Spring Cloud Gateway重试机制

前言

重试，我相信你们并不陌生。在咱们调用Http接口的时候，总会由于某种缘由调用失败，这个时候咱们能够经过重试的方式，来从新请求接口。

生活中这样的事例不少，好比打电话，对方正在通话中啊，信号很差啊等等缘由，你总会打不通，当你第一次没打通以后，你会打第二次，第三次...第四次就通了。

重试也要注意应用场景，读数据的接口比较适合重试的场景，写数据的接口就须要注意接口的幂等性了。还有就是重试次数若是太多的话会致使请求量加倍，给后端形成更大的压力，设置合理的重试机制才是最关键的。

今天咱们来简单的了解下Spring Cloud Gateway中的重试机制和使用。

使用讲解

RetryGatewayFilter是Spring Cloud Gateway对请求重试提供的一个GatewayFilter Factory。

配置方式：

spring:
  cloud:
    gateway:
      routes:
      - id: fsh-house
        uri: lb://fsh-house
        predicates:
        - Path=/house/**
        filters:
        - name: Retry
          args:
           retries: 3
           series:
            - SERVER_ERROR
           statuses:
            - OK
           methods:
            - GET
            - POST
           exceptions:
            - java.io.IOException
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配置讲解

配置类源码org.springframework.cloud.gateway.filter.factory.RetryGatewayFilterFactory.RetryConfig：

public static class RetryConfig {
    private int retries = 3;
		
    private List<Series> series = toList(Series.SERVER_ERROR);
		
    private List<HttpStatus> statuses = new ArrayList<>();
		
    private List<HttpMethod> methods = toList(HttpMethod.GET);

    private List<Class<? extends Throwable>> exceptions = toList(IOException.class);
        
    // .....
}
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• retries：重试次数，默认值是3次
• series：状态码配置（分段），符合的某段状态码才会进行重试逻辑，默认值是SERVER_ERROR，值是5，也就是5XX(5开头的状态码)，共有5个值：

public enum Series {
    INFORMATIONAL(1),
    SUCCESSFUL(2),
    REDIRECTION(3),
    CLIENT_ERROR(4),
    SERVER_ERROR(5);
}
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• statuses：状态码配置，和series不一样的是这边是具体状态码的配置，取值请参考：org.springframework.http.HttpStatus
• methods：指定哪些方法的请求须要进行重试逻辑，默认值是GET方法，取值以下：

public enum HttpMethod {
	GET, HEAD, POST, PUT, PATCH, DELETE, OPTIONS, TRACE;
}
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• exceptions：指定哪些异常须要进行重试逻辑，默认值是java.io.IOException

代码测试

就写个接口，在接口中记录请求次数，而后抛出一个异常模拟500，经过网关访问这个接口，若是你配置了重试次数是3，那么接口中会输出4次结果才是对的，证实重试生效了。

AtomicInteger ac = new AtomicInteger();

@GetMapping("/data")
public HouseInfo getData(@RequestParam("name") String name) {
	if (StringUtils.isBlank(name)) {
		throw new RuntimeException("error");
	}
	System.err.println(ac.addAndGet(1));
	return new HouseInfo(1L, "上海", "虹口", "XX小区");
}
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更多Spring Cloud代码尽在：github.com/yinjihuan/s…

源码欣赏

@Override
	public GatewayFilter apply(RetryConfig retryConfig) {
        // 验证重试配置格式是否正确
		retryConfig.validate();

		Repeat<ServerWebExchange> statusCodeRepeat = null;
		if (!retryConfig.getStatuses().isEmpty() || !retryConfig.getSeries().isEmpty()) {
			Predicate<RepeatContext<ServerWebExchange>> repeatPredicate = context -> {
				ServerWebExchange exchange = context.applicationContext();
                // 判断重试次数是否已经达到了配置的最大值
				if (exceedsMaxIterations(exchange, retryConfig)) {
					return false;
				}
                // 获取响应的状态码
				HttpStatus statusCode = exchange.getResponse().getStatusCode();
                // 获取请求方法类型
				HttpMethod httpMethod = exchange.getRequest().getMethod();
                // 判断响应状态码是否在配置中存在
				boolean retryableStatusCode = retryConfig.getStatuses().contains(statusCode);

				if (!retryableStatusCode && statusCode != null) { // null status code might mean a network exception?
					// try the series
					retryableStatusCode = retryConfig.getSeries().stream()
							.anyMatch(series -> statusCode.series().equals(series));
				}
                // 判断方法是否包含在配置中
				boolean retryableMethod = retryConfig.getMethods().contains(httpMethod);
                // 决定是否要进行重试
				return retryableMethod && retryableStatusCode;
			};

			statusCodeRepeat = Repeat.onlyIf(repeatPredicate)
					.doOnRepeat(context -> reset(context.applicationContext()));
		}

		//TODO: support timeout, backoff, jitter, etc... in Builder

		Retry<ServerWebExchange> exceptionRetry = null;
		if (!retryConfig.getExceptions().isEmpty()) {
			Predicate<RetryContext<ServerWebExchange>> retryContextPredicate = context -> {
				if (exceedsMaxIterations(context.applicationContext(), retryConfig)) {
					return false;
				}
                // 异常判断
				for (Class<? extends Throwable> clazz : retryConfig.getExceptions()) {             
					if (clazz.isInstance(context.exception())) {
						return true;
					}
				}
				return false;
			};
            // 使用reactor extra的retry组件
			exceptionRetry = Retry.onlyIf(retryContextPredicate)
					.doOnRetry(context -> reset(context.applicationContext()))
					.retryMax(retryConfig.getRetries());
		}


		return apply(statusCodeRepeat, exceptionRetry);
	}

	public boolean exceedsMaxIterations(ServerWebExchange exchange, RetryConfig retryConfig) {
		Integer iteration = exchange.getAttribute(RETRY_ITERATION_KEY);

		//TODO: deal with null iteration
		return iteration != null && iteration >= retryConfig.getRetries();
	}

	public void reset(ServerWebExchange exchange) {
		//TODO: what else to do to reset SWE?
		exchange.getAttributes().remove(ServerWebExchangeUtils.GATEWAY_ALREADY_ROUTED_ATTR);
	}

	public GatewayFilter apply(Repeat<ServerWebExchange> repeat, Retry<ServerWebExchange> retry) {
		return (exchange, chain) -> {
			if (log.isTraceEnabled()) {
				log.trace("Entering retry-filter");
			}

			// chain.filter returns a Mono<Void>
            Publisher<Void> publisher = chain.filter(exchange)
                    //.log("retry-filter", Level.INFO)
                    .doOnSuccessOrError((aVoid, throwable) -> {
                        // 获取已经重试的次数，默认值为-1
                        int iteration = exchange.getAttributeOrDefault(RETRY_ITERATION_KEY, -1);
                        // 增长重试次数
                        exchange.getAttributes().put(RETRY_ITERATION_KEY, iteration + 1);
                    });

            if (retry != null) {
				// retryWhen returns a Mono<Void>
				// retry needs to go before repeat
				publisher = ((Mono<Void>)publisher).retryWhen(retry.withApplicationContext(exchange));
			}
			if (repeat != null) {
            	// repeatWhen returns a Flux<Void>
				// so this needs to be last and the variable a Publisher<Void>
				publisher = ((Mono<Void>)publisher).repeatWhen(repeat.withApplicationContext(exchange));
			}

            return Mono.fromDirect(publisher);
		};
	}

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