Ribbon
Ribbon是一个基于HTTP和TCP客户端的负载均衡器。Feign中也使用Ribbon，后续会介绍Feign的使用。
Ribbon可以在通过客户端中配置的ribbonServerList服务端列表去轮询访问以达到均衡负载的作用。
当Ribbon与Eureka联合使用时，ribbonServerList会被DiscoveryEnabledNIWSServerList重写，扩展成从Eureka注册中心中获取服务端列表。同时它也会用NIWSDiscoveryPing来取代IPing，它将职责委托给Eureka来确定服务端是否已经启动。
下面我们通过实例看看如何使用Ribbon来调用服务，并实现客户端的均衡负载。
准备工作
- 启动Chapter-9-1-1中的服务注册中心：eureka-server
- 启动Chapter-9-1-1中的服务提供方：compute-service
- 修改compute-service中的server-port为2223，再启动一个服务提供方：compute-service
可以看到COMPUTE-SERVICE服务有两个单元正在运行：
- 192.168.21.101:compute-service:2222
- 192.168.21.101:compute-service:2223
使用Ribbon实现客户端负载均衡的消费者
构建一个基本Spring Boot项目，并在pom.xml中加入如下内容：
| ``` org.springframework.boot spring-boot-starter-parent 1.3.5.RELEASE org.springframework.cloud spring-cloud-starter-ribbon org.springframework.cloud spring-cloud-starter-eureka org.springframework.boot spring-boot-starter-web org.springframework.boot spring-boot-starter-test test org.springframework.cloud spring-cloud-dependencies Brixton.RELEASE pom import
| ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
在应用主类中，通过`@EnableDiscoveryClient`注解来添加发现服务能力。创建RestTemplate实例，并通过`@LoadBalanced`注解开启均衡负载能力。
| ```
@SpringBootApplication @EnableDiscoveryClient public class RibbonApplication { @Bean @LoadBalanced RestTemplate restTemplate() { return new RestTemplate(); } public static void main(String[] args) { SpringApplication.run(RibbonApplication.class, args); } }
``` |
| --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
创建`ConsumerController`来消费`COMPUTE-SERVICE`的add服务。通过直接RestTemplate来调用服务，计算10 + 20的值。
| ```
@RestController public class ConsumerController { @Autowired RestTemplate restTemplate; @RequestMapping(value = "/add", method = RequestMethod.GET) public String add() { return restTemplate.getForEntity("http://COMPUTE-SERVICE/add?a=10&b=20", String.class).getBody(); } }
``` |
| ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
`application.properties`中配置eureka服务注册中心
| ```
spring.application.name=ribbon-consumer server.port=3333 eureka.client.serviceUrl.defaultZone=http://localhost:1111/eureka/
``` |
| -------------------------------------------------------------------------------------------------------------------------------------- |
启动该应用，并访问两次：<http://localhost:3333/add>
然后，打开compute-service的两个服务提供方，分别输出了类似下面的日志内容：
- 端口为`2222`服务提供端的日志：
| ```
2016-06-02 11:16:26.787 INFO 90014 --- [io-2222-exec-10] com.didispace.web.ComputeController : /add, host:192.168.21.101, service_id:compute-service, result:30
``` |
| ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
- 端口为`2223`服务提供端的日志：
| ```
2016-06-02 11:19:41.241 INFO 90122 --- [nio-2223-exec-1] com.didispace.web.ComputeController : /add, host:192.168.21.101, service_id:compute-service, result:30
``` |
| ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
可以看到，之前启动的两个compute-service服务端分别被调用了一次。到这里，我们已经通过Ribbon在客户端已经实现了对服务调用的均衡负载。
完整示例可参考：[Chapter9-1-2/eureka-ribbon](http://git.oschina.net/didispace/SpringBoot-Learning/)
## [](https://blog.didispace.com/springcloud2/#Feign "Feign")Feign
Feign是一个声明式的Web Service客户端，它使得编写Web Serivce客户端变得更加简单。我们只需要使用Feign来创建一个接口并用注解来配置它既可完成。它具备可插拔的注解支持，包括Feign注解和JAX-RS注解。Feign也支持可插拔的编码器和解码器。Spring Cloud为Feign增加了对Spring MVC注解的支持，还整合了Ribbon和Eureka来提供均衡负载的HTTP客户端实现。
下面，通过一个例子来展现Feign如何方便的声明对上述computer-service服务的定义和调用。
创建一个Spring Boot工程，配置`pom.xml`，将上述的配置中的ribbon依赖替换成feign的依赖即可，具体如下：
| ```
<parent> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-parent</artifactId> <version>1.3.5.RELEASE</version> <relativePath/> <!-- lookup parent from repository --> </parent> <dependencies> <dependency> <groupId>org.springframework.cloud</groupId> <artifactId>spring-cloud-starter-feign</artifactId> </dependency> <dependency> <groupId>org.springframework.cloud</groupId> <artifactId>spring-cloud-starter-eureka</artifactId> </dependency> <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-web</artifactId> </dependency> <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-test</artifactId> <scope>test</scope> </dependency> </dependencies> <dependencyManagement> <dependencies> <dependency> <groupId>org.springframework.cloud</groupId> <artifactId>spring-cloud-dependencies</artifactId> <version>Brixton.RELEASE</version> <type>pom</type> <scope>import</scope> </dependency> </dependencies> </dependencyManagement>
``` |
| ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
在应用主类中通过`@EnableFeignClients`注解开启Feign功能，具体如下：
| ```
@SpringBootApplication @EnableDiscoveryClient @EnableFeignClients public class FeignApplication { public static void main(String[] args) { SpringApplication.run(FeignApplication.class, args); } }
``` |
| ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
定义`compute-servic`e服务的接口，具体如下：
| ```
@FeignClient("compute-service") public interface ComputeClient { @RequestMapping(method = RequestMethod.GET, value = "/add") Integer add(@RequestParam(value = "a") Integer a, @RequestParam(value = "b") Integer b); }
``` |
| ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
- 使用`@FeignClient("compute-service")`注解来绑定该接口对应compute-service服务
- 通过Spring MVC的注解来配置compute-service服务下的具体实现。
在web层中调用上面定义的`ComputeClient`，具体如下：
| ```
@RestController public class ConsumerController { @Autowired ComputeClient computeClient; @RequestMapping(value = "/add", method = RequestMethod.GET) public Integer add() { return computeClient.add(10, 20); } }
``` |
| ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
`application.properties`中不用变，指定eureka服务注册中心即可，如：
| ```
spring.application.name=feign-consumer server.port=3333 eureka.client.serviceUrl.defaultZone=http://localhost:1111/eureka/
``` |
| ------------------------------------------------------------------------------------------------------------------------------------ |
启动该应用，访问几次：<http://localhost:3333/add>
再观察日志，可以得到之前使用Ribbon时一样的结果，对服务提供方实现了均衡负载。
这一节我们通过Feign以接口和注解配置的方式，轻松实现了对compute-service服务的绑定，这样我们就可以在本地应用中像本地服务一下的调用它，并且做到了客户端均衡负载。
