且标记了@Qualifier(基于Spring Boot的自动配置机制),我们可以溯源到LoadBalancerAutoConfiguration
LoadBalancerAutoConfiguration
/** * 功能区的自动配置（客户端负载平衡） */ @Configuration @ConditionalOnClass(RestTemplate.class) @ConditionalOnBean(LoadBalancerClient.class) @EnableConfigurationProperties(LoadBalancerRetryProperties.class) public class LoadBalancerAutoConfiguration { @LoadBalanced @Autowired(required = false) private List<RestTemplate> restTemplates = Collections.emptyList(); //这里持有@LoadBalanced标记的RestTemplate实例 @Autowired(required = false) private List<LoadBalancerRequestTransformer> transformers = Collections.emptyList(); @Bean public SmartInitializingSingleton loadBalancedRestTemplateInitializerDeprecated( final ObjectProvider<List<RestTemplateCustomizer>> restTemplateCustomizers) { return () -> restTemplateCustomizers.ifAvailable(customizers -> { for (RestTemplate restTemplate : LoadBalancerAutoConfiguration.this.restTemplates) { for (RestTemplateCustomizer customizer : customizers) { //为restTemplate添加定制 customizer.customize(restTemplate); } } }); } // ... /** * 以下针对classpath存在RetryTemplate.class的情况配置，先忽略 */ @Configuration @ConditionalOnClass(RetryTemplate.class) public static class RetryAutoConfiguration { @Bean @ConditionalOnMissingBean public LoadBalancedRetryFactory loadBalancedRetryFactory() { return new LoadBalancedRetryFactory() { }; } } // ... }@LoadBalanced和@Autowried结合使用,意思就是这里注入的RestTempate Bean是所有加有@LoadBalanced注解标记的(持有@LoadBalanced标记的RestTemplate实例)
这段自动装配的代码的含义不难理解，就是利用了RestTempllate的拦截器，使用RestTemplateCustomizer对所有标注了@LoadBalanced的RestTemplate Bean添加了一个LoadBalancerInterceptor拦截器，而这个拦截器的作用就是对请求的URI进行转换获取到具体应该请求哪个服务实例ServiceInstance。
关键问下自己：为什么？
- RestTemplate实例是怎么被收集的？
- 怎样通过负载均衡规则获取具体的具体的server？
继续扒看源码>
上面可以看出，会LoadBalancerAutoConfiguration类对我们加上@LoadBalanced注解的bean 添加loadBalancerInterceptor拦截器
LoadBalancerInterceptor
/** * 功能区的自动配置（客户端负载平衡）。 */ 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) { // for backwards compatibility this(loadBalancer, new LoadBalancerRequestFactory(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(); Assert.state(serviceName != null, "Request URI does not contain a valid hostname: " + originalUri); return this.loadBalancer.execute(serviceName, this.requestFactory.createRequest(request, body, execution)); } }重点看intercept方法 当我们restTemplate执行请求操作时，就会被拦截器拦截进入intercept方法,而loadBalancer是LoadBalancerClient的具体实现
RibbonLoadBalancerClient
public <T> T execute(String serviceId, LoadBalancerRequest<T> request, Object hint) throws IOException { ILoadBalancer loadBalancer = getLoadBalancer(serviceId); Server server = getServer(loadBalancer, hint); 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); }看到这里相信都遇到过类似的错误，恍然大悟
No instances available for xxxxx总结
- 1.根据serviceId 获取对应的loadBalancer
- 2.根据loadBalancer获取具体的server（这里根据负载均衡规则，获取到具体的服务实例）
- 3.创建RibbonServer
- 4.执行具体请求
这里
注意: @LoadBalanced 标记注解获取到最后通过负载均衡规则获取具体的具体的server来发起请求
案例
/** * 服务注册中心配置 * * @author <a href="mailto:[email protected]">iByte</a> * @since 1.0.1 */ @Configuration @EnableConfigurationProperties(ModuleMappingHelper.class) public class DiscoveryConfig { @Autowired Environment environment; /** * DiscoveryHeaderHelper默认bean * @return */ @Bean public DiscoveryHeaderHelper discoveryHeaderHelper() { DiscoveryHeaderHelper discoveryHeaderHelper = new DiscoveryHeaderHelper(environment); DiscoveryHeaderHelper.INSTANCE = discoveryHeaderHelper; return discoveryHeaderHelper; } /** * resttemplate构建 */ @Resource private RestTemplateBuilder restTemplateBuilder; /** * resttemplate请求bean,更改系统本身的builder * @return */ @Bean @LoadBalanced public RestTemplate restTemplate() { RestTemplate restTemplate = restTemplateBuilder.configure(new RestTemplate()); //RestTemplate interceptors 远程调用请求增加头部信息处理 restTemplate.getInterceptors().add(new RestApiHeaderInterceptor()); //RestTemplate Set the error handler 错误处理 restTemplate.setErrorHandler(new RestResponseErrorHandler()); return restTemplate; } @Bean public DiscoveryClient.DiscoveryClientOptionalArgs discoveryClientOptionalArgs() { DiscoveryClient.DiscoveryClientOptionalArgs discoveryClientOptionalArgs = new DiscoveryClient.DiscoveryClientOptionalArgs(); discoveryClientOptionalArgs.setAdditionalFilters(Collections.singletonList(new DiscoveryHeaderClientFilter())); discoveryClientOptionalArgs.setEventListeners(Collections.singleton(new EurekaClientEventListener())); return discoveryClientOptionalArgs; } }源码地址 > DiscoveryConfig
