前言：
本篇文章主要介绍Ribbon负载均衡的使用，策略及原理，希望能加深自己的印象以及帮助各位大佬🤞🤞
如果文章有什么需要改进的地方还请大佬多多指正✨✨
小威在这里先感谢各位大佬了🎊🎊

@[toc]

🚗Ribbon负载均衡使用

接着上一篇文章的来，运用负载均衡的注解 @LoadBalanced进行管理

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

🚜Ribbon项目使用流程

• 拦截我们的RestTemplate请求http://userservice/user/1
• RibbonLoadBalancerClient会从请求url中获取服务名称，也就是user-service
• DynamicServerListLoadBalancer根据user-service到eureka拉取服务列表
• eureka返回列表，localhost:8081、localhost:8082
• IRule利用内置负载均衡规则，从列表中选择一个，例如localhost:8081
• RibbonLoadBalancerClient修改请求地址，用localhost:8081替代userservice，得到http://localhost:8081/user/1，发起真实请求

🚓Ribbon负载均衡分析

负载均衡拦截器实现客户端http请求拦截器（ClientHttpRequestInterceptor）并重写方法

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));
    }
}

找到Eureka服务列表并根据负载均衡算法选取一个服务

public <T> T execute(String serviceId, LoadBalancerRequest<T> request, Object hint) throws IOException {
        ILoadBalancer loadBalancer = this.getLoadBalancer(serviceId);//根据服务名称找Eureka服务列表
        Server server = this.getServer(loadBalancer, hint);//根据负载均衡算法从Eureka的server列表中选出一个   
             if (server == null) {
            throw new IllegalStateException("No instances available for " + serviceId);
        } else {
            RibbonLoadBalancerClient.RibbonServer ribbonServer = new RibbonLoadBalancerClient.RibbonServer(serviceId, server, this.isSecure(server, serviceId), this.serverIntrospector(serviceId).getMetadata(server));
            return this.execute(serviceId, (ServiceInstance)ribbonServer, (LoadBalancerRequest)request);
        }
    }

getLoadBalancer(serviceId)：根据服务id获取ILoadBalancer，而ILoadBalancer会拿着服务id去eureka中获取服务列表并保存起来。getServer(loadBalancer)：利用内置的负载均衡算法，从服务列表中选择一个。

在BaseLoadBalancer.class的类下，利用IRule实现负载均衡的策略

跟进上面的getServer方法，继续跟踪源码在BaseLoadBalancer类下找到chooseServer方法

 public Server chooseServer(Object key) {
        if (this.counter == null) {
            this.counter = this.createCounter();
        }

        this.counter.increment();
        if (this.rule == null) {
            return null;
        } else {
            try {
                return this.rule.choose(key);//进行服务选择的Key
            } catch (Exception var3) {
                logger.warn("LoadBalancer [{}]:  Error choosing server for key {}", new Object[]{this.name, key, var3});
                return null;
            }
        }
    }