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Circuit Breaker
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=> It is one of the most famous design pattern in microservices.

=> It is used to implement fault tolerent systems.

=> Fault Tolerant systems also called as Resillence systems.

Note: If main logic is failed to execute then we have to execute fallback logic.


Usecase : 

main logic : retrieve data from redis cache

fallback logic : retrieve data from database


=> In springboot, we can implement circuit breaker in 2 ways

			1) hystrix (outdated)

			2) Spring Cloud Resillence4J (trending)

=> Circuit Breaker works based on 3 states

			1) CLOSED

			2) OPEN

			3) HALF_OPEN


CLOSED : System is having normal flow. System working as expected.

OPEN : The circuit interrupted, some fault occured hence normal flow will be stopped and fallback logic will be executed for given amount time.	

HALF_OPEN : System will try for normal flow, if normal flow working as expected then Circuit will go to CLOSED state else it will go to OPEN state.	


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Circuit Breaker Implementation
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#### 1) Create Spring Boot project with below dependencies

		a) web-starter
		b) actuator
		c) aop
		d) resillence4J

<dependency>
	<groupId>org.springframework.boot</groupId>
	<artifactId>spring-boot-starter-aop</artifactId>
</dependency>

### 2) Create Rest Controller with required methods		

@RestController
public class DataRestController {

	@GetMapping("/data")
	@CircuitBreaker(fallbackMethod = "getDataFromDB", name = "ashokit")
	public String getDataFromRedis() {
		System.out.println("*** redis() method called.. ***");
		int i = 10/0;
		return "Retrieved Data From Redis";
	}

	public String getDataFromDB() {
		System.out.println("*** db() method called.. ***");
		return "Retrieved Data From Database";
	}

}

#### 3) Configure Circuit Breaker Properties in application.yml file

spring:
  application:
    name: 03_SB_CircuitBreaker_App
management:
  endpoints:
    web:
      exposure:
        include: '*'
  endpoint:
    health:
      show-details: always
  health:
    circuitbreakers:
      enabled: true

resilience4j:
  circuitbreaker:
    configs:
      default: 
        registerHealthIndicator: true
        slidingWindowSize: 10
        minimumNumberOfCalls: 5
        permittedNumberOfCallsInHalfOpenState: 3
        automaticTransitionFromOpenToHalfOpenEnabled: true
        waitDurationInOpenState: 100s
        failureRateThreshold: 50

#### 4) Test The application and monitor actuator health endpoint to observe Circuit Breaker status.  


		URL-1 : http://localhost:8080/data

		URL-2 : http://localhost:8080/actuator/health


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Spring Batch : https://youtu.be/2sdY-fKaR2o

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