详解spring cloud中使用Ribbon实现客户端的软负载均衡

开篇

本例是在springboot整合H2内存数据库，实现单元测试与数据库无关性和使用RestTemplate消费spring boot的Restful服务两个示例的基础上改造而来

在使用RestTemplate来消费spring boot的Restful服务示例中，我们提到，调用spring boot服务的时候，需要将服务的URL写死或者是写在配置文件中，但这两种方式，无论哪一种，一旦ip地址发生了变化，都需要改动程序，并重新部署服务，使用Ribbon的时候，可以有效的避免这个问题。

前言：

软负载均衡的实现方式有两种，分别是服务端的负载均衡和客户端的负载均衡

服务端负载均衡：当浏览器向后台发出请求的时候，会首先向反向代理服务器发送请求，反向代理服务器会根据客户端部署的ip：port映射表以及负载均衡策略，来决定向哪台服务器发送请求，一般会使用到nginx反向代理技术。

客户端负载均衡：当浏览器向后台发出请求的时候，客户端会向服务注册器(例如：Eureka Server)，拉取注册到服务器的可用服务信息，然后根据负载均衡策略，直接命中哪台服务器发送请求。这整个过程都是在客户端完成的，并不需要反向代理服务器的参与。

一、启动Eureka Server

请参考该例：spring cloud中启动Eureka Server

二、启动微服务，并注册到Eureka Server上

spring cloud-将spring boot服务注册到Eureka Server上

为了演示负载均衡的效果，再启动一个为服务，注意需要将端口号改成不一致

三、添加Ribbon支持

1、添加Ribbon的依赖

2、添加负载均衡支持

package com.chhliu.springboot.restful;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.web.client.RestTemplateBuilder;
import org.springframework.cloud.client.loadbalancer.LoadBalanced;
import org.springframework.cloud.netflix.eureka.EnableEurekaClient;
import org.springframework.context.annotation.Bean;
import org.springframework.web.client.RestTemplate;
@SpringBootApplication
@EnableEurekaClient
public class SpringbootRestTemplateApplication { 

  @Autowired
  private RestTemplateBuilder builder; 

  @Bean
  @LoadBalanced // 添加负载均衡支持，很简单，只需要在RestTemplate上添加@LoadBalanced注解，那么RestTemplate即具有负载均衡的功能,如果不加@LoadBalanced注解的话，会报java.net.UnknownHostException:springboot-h2异常，此时无法通过注册到Eureka Server上的服务名来调用服务，因为RestTemplate是无法从服务名映射到ip:port的，映射的功能是由LoadBalancerClient来实现的。
  public RestTemplate restTemplate() {
    return builder.build();
  } 

  public static void main(String[] args) {
    SpringApplication.run(SpringbootRestTemplateApplication.class, args);
  }
}

3、修改调用微服务的URL

package com.chhliu.springboot.restful.controller;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.client.RestTemplate;
import com.chhliu.springboot.restful.vo.User; 

@RestController
public class RestTemplateController {
  @Autowired
  private RestTemplate restTemplate; 

  @GetMapping("/template/{id}")
  public User findById(@PathVariable Long id) {// 将原来的ip:port的形式，改成注册到Eureka Server上的应用名即可
    User u = this.restTemplate.getForObject("http://springboot-h2/user/" + id, User.class);
    System.out.println(u);
    return u;
  }
}

四、查看Eureka Server状态

五，在浏览器中，多次刷新http://localhost:7904/template/2地址

六、测试结果

7900端口服务：

Hibernate: select user0_.id as id1_0_0_, user0_.age as age2_0_0_, user0_.balance as balance3_0_0_, user0_.name as name4_0_0_, user0_.username as username5_0_0_ from user user0_ where user0_.id=?
Hibernate: select user0_.id as id1_0_0_, user0_.age as age2_0_0_, user0_.balance as balance3_0_0_, user0_.name as name4_0_0_, user0_.username as username5_0_0_ from user user0_ where user0_.id=?
Hibernate: select user0_.id as id1_0_0_, user0_.age as age2_0_0_, user0_.balance as balance3_0_0_, user0_.name as name4_0_0_, user0_.username as username5_0_0_ from user user0_ where user0_.id=?
Hibernate: select user0_.id as id1_0_0_, user0_.age as age2_0_0_, user0_.balance as balance3_0_0_, user0_.name as name4_0_0_, user0_.username as username5_0_0_ from user user0_ where user0_.id=? 

7901端口服务：

Hibernate: select user0_.id as id1_0_0_, user0_.age as age2_0_0_, user0_.balance as balance3_0_0_, user0_.name as name4_0_0_, user0_.username as username5_0_0_ from user user0_ where user0_.id=?
Hibernate: select user0_.id as id1_0_0_, user0_.age as age2_0_0_, user0_.balance as balance3_0_0_, user0_.name as name4_0_0_, user0_.username as username5_0_0_ from user user0_ where user0_.id=?
Hibernate: select user0_.id as id1_0_0_, user0_.age as age2_0_0_, user0_.balance as balance3_0_0_, user0_.name as name4_0_0_, user0_.username as username5_0_0_ from user user0_ where user0_.id=?
Hibernate: select user0_.id as id1_0_0_, user0_.age as age2_0_0_, user0_.balance as balance3_0_0_, user0_.name as name4_0_0_, user0_.username as username5_0_0_ from user user0_ where user0_.id=?
Hibernate: select user0_.id as id1_0_0_, user0_.age as age2_0_0_, user0_.balance as balance3_0_0_, user0_.name as name4_0_0_, user0_.username as username5_0_0_ from user user0_ where user0_.id=? 

7904端口服务：

User [id=2, username=user2, name=李四, age=20, balance=100.00]
2017-01-23 09:58:05.682 INFO 7436 --- [erListUpdater-0] c.netflix.config.ChainedDynamicProperty : Flipping property: springboot-h2.ribbon.ActiveConnectionsLimit to use NEXT property: niws.loadbalancer.availabilityFilteringRule.activeConnectionsLimit = 2147483647
User [id=2, username=user2, name=李四, age=20, balance=100.00]
User [id=2, username=user2, name=李四, age=20, balance=100.00]
User [id=2, username=user2, name=李四, age=20, balance=100.00]
User [id=2, username=user2, name=李四, age=20, balance=100.00]
User [id=2, username=user2, name=李四, age=20, balance=100.00]
User [id=2, username=user2, name=李四, age=20, balance=100.00]
User [id=2, username=user2, name=李四, age=20, balance=100.00]
User [id=2, username=user2, name=李四, age=20, balance=100.00] 

从上面的测试结果可以看出，总共调了7904端口服务9次，其中7904端口服务调7900端口服务4次，调7901端口5次，刚好是9次

经过上面的几个步骤，就基本使用Ribbon实现了客户端负载均衡的功能

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