How to communicate microservices

• @FeignClient (ApiClient)
• Kafka (Use Event)
• RestTemplate (communicate with REST API)
• Circuit Breaker (Fault Tolerant Design Pattern)

Spring Cloud

a framework for building robust cloud(strong and healthy) applications. (= microservices)

Spring Cloud Stream

A lightweight event-driven microservices framework to quickly build applications that can connect to external systems. Simple declarative model to send and receive messages using Apache Kafka(middleware) or RabbitMQ between Spring Boot apps.

@FeignClient

• Microservices communication method in Spring Boot
• @EnableFeignClients

user-service

build.gradle

1
2
3

dependencies {
    implementation group: 'org.springframework.cloud', name: 'spring-cloud-starter-openfeign', version: '2.1.1.RELEASE'
}

user-service

1
2
3
4

@EnableFeignClients
public class ApiClientConfiguration {
    ...
}

departments-service

1
2
3
4
5
6
7
8

@FeignClient(name = "user", contextId = "userEnvironmentSetting", url = "${...}")
@RequestMapping(path="/users", consumes = "application/json")
interface UserEnvironmentSettingClient {

    @Override
    @GetMapping("/users/environmentSetting")
    Map<String, String> getEnvironmentSetting();
}

• name : LoadBalancer client name
• contextId : bean name instead of name if present
  • issue : 동일 name 으로 두개의 class 에서 지정하면 오류 발생
• reference: https://techblog.woowahan.com/2630/

RestTemplate

• Spring Http communication template (Rest API 서비스를 요청 후 응답받을 때 주로 사용)
• For communiating over the microservices
• Synchronize (Async : org.springframework.web.client.AsyncRestTemplate)
• will be deprecated

build.gradle

1

implementation 'org.springframework.boot:spring-boot-starter-web'

RestTemplate VS WebCLient

• reference

https://backtony.github.io/spring/2021-07-12-spring-basic-8/
https://blog.naver.com/hj_kim97/222295259904
https://sjh836.tistory.com/141

Blocking VS Non-Blocking VS Sync VS Async

• Blocking / Non-blocking

  다른 주체가 작업할 때 자신에게 자신의 작업에 대한 제어권이 있는지 없는지

• Sync / Async

  그것을 요청한 순서가 지켜지는가 아닌가 (호출되는 함수의 작업 완료 여부를 누가 신경쓰냐)

• common used : sync-blocking, sync-nonblocking, async-nonblocking (most efficient)

• reference

https://studyandwrite.tistory.com/486
https://baek-kim-dev.site/38

CircuitBreaker (Design pattern using netfilx hystrix)

• Hystrix

  library form Netflix (spring-cloud-starter-netflix), isolates the points of access between the services, stops cascading failures across thhem and provides the fallback options

  • Fallback

    provides an alternative solution during a service request failure

• Circuit Breaker Pattern

  prevents failure cascading and gives a default behavior when services fail

  • Netflix Hystrix

    allows us to introduce fault tolerance and latency tolerance by isolating failure and by preventing them from cascading into the other part of the system building a more robust distributed application.

1
2
3
4
5
6
7
8
9
10
11
12
13
14

@Service
public class GreetingService {

    @HystrixCommand(fallbackMethod = "defaultGreeting")
    public String getGreeting(String username) {
        return new RestTemplate()
          .getForObject("http://localhost:9090/greeting/{username}", 
          String.class, username);
    }
 
    private String defaultGreeting(String username) {
        return "Hello User!";
    }
}

• @EnableCircuitBreaker

  will scan the classpath for any compatible Circuit Breaker implementation.

1
2
3
4
5
6
7

@SpringBootApplication
@EnableCircuitBreaker
public class RestConsumerApplication {
    public static void main(String[] args) {
        SpringApplication.run(RestConsumerApplication.class, args);
    }
}

Hystrix VS Resilience4J

• common feature : fault tolerant library

• reference

https://engineering.linecorp.com/ko/blog/circuit-breakers-for-distributed-services/
https://www.baeldung.com/spring-cloud-netflix-hystrix
https://digitalvarys.com/what-is-circuit-breaker-design-pattern/

API Gateway (Load balancing)

Service : Cloud-gateway

spring-cloud-starter-gateway

bulid.gradle

1
2
3

dependencies {
    compile 'org.springframework.cloud:spring-cloud-starter-gateway:2.1.0.RELEASE'
}

application.yml

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24

server:
    port: 9191

spring:
  cloud:
    gateway:
      routes:
        - id: USER-SERVICE
          url: ${cloud-gateway.user-url}
          predicates:
            - Path=/users/ **
        - id: DEPARTMENT-SERVICE
          url: ${cloud-gateway.department-url}
          predicates:
            - Path=/departments/ **
          filter:
            - name: CircuitBreaker
              args:
                name: USER-SERVICE
                fallbackuri: forward:/userServiceFallBack

cloud-gateway:
    user-url: http://cloud-user-service:9001
    department-url: http://cloud-department-service:9002

Meaning:

• when you call http://localhost:9191/users/1, it goes to http://localhost:9001/users/1

• when you call http://localhost:9191/departments/1, it goes to http://localhost:9002/departments/1

• reference : https://cloud.spring.io/spring-cloud-gateway/reference/html/#configuration

Spring-boot-starter-actuator

bulid.gradle

1
2
3

dependencies { 
  compile("org.springframework.boot:spring-boot-starter-actuator")
}

application.yml

From spring boot 2.x, you have to custom management part in application.yml because the default setting is not exposing most datas.

1
2
3
4
5

management:
  endpoints:
    web:
      exposure:
        include: health

Meaning : expose the health information

1
2
3
4
5

management:
    endpoints:
        web:
            exposure:
                exclude: env, beans

Meaning : expose the information except for env, beans

the property exclude is prior than include, so if you declared some information on exclude, you can’t see it even if you declared that on include.

• reference : https://jeong-pro.tistory.com/160

Spring Cloud Stream

• 이벤트 중심 microservcie를 구축하기 위한 프레임워크
• Apache Kafka 또는 RabbitMQ 등을 사용하여 Spring Boot 어플리케이션과 메세지를 보내고 받음.

https://saramin.github.io/2019-08-28-2/

Eureke Client

• reference : https://www.baeldung.com/spring-cloud-netflix-eureka