CircuitBreaker斷路器Fallback如何獲取異常

在Spring Cloud 2020新版裡, 可以使用新版的 CircuitBreaker 斷路器, 可以配置Fallback, 可以是內部的, 也可以是外部的Fallback.

內部 Fallback

這裡搭建一個內部fallback, 配置如下:

server:
  port: 8900

spring:
  application:
    name: ms-gateway
  main:
    allow-bean-definition-overriding: true
  cloud:
    zookeeper:
      connect-string: ${ZK_HOSTS:127.0.0.1:2181}
      discovery:
        enabled: true
        preferIpAddress: true
    loadbalancer:
      ribbon:
        enabled: false
    gateway:
      discovery:
        locator:
          lowerCaseServiceId: true
          enabled: true
      routes:
        - id: default
          uri: lb://ms-fundmain-service
          predicates:
            - Path=/fundmain/**
          filters:
            - StripPrefix=1
            - name: CircuitBreaker
              args:
                name: fetchIngredients
                fallbackUri: forward:/defaultfallback
            - name: Retry
              args:
                retries: 2
                series: SERVER_ERROR

fallback攔截了服務端異常, defaultfallback的實現如下:

@RestController
public class DefaultFallback {

    private static final Logger logger = LoggerFactory.getLogger(GatewayErrorAttributes.class);

    @RequestMapping("/defaultfallback")
    @ResponseStatus(HttpStatus.INTERNAL_SERVER_ERROR)
    public Map<String, Object> defaultfallback() {

        Map<String, Object> map = new HashMap<>();
        map.put("code", 5001);
        map.put("msg", "服務異常");
        return map;
    }

}

這樣就可以實現一個最簡單的fallback, 如果要獲取服務的實際錯誤資訊, 則改造如下:

@RestController
public class DefaultFallback {

    private static final Logger logger = LoggerFactory.getLogger(GatewayErrorAttributes.class);
    
    @RequestMapping(value = "/defaultfallback")
    @ResponseStatus
    public Mono<Map<String, Object>> fallback(ServerWebExchange exchange) {
        Map<String, Object> result = new HashMap<>(4);
        result.put("code", 5001);

        Exception exception = exchange.getAttribute(ServerWebExchangeUtils.CIRCUITBREAKER_EXECUTION_EXCEPTION_ATTR);

        ServerWebExchange delegate = ((ServerWebExchangeDecorator) exchange).getDelegate();
        logger.error("服務呼叫失敗，URL={}", delegate.getRequest().getURI(), exception);

        result.put("uri", delegate.getRequest().getURI());

        if (exception instanceof TimeoutException) {
            result.put("msg", "服務超時");
        }
        else if (exception != null && exception.getMessage() != null) {
            result.put("msg", "服務錯誤: " + exception.getMessage());
        }
        else {
            result.put("msg", "服務錯誤");
        }
        return Mono.just(result);
    }
}

這次注入了一個 ServerWebExchange, 然後獲取儲存在 CIRCUITBREAKER_EXECUTION_EXCEPTION_ATTR 裡面的例項, 拿到相應的異常資訊.

外部Fallback

請參考 https://docs.spring.io/spring-cloud-gateway/docs/current/reference/html/, 使用 FallbackHeaders 即可.

spring:
  application:
    name: ms-gateway
  main:
    allow-bean-definition-overriding: true
  redis:
    host: 127.0.0.1
    port: 6379
    database: 0
  cloud:
    zookeeper:
      connect-string: ${ZK_HOSTS:127.0.0.1:2181}
      discovery:
        enabled: true
        preferIpAddress: true
    loadbalancer:
      ribbon:
        enabled: false
    gateway:
      discovery:
        locator:
          lowerCaseServiceId: true
          enabled: true
      routes:
        - id: testoutsidefallback
          uri: lb://ms-fundmain-service
          predicates:
            - Path=/fundmain2/**
          filters:
            - StripPrefix=1
            - name: CircuitBreaker
              args:
                name: fetchIngredients2
                fallbackUri: forward:/externalfallback
        - id: ingredients-fallback
          uri: lb://ms-fundmain-service
          predicates:
            - Path=/externalfallback
          filters:
            - name: FallbackHeaders
              args:
                executionExceptionTypeHeaderName: Exception-Type
                executionExceptionMessageHeaderName: Exception-Message

支援四個欄位:

• executionExceptionTypeHeaderName ("Execution-Exception-Type")
• executionExceptionMessageHeaderName ("Execution-Exception-Message")
• rootCauseExceptionTypeHeaderName ("Root-Cause-Exception-Type")
• rootCauseExceptionMessageHeaderName ("Root-Cause-Exception-Message")

實現示例 Fallback如下: (注意, 此Controller不能放在閘道器專案裡, 否則就要用內部Fallback才行)

@RestController
public class ExternalFallback {

    private static final Logger logger = LoggerFactory.getLogger(ExternalFallback.class);

    private static final String EXECUTION_EXCEPTION_TYPE = "Exception-Type";
    private static final String EXECUTION_EXCEPTION_MESSAGE = "Exception-Message";

    @RequestMapping("/externalfallback")
    @ResponseStatus
    public Map<String, Object> externalfallback(HttpServletRequest request) {

        Map<String, Object> map = new HashMap<>();
        map.put("code", 5002);
        map.put("msg", "服務異常");

        map.put(EXECUTION_EXCEPTION_TYPE, request.getHeader(EXECUTION_EXCEPTION_TYPE));
        map.put(EXECUTION_EXCEPTION_MESSAGE, request.getHeader(EXECUTION_EXCEPTION_MESSAGE));

        return map;
    }
}

這次注入了一個 HttpServletRequest, 然後獲取儲存在 Header 裡面的資訊, 拿到相應的異常.