Spring Cloud Gateway重試機制

前言

重試，我相信你們並不陌生。在咱們調用Http接口的時候，總會由於某種緣由調用失敗，這個時候咱們能夠經過重試的方式，來從新請求接口。

生活中這樣的事例不少，好比打電話，對方正在通話中啊，信號很差啊等等緣由，你總會打不通，當你第一次沒打通以後，你會打第二次，第三次...第四次就通了。

重試也要注意應用場景，讀數據的接口比較適合重試的場景，寫數據的接口就須要注意接口的冪等性了。還有就是重試次數若是太多的話會致使請求量加倍，給後端形成更大的壓力，設置合理的重試機制纔是最關鍵的。

今天咱們來簡單的瞭解下Spring Cloud Gateway中的重試機制和使用。

使用講解

RetryGatewayFilter是Spring Cloud Gateway對請求重試提供的一個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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配置講解

配置類源碼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：重試次數，默認值是3次
• series：狀態碼配置（分段），符合的某段狀態碼纔會進行重試邏輯，默認值是SERVER_ERROR，值是5，也就是5XX(5開頭的狀態碼)，共有5個值：

public enum Series {
    INFORMATIONAL(1),
    SUCCESSFUL(2),
    REDIRECTION(3),
    CLIENT_ERROR(4),
    SERVER_ERROR(5);
}
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• statuses：狀態碼配置，和series不一樣的是這邊是具體狀態碼的配置，取值請參考：org.springframework.http.HttpStatus
• methods：指定哪些方法的請求須要進行重試邏輯，默認值是GET方法，取值以下：

public enum HttpMethod {
	GET, HEAD, POST, PUT, PATCH, DELETE, OPTIONS, TRACE;
}
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• exceptions：指定哪些異常須要進行重試邏輯，默認值是java.io.IOException

代碼測試

就寫個接口，在接口中記錄請求次數，而後拋出一個異常模擬500，經過網關訪問這個接口，若是你配置了重試次數是3，那麼接口中會輸出4次結果纔是對的，證實重試生效了。

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小區");
}
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更多Spring Cloud代碼盡在：github.com/yinjihuan/s…

源碼欣賞

@Override
	public GatewayFilter apply(RetryConfig retryConfig) {
        // 驗證重試配置格式是否正確
		retryConfig.validate();

		Repeat<ServerWebExchange> statusCodeRepeat = null;
		if (!retryConfig.getStatuses().isEmpty() || !retryConfig.getSeries().isEmpty()) {
			Predicate<RepeatContext<ServerWebExchange>> repeatPredicate = context -> {
				ServerWebExchange exchange = context.applicationContext();
                // 判斷重試次數是否已經達到了配置的最大值
				if (exceedsMaxIterations(exchange, retryConfig)) {
					return false;
				}
                // 獲取響應的狀態碼
				HttpStatus statusCode = exchange.getResponse().getStatusCode();
                // 獲取請求方法類型
				HttpMethod httpMethod = exchange.getRequest().getMethod();
                // 判斷響應狀態碼是否在配置中存在
				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));
				}
                // 判斷方法是否包含在配置中
				boolean retryableMethod = retryConfig.getMethods().contains(httpMethod);
                // 決定是否要進行重試
				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;
				}
                // 異常判斷
				for (Class<? extends Throwable> clazz : retryConfig.getExceptions()) {             
					if (clazz.isInstance(context.exception())) {
						return true;
					}
				}
				return false;
			};
            // 使用reactor extra的retry組件
			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) -> {
                        // 獲取已經重試的次數，默認值爲-1
                        int iteration = exchange.getAttributeOrDefault(RETRY_ITERATION_KEY, -1);
                        // 增長重試次數
                        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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