【设计模式】JAVA Design Patterns——Circuit Breaker（断路器模式）

@Slf4j
public class App {
 
  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
 
  /**
   * Program entry point.
   *
   * @param args command line args
   */
  public static void main(String[] args) {
 
    var serverStartTime = System.nanoTime();
 
    var delayedService = new DelayedRemoteService(serverStartTime, 5);
    var delayedServiceCircuitBreaker = new DefaultCircuitBreaker(delayedService, 3000, 2,
        2000 * 1000 * 1000);
 
    var quickService = new QuickRemoteService();
    var quickServiceCircuitBreaker = new DefaultCircuitBreaker(quickService, 3000, 2,
        2000 * 1000 * 1000);
 
    // 创建一个可以进行本地和远程调用的监控服务对象
    var monitoringService = new MonitoringService(delayedServiceCircuitBreaker,
        quickServiceCircuitBreaker);
 
    // 获取本地资源
    LOGGER.info(monitoringService.localResourceResponse());
 
    // 从延迟服务中获取响应 2 次，以满足失败阈值
    LOGGER.info(monitoringService.delayedServiceResponse());
    LOGGER.info(monitoringService.delayedServiceResponse());
 
    // 在超过故障阈值限制后获取延迟服务断路器的当前状态
    // 现在是打开状态
    LOGGER.info(delayedServiceCircuitBreaker.getState());
 
     // 同时，延迟服务宕机，从健康快速服务获取响应
    LOGGER.info(monitoringService.quickServiceResponse());
    LOGGER.info(quickServiceCircuitBreaker.getState());
 
    // 等待延迟的服务响应
    try {
      LOGGER.info("Waiting for delayed service to become responsive");
      Thread.sleep(5000);
    } catch (InterruptedException e) {
      e.printStackTrace();
    }
    // 检查延时断路器的状态，应该是HALF_OPEN
    LOGGER.info(delayedServiceCircuitBreaker.getState());
 
    // 从延迟服务中获取响应，现在应该是健康的
    LOGGER.info(monitoringService.delayedServiceResponse());
    // 获取成功响应后，它的状态应该是关闭。
    LOGGER.info(delayedServiceCircuitBreaker.getState());
  }
}

public class MonitoringService {
 
  private final CircuitBreaker delayedService;
 
  private final CircuitBreaker quickService;
 
  public MonitoringService(CircuitBreaker delayedService, CircuitBreaker quickService) {
    this.delayedService = delayedService;
    this.quickService = quickService;
  }
 
  // 假设：本地服务不会失败，无需将其包装在断路器逻辑中
  public String localResourceResponse() {
    return "Local Service is working";
  }
 
  /**
   * Fetch response from the delayed service (with some simulated startup time).
   *
   * @return response string
   */
  public String delayedServiceResponse() {
    try {
      return this.delayedService.attemptRequest();
    } catch (RemoteServiceException e) {
      return e.getMessage();
    }
  }
 
  /**
   * Fetches response from a healthy service without any failure.
   *
   * @return response string
   */
  public String quickServiceResponse() {
    try {
      return this.quickService.attemptRequest();
    } catch (RemoteServiceException e) {
      return e.getMessage();
    }
  }
}

public class DefaultCircuitBreaker implements CircuitBreaker {
 
    private final long timeout;
    private final long retryTimePeriod;
    private final RemoteService service;
    long lastFailureTime;
    private String lastFailureResponse;
    int failureCount;
    private final int failureThreshold;
    private State state;
    private final long futureTime = 1000 * 1000 * 1000 * 1000;
 
    /**
     * Constructor to create an instance of Circuit Breaker.
     *
     * @param timeout          Timeout for the API request. Not necessary for this simple example
     * @param failureThreshold Number of failures we receive from the depended service before changing
     *                         state to 'OPEN'
     * @param retryTimePeriod  Time period after which a new request is made to remote service for
     *                         status check.
     */
    DefaultCircuitBreaker(RemoteService serviceToCall, long timeout, int failureThreshold,
                          long retryTimePeriod) {
        this.service = serviceToCall;
        //  我们从关闭状态开始希望一切都是正常的
        this.state = State.CLOSED;
        this.failureThreshold = failureThreshold;
        // API的超时时间.
        // 用于在超过限制时中断对远程资源的调用
        this.timeout = timeout;
        this.retryTimePeriod = retryTimePeriod;
        //An absurd amount of time in future which basically indicates the last failure never happened
        this.lastFailureTime = System.nanoTime() + futureTime;
        this.failureCount = 0;
    }
 
    // 重置所有
    @Override
    public void recordSuccess() {
        this.failureCount = 0;
        this.lastFailureTime = System.nanoTime() + futureTime;
        this.state = State.CLOSED;
    }
 
    @Override
    public void recordFailure(String response) {
        failureCount = failureCount + 1;
        this.lastFailureTime = System.nanoTime();
        // Cache the failure response for returning on open state
        this.lastFailureResponse = response;
    }
 
    // 根据 failureThreshold、failureCount 和 lastFailureTime 评估当前状态。
    protected void evaluateState() {
        if (failureCount >= failureThreshold) { //Then something is wrong with remote service
            if ((System.nanoTime() - lastFailureTime) > retryTimePeriod) {
                // 我们已经等得够久了，应该尝试检查服务是否已启动
                state = State.HALF_OPEN;
            } else {
                // 服务可能仍会出现故障
                state = State.OPEN;
            }
        } else {
            // 一切正常
            state = State.CLOSED;
        }
    }
 
    @Override
    public String getState() {
        evaluateState();
        return state.name();
    }
 
    /**
     * Break the circuit beforehand if it is known service is down Or connect the circuit manually if
     * service comes online before expected.
     *
     * @param state State at which circuit is in
     */
    @Override
    public void setState(State state) {
        this.state = state;
        switch (state) {
            case OPEN -> {
                this.failureCount = failureThreshold;
                this.lastFailureTime = System.nanoTime();
            }
            case HALF_OPEN -> {
                this.failureCount = failureThreshold;
                this.lastFailureTime = System.nanoTime() - retryTimePeriod;
            }
            default -> this.failureCount = 0;
        }
    }
 
    /**
     * Executes service call.
     *
     * @return Value from the remote resource, stale response or a custom exception
     */
    @Override
    public String attemptRequest() throws RemoteServiceException {
        evaluateState();
        if (state == State.OPEN) {
            // 如果电路处于打开状态，则返回缓存的响应
            return this.lastFailureResponse;
        } else {
            // 如果电路未打开，则发出 API 请求
            try {
                //在实际应用程序中，这将在线程中运行，并且将利用断路器的超时参数来了解服务
                // 是否正在工作。 在这里，我们根据服务器响应本身模拟
                var response = service.call();
                // api 响应正常，重置所有。
                recordSuccess();
                return response;
            } catch (RemoteServiceException ex) {
                recordFailure(ex.getMessage());
                throw ex;
            }
        }
    }
}