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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