什么是负载均衡？

如果一个服务对应多个实例，我们需要把流量合理的分配给多个实例；当服务流量增⼤时, 通常会采⽤增加机器的⽅式进⾏扩容, 负载均衡就是⽤来在多个机器或者其他资源，中, 按照⼀定的规则合理分配负载.

服务端负载均衡

客户端负载均衡

使用Spring Cloud LoadBalancer实现负载均衡

1.给RestTemplate这个Bean添加@LoadBalanced注解。

2.修改远程调用代码的url的IP端口号为服务名称。

负载均衡策略

负载均衡策略是一种思想，无论哪种负载均衡器，它们的负载均衡策略都是相似的。Spring Cloud LoadBalancer仅支持两种负载均衡策略：轮询策略和随机策略。

1. 轮询（Round Robin）：轮询策略是指服务器轮流处理用户请求，这是一种实现最简单，也是最常用的策略。
2. 随机选择（Random）：这个策略是指随机选择一个后端服务器来处理新的请求。

自定义负载均衡策略

1. 自定义随机算法对象, 通过 @Bean 将其加载到 Spring 容器中

public class LoadBalancerConfig {

        @Bean

        ReactorLoadBalancer<ServiceInstance> randomLoadBalancer (Environment

environment,

LoadBalancerClientFactory loadBalancerClientFactory) {

                String name =

environment.getProperty(LoadBalancerClientFactory. PROPERTY_NAME );

                System.out.println( "==============" +name);

                return new

RandomLoadBalancer (loadBalancerClientFactory.getLazyProvider(name,

ServiceInstanceListSupplier.class), name);

        }

}

   2.使⽤ @LoadBalancerClient 或者 @LoadBalancerClients 注解在RestTemplate 配置类上⽅, 使⽤ @LoadBalancerClient 或 @LoadBalancerClients 注解, 可以对不同的服务提供⽅配置不同的客⼾端负载均衡算法策略. 

@LoadBalancerClient(name = "product-service", configuration = LoadBalancerConfig.class)

@Configuration

public class BeanConfig {

        @Bean

        @LoadBalanced

        public RestTemplate restTemplate (){

                return new RestTemplate ();

        }

}

@LoadBalancerClient 注解说明

1. name: 该负载均衡策略对哪个服务⽣效(服务提供⽅)

2. configuration : 该负载均衡策略 ⽤哪个负载均衡策略实现.

LoadBalancer原理

public class LoadBalancerInterceptor implements ClientHttpRequestInterceptor {
    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));
    }
}

可以看到这里的intercept方法，拦截了用户的HttpRequest请求，然后做了几件事：

public class BlockingLoadBalancerClient implements LoadBalancerClient {    
    public <T> T execute(String serviceId, LoadBalancerRequest<T> request) throws IOException {
        String hint = this.getHint(serviceId);
        LoadBalancerRequestAdapter<T, TimedRequestContext> lbRequest = new LoadBalancerRequestAdapter(request, this.buildRequestContext(request, hint));
        Set<LoadBalancerLifecycle> supportedLifecycleProcessors = this.getSupportedLifecycleProcessors(serviceId);
        supportedLifecycleProcessors.forEach((lifecycle) -> {
            lifecycle.onStart(lbRequest);
        });
        //根据serviceId,和负载均衡策略, 选择处理的服务
        ServiceInstance serviceInstance = this.choose(serviceId, lbRequest);
        if (serviceInstance == null) {
            supportedLifecycleProcessors.forEach((lifecycle) -> {
                lifecycle.onComplete(new CompletionContext(Status.DISCARD, lbRequest, new EmptyResponse()));
            });
            throw new IllegalStateException("No instances available for " + serviceId);
        } else {
            return this.execute(serviceId, serviceInstance, lbRequest);
        }
    }
//根据serviceId,和负载均衡策略, 选择处理的服务
    public <T> ServiceInstance choose(String serviceId, Request<T> request) {
        //获取负载均衡器
        ReactiveLoadBalancer<ServiceInstance> loadBalancer = this.loadBalancerClientFactory.getInstance(serviceId);
        if (loadBalancer == null) {
            return null;
        } else {
//根据负载均衡算法, 在列表中选择⼀个服务实例
            Response<ServiceInstance> loadBalancerResponse = (Response)Mono.from(loadBalancer.choose(request)).block();
            return loadBalancerResponse == null ? null : (ServiceInstance)loadBalancerResponse.getServer();
        }
    }

 负载均衡跟LoadBalancer就介绍到这里，如果想深入了解，可以去继续阅读源码~