2.3.1 Ribbon

Ribbon介绍

Ribbon是Netflix发布的开源项目，主要功能是提供客户端的软件负载均衡算法，将Netflix的中间层服务连接在一起。Ribbon客户端组件提供一系列完善的配置项如连接超时，重试等。简单的说，就是在配置文件中列出Load Balancer后面所有的机器，Ribbon会自动的帮助你基于某种规则（如简单轮询，随机连接等）去连接这些机器。我们也很容易使用Ribbon实现自定义的负载均衡算法。简单地说，Ribbon是一个客户端负载均衡器。

Ribbon工作时分为两步：第一步先选择 Eureka Server, 它优先选择在同一个Zone且负载较少的Server；第二步再根据用户指定的策略，在从Server取到的服务注册列表中选择一个地址。其中Ribbon提供了多种策略，例如轮询、随机、根据响应时间加权等。

Ribbon代码示例

创建一个Maven项目，并在pom.xml中加入如下内容：

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <artifactId>microservice-consumer-movie-ribbon</artifactId>
  <packaging>jar</packaging>
  <parent>
    <groupId>com.itmuch.cloud</groupId>
    <artifactId>spring-cloud-microservice-study</artifactId>
    <version>0.0.1-SNAPSHOT</version>
  </parent>
  <dependencies>
    <dependency>
      <groupId>org.springframework.cloud</groupId>
      <artifactId>spring-cloud-starter-eureka</artifactId>
    </dependency>
    <!-- 整合ribbon -->
    <dependency>
      <groupId>org.springframework.cloud</groupId>
      <artifactId>spring-cloud-starter-ribbon</artifactId>
    </dependency>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-actuator</artifactId>
    </dependency>
  </dependencies>
</project>

启动类：MovieRibbonApplication.java。使用@LoadBalanced注解，为RestTemplate开启负载均衡的能力。

@SpringBootApplication
@EnableDiscoveryClient
public class MovieRibbonApplication {
  /**
   * 实例化RestTemplate，通过@LoadBalanced注解开启均衡负载能力.
   * @return restTemplate
   */
  @Bean
  @LoadBalanced
  public RestTemplate restTemplate() {
    return new RestTemplate();
  }
  public static void main(String[] args) {
    SpringApplication.run(MovieRibbonApplication.class, args);
  }
}

实体类：User.java

public class User {
  private Long id;
  private String username;
  private Integer age;
  ...
  // getters and setters
}

Ribbon的测试类：RibbonService.java

@Service
public class RibbonService {
  @Autowired
  private RestTemplate restTemplate;
  public User findById(Long id) {
    // http://服务提供者的serviceId/url
    return this.restTemplate.getForObject("http://microservice-provider-user/" + id, User.class);
  }
}

controller：RibbonController.java

@RestController
public class RibbonController {
  @Autowired
  private RibbonService ribbonService;
  @GetMapping("/ribbon/{id}")
  public User findById(@PathVariable Long id) {
    return this.ribbonService.findById(id);
  }
}

application.yml

server:
  port: 8010
spring:
  application:
    name: microservice-consumer-movie-ribbon
eureka:
  client:
    serviceUrl:
      defaultZone: http://discovery:8761/eureka/
  instance:
    preferIpAddress: true

启动后，访问多次http://localhost:8010/ribbon/1，返回结果：

{
    "id": 1,
    "username": "Tom",
    "age": 12
}

然后打开两个microservice-provider-user实例的控制台，发现两个实例都输出了类似如下的日志内容：

Hibernate: select user0_.id as id1_0_0_, user0_.age as age2_0_0_, user0_.username as username3_0_0_ from user user0_ where user0_.id=?
2016-09-13 21:38:56.719 TRACE 17404 --- [nio-8000-exec-1] o.h.type.descriptor.sql.BasicBinder      : binding parameter [1] as [BIGINT] - [1]
2016-09-13 21:38:56.720 TRACE 17404 --- [nio-8000-exec-1] o.h.type.descriptor.sql.BasicExtractor   : extracted value ([age2_0_0_] : [INTEGER]) - [12]
2016-09-13 21:38:56.720 TRACE 17404 --- [nio-8000-exec-1] o.h.type.descriptor.sql.BasicExtractor   : extracted value ([username3_0_0_] : [VARCHAR]) - [Tom]
2016-09-13 21:39:10.588  INFO 17404 --- [trap-executor-0] c.n.d.s.r.aws.ConfigClusterResolver      : Resolving eureka endpoints via configuration

至此，我们已经通过Ribbon在客户端侧实现了均衡负载。

代码地址（任选其一）：

Ribbon代码地址：

http://git.oschina.net/itmuch/spring-cloud-study/tree/master/microservice-consumer-movie-ribbon
https://github.com/eacdy/spring-cloud-study/tree/master/microservice-consumer-movie-ribbon