[Demo] Spring Cloud Ribbon + RestTemplate

What is Ribbon

Ribbon is a client-side load balancer that gives you a lot of control over the behavior of HTTP and TCP clients. Feign already uses Ribbon, so, if you use @FeignClient, this section also applies.

A central concept in Ribbon is that of the named client. Each load balancer is part of an ensemble of components that work together to contact a remote server on demand, and the ensemble has a name that you give it as an application developer (for example, by using the @FeignClient annotation). On demand, Spring Cloud creates a new ensemble as an ApplicationContext for each named client by using RibbonClientConfiguration. This contains (amongst other things) an ILoadBalancer, a RestClient, and a ServerListFilter.

Reference : Client Side Load Balancer: Ribbon

RestTemplate

RestTemplate class is Spring’s central class for synchronous client-side HTTP access.

Now let’s have a look at the the main code of the Crash Course - Spring Cloud Netflix Eureka - Part 3/3 ~~ Eureka Consumer again:

@SpringBootApplication
@Configuration
@EnableAutoConfiguration
public class DemoApplication {

	@Autowired
	RestTemplate restTemplate;

	private @RestController class Controller {

		@RequestMapping("/sayHelo/{name}")
		public String greeting(@PathVariable String name) {
			return restTemplate.getForObject("http://eureka-service/greeting/{name}", String.class, name);
		}

		@Bean
		@LoadBalanced
		public RestTemplate restTemplate() {
			return new RestTemplate();
		}
	}

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

Notice for the following two items which are used the in above code block.

• RestTemplate restTemplate;
• @LoadBalanced

RestTemplate

If we make a little modification, remove the annotation @LoadBalanced.

@Bean
@LoadBalanced
public RestTemplate restTemplate() {
        return new RestTemplate();
}

Try to access the url http://eureka-web:8081/sayHelo/Tracy

Whitelabel Error Page
This application has no explicit mapping for /error, so you are seeing this as a fallback.

Tue Aug 21 14:26:02 CST 2018
There was an unexpected error (type=Internal Server Error, status=500).
I/O error on GET request for "http://eureka-service/greeting/Tracy": eureka-service; nested exception is java.net.UnknownHostException: eureka-service

Does it means the RestTemplate must be used together with the @LoadBalanced?

Of course, NO!

RestTemplate is a pure client side library for http accessing, to use it without eureka you just use the real URL - http://eureka-service-p1:8071/greeting/{name}

@LoadBalanced

The annotation @LoadBalanced is used to mark a RestTemplate bean to be configured to use a RibbonLoadBalancerClient. It implicitly include the Ribbon to locate & indicate with the eureka services.

Multiple Services for Load Balance

We start three instances of the same service at three different ports, look at the source code.

New service API will print a simple message, indicates which service instace is accessed:

public class DemoApplication {

        @RestController
        class ServiceInstanceRestController {

                @Value("${server.port}")
                String port;

                @RequestMapping("/greeting/{name}")
                public String greeting(@PathVariable String name) {
                        return "Hello " + name + "! I'm service from port : " + port;
                }
        }

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

Start the consumer app

Now, what we expect if we access our consumer URL - http://eureka-web:8081/sayHelo/Tracy??? Our access will be balanced to three service instances:

[
   {
      "0": "Hello Tracy! I'm service from port : 10071"
   },
   {
      "0": "Hello Tracy! I'm service from port : 10073"
   },
   {
      "0": "Hello Tracy! I'm service from port : 10072"
   },
   {
      "0": "Hello Tracy! I'm service from port : 10071"
   }
   //.....
]

Well, this is not the end. If you have read the official document of : Reference - Client Side Load Balancer: Ribbon, you may know:

• Ribbon can be used with eureka – As I used in this post.
• Ribbon can be used without eureka.

Another thing that I would like to highlight about the different of @RibbonClient and @LoadBalanced.

In next post, I would try to introduce you how to use Ribbon without eureka.