深入理解@LoadBalanced註解的實現原理與客戶端負載均衡
- 2019 年 11 月 14 日
- 筆記
前提
在閱讀這篇部落格之前,希望你對SpringCloud套件熟悉和理解,更希望關注下微服務開發平台
概述
在使用springcloud ribbon客戶端負載均衡的時候，可以給RestTemplate bean 加一個@LoadBalanced註解，就能讓這個RestTemplate在請求時擁有客戶端負載均衡的能力,先前有細嚼過但是沒有做過筆記,剛好處理此類問題記錄下
@LoadBalanced
/** * 注釋將RestTemplate bean標記為配置為使用LoadBalancerClient。 */ @Target({ ElementType.FIELD, ElementType.PARAMETER, ElementType.METHOD }) @Retention(RetentionPolicy.RUNTIME) @Documented @Inherited @Qualifier public @interface LoadBalanced { }通過源碼可以發現這是一個LoadBalanced標記註解並且標記了@Qualifier(基於Spring Boot的自動配置機制),我們可以溯源到LoadBalancerAutoConfiguration
LoadBalancerAutoConfiguration
/** * 功能區的自動配置（客戶端負載平衡） */ @Configuration @ConditionalOnClass(RestTemplate.class) @ConditionalOnBean(LoadBalancerClient.class) @EnableConfigurationProperties(LoadBalancerRetryProperties.class) public class LoadBalancerAutoConfiguration { @LoadBalanced @Autowired(required = false) private List<RestTemplate> restTemplates = Collections.emptyList(); //這裡持有@LoadBalanced標記的RestTemplate實例 @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) { //為restTemplate添加訂製 customizer.customize(restTemplate); } } }); } // ... /** * 以下針對classpath存在RetryTemplate.class的情況配置，先忽略 */ @Configuration @ConditionalOnClass(RetryTemplate.class) public static class RetryAutoConfiguration { @Bean @ConditionalOnMissingBean public LoadBalancedRetryFactory loadBalancedRetryFactory() { return new LoadBalancedRetryFactory() { }; } } // ... }@LoadBalanced和@Autowried結合使用,意思就是這裡注入的RestTempate Bean是所有加有@LoadBalanced註解標記的(持有@LoadBalanced標記的RestTemplate實例)
這段自動裝配的程式碼的含義不難理解，就是利用了RestTempllate的攔截器，使用RestTemplateCustomizer對所有標註了@LoadBalanced的RestTemplate Bean添加了一個LoadBalancerInterceptor攔截器，而這個攔截器的作用就是對請求的URI進行轉換獲取到具體應該請求哪個服務實例ServiceInstance。
關鍵問下自己：為什麼？
- RestTemplate實例是怎麼被收集的？
- 怎樣通過負載均衡規則獲取具體的具體的server？
繼續扒看源碼>
上面可以看出，會LoadBalancerAutoConfiguration類對我們加上@LoadBalanced註解的bean 添加loadBalancerInterceptor攔截器
LoadBalancerInterceptor
/** * 功能區的自動配置（客戶端負載平衡）。 */ 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)); } }重點看intercept方法 當我們restTemplate執行請求操作時，就會被攔截器攔截進入intercept方法,而loadBalancer是LoadBalancerClient的具體實現
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); }看到這裡相信都遇到過類似的錯誤，恍然大悟
No instances available for xxxxx總結
- 1.根據serviceId 獲取對應的loadBalancer
- 2.根據loadBalancer獲取具體的server（這裡根據負載均衡規則，獲取到具體的服務實例）
- 3.創建RibbonServer
- 4.執行具體請求
這裡
注意: @LoadBalanced 標記註解獲取到最後通過負載均衡規則獲取具體的具體的server來發起請求
案例
/** * 服務註冊中心配置 * * @author <a href="mailto:[email protected]">iByte</a> * @since 1.0.1 */ @Configuration @EnableConfigurationProperties(ModuleMappingHelper.class) public class DiscoveryConfig { @Autowired Environment environment; /** * DiscoveryHeaderHelper默認bean * @return */ @Bean public DiscoveryHeaderHelper discoveryHeaderHelper() { DiscoveryHeaderHelper discoveryHeaderHelper = new DiscoveryHeaderHelper(environment); DiscoveryHeaderHelper.INSTANCE = discoveryHeaderHelper; return discoveryHeaderHelper; } /** * resttemplate構建 */ @Resource private RestTemplateBuilder restTemplateBuilder; /** * resttemplate請求bean,更改系統本身的builder * @return */ @Bean @LoadBalanced public RestTemplate restTemplate() { RestTemplate restTemplate = restTemplateBuilder.configure(new RestTemplate()); //RestTemplate interceptors 遠程調用請求增加頭部資訊處理 restTemplate.getInterceptors().add(new RestApiHeaderInterceptor()); //RestTemplate Set the error handler 錯誤處理 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; } }源碼地址 > DiscoveryConfig
