Spring Cloud Gateway重試機制

前言

重試，我相信大家并不陌生。在我們調用Http接口的時候，總會因為某種原因調用失敗，這個時候我們可以通過重試的方式，來重新請求接口。

生活中這樣的事例很多，比如打電話，對方正在通話中啊，信号不好啊等等原因，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，讀資料的接口比較适合重試的場景，寫資料的接口就需要注意接口的幂等性了。還有就是重試次數如果太多的話會導緻請求量加倍，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。

今天我們來簡單的了解下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);

    // .....

}
           

● retries：重試次數，預設值是3次

● series：狀态碼配置（分段），符合的某段狀态碼才會進行重試邏輯，預設值是SERVER_ERROR，值是5，也就是5XX(5開頭的狀态碼)，共有5個值：

public enum Series {

    INFORMATIONAL(1),

    SUCCESSFUL(2),

    REDIRECTION(3),

    CLIENT_ERROR(4),

    SERVER_ERROR(5);

}
           

● statuses：狀态碼配置，和series不同的是這邊是具體狀态碼的配置，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：

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

代碼測試

就寫個接口，在接口中記錄請求次數，然後抛出一個異常模拟500，通過網關通路這個接口，如果你配置了重試次數是3，那麼接口中會輸出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");

}
           

更多Spring Cloud代碼盡在：

https://github.com/yinjihuan/spring-cloud

源碼欣賞

@Override
public GatewayFilter apply(RetryConfig retryConfig) {

    // 驗證重試配置格式是否正确

    retryConfig.validate();

    Repeat<ServerWebExchange> statusCodeRepeat = null;

    if (!retryConfig.getStatuses().isEmpty() || !retryConfig.getSeries().isEmpty()) {

        Predicate<RepeatContext<ServerWebExchange>> repeatPredicate = context -> {

            ServerWebExchange exchange = context.applicationContext();

            // 判斷重試次數是否已經達到了配置的最大值

            if (exceedsMaxIterations(exchange, retryConfig)) {

                return false;

            }

            // 擷取響應的狀态碼

            HttpStatus statusCode = exchange.getResponse().getStatusCode();

            // 擷取請求方法類型

            HttpMethod httpMethod = exchange.getRequest().getMethod();

            // 判斷響應狀态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) -> {

                    // 擷取已經重試的次數，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);

    };

}