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Circuit Breaker Implementation Guide

This document explains the steps taken to implement a Circuit Breaker pattern in the bigkart_customer service, allowing it to gracefully handle failures when the bigkart_admin service is unreachable. We used Spring Cloud Circuit Breaker with Resilience4j.

1. Added Dependencies (pom.xml)​

To enable Resilience4j and its features, we added three crucial dependencies to the project's pom.xml:

  • spring-cloud-starter-circuitbreaker-resilience4j: The core dependency that brings in the Resilience4j library for Circuit Breakers.
  • spring-boot-starter-aop: Resilience4j uses Spring AOP (Aspect-Oriented Programming) under the hood to intercept method calls annotated with @CircuitBreaker. Without this dependency, the circuit breaker annotations will not function. We explicitly pinned version 3.1.2 to resolve a dependency tree build issue.
  • spring-boot-starter-actuator: This exposes health and metrics endpoints for the application, allowing us to monitor the current state (closed, open, half-open) of the circuit breaker.

2. Added Circuit Breaker Configuration (application.properties)​

We defined the behavior of our circuit breaker in application.properties:

# Exposes actuator endpoints to monitor the circuit breaker's status
management.endpoints.web.exposure.include=health,info,metrics,circuitbreakers
management.endpoint.health.show-details=always

# Defines the URL for the external admin service
bigkart.admin.url=http://localhost:8081

# Resilience4j Rules for "adminService" instance
resilience4j.circuitbreaker.instances.adminService.registerHealthIndicator=true
resilience4j.circuitbreaker.instances.adminService.slidingWindowType=COUNT_BASED
resilience4j.circuitbreaker.instances.adminService.slidingWindowSize=10
resilience4j.circuitbreaker.instances.adminService.minimumNumberOfCalls=5
resilience4j.circuitbreaker.instances.adminService.waitDurationInOpenState=5s
resilience4j.circuitbreaker.instances.adminService.failureRateThreshold=50
resilience4j.circuitbreaker.instances.adminService.permittedNumberOfCallsInHalfOpenState=3

Explanation of properties:

  • slidingWindowSize=10: The circuit breaker evaluates the last 10 calls to determine the failure rate.
  • minimumNumberOfCalls=5: It requires at least 5 calls to happen before calculating the failure rate.
  • failureRateThreshold=50: If 50% or more of the calls fail (e.g., timeout or service down), the circuit opens, rejecting all subsequent requests instantly.
  • waitDurationInOpenState=5s: When open, the circuit breaker waits 5 seconds before transitioning to the "Half-Open" state to test if the service has recovered.

3. Created the REST Client (AdminServiceClient.java)​

We created a dedicated service layer to handle communication with the bigkart_admin microservice:

  • Used @Service to register it as a Spring Bean.
  • Injected the bigkart.admin.url property using @Value.
  • Created the getAdminHello() method, which uses RestTemplate to make an HTTP GET request to the admin service.
  • Circuit Breaker Logic: We annotated this method with @CircuitBreaker(name = "adminService", fallbackMethod = "adminServiceFallback").
  • Fallback Method: We defined adminServiceFallback(Exception e). If bigkart_admin is down or times out, this method executes immediately (or intercepts open-circuit calls) to return a safe fallback message: "Admin service is currently down. Please try again later. (Fallback Response)".

4. Configured RestTemplate (BigkartCustomerApplication.java)​

RestTemplate is not auto-configured by default in a way that we can just inject it. So, we defined a @Bean method returning new RestTemplate() in the main application class. This makes it available for auto-wiring inside AdminServiceClient.

5. Exposed a Test Endpoint (HelloController.java)​

Finally, to test the entire integration, we added a new endpoint /admin-hello inside HelloController. This endpoint auto-wires AdminServiceClient and invokes its getAdminHello() method.

How to Test​

  1. Start both the bigkart_admin (port 8081) and bigkart_customer (port 8080) applications.
  2. Navigate to http://localhost:8080/admin-hello. You will see the successful response from the admin service.
  3. Stop the bigkart_admin service and hit the same URL again.
  4. You will immediately receive the fallback message because the circuit breaker handles the connection failure and protects the system from hanging indefinitely.