前言

重試，我相信大家並不陌生。在我們呼叫Http介面的時候，總會因為某種原因呼叫失敗，這個時候我們可以通過重試的方式，來重新請求介面。

生活中這樣的事例很多，比如打電話，對方正在通話中啊，訊號不好啊等等原因，你總會打不通，當你第一次沒打通之後，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，讀資料的介面比較適合重試的場景，寫資料的介面就需要注意介面的冪等性了。還有就是重試次數如果太多的話會導致請求量加倍，給後端造成更大的壓力，設定合理的重試機制才是最關鍵的。

今天我們來簡單的瞭解下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

配置講解

配置類原始碼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);

    // .....
}

• retries：重試次數，預設值是3次
• series：狀態碼配置（分段），符合的某段狀態碼才會進行重試邏輯，預設值是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);
}

• statuses：狀態碼配置，和series不同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需要進行重試邏輯，預設值是GET方法，取值如下：

public enum HttpMethod {
    GET, HEAD, POST, PUT, PATCH, DELETE, OPTIONS, TRACE;
}

• exceptions：指定哪些異常需要進行重試邏輯，預設值是java.io.IOException

程式碼測試

就寫個介面，在介面中記錄請求次數，然後丟擲一個異常模擬500，通過閘道器訪問這個介面，如果你配置了重試次數是3，那麼介面中會輸出4次結果才是對的，證明重試生效了。

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");
}

原始碼欣賞

    @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();
                // 獲取請求方法型別
                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元件
            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，預設值為-1
                        int iteration = exchange.getAttributeOrDefault(RETRY_ITERATION_KEY, -1);
                        // 增加重試次數
                        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);
        };
    }