Spring Cloud Gateway重试机制

前言

重试，我相信大家并不陌生。在我们调用Http接口的时候，总会因为某种原因调用失败，这个时候我们可以通过重试的方式，来重新请求接口。

生活中这样的事例很多，比如打电话，对方正在通话中啊，信号不好啊等等原因，你总会打不通，当你第一次没打通之后，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，写数据的接口就需要注意接口的幂等性了。还有就是重试次数如果太多的话会导致请求量加倍，给后端造成更大的压力，设置合理的重试机制才是最关键的。

今天我们来简单的了解下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

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);
        
    // .....
}

• 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);
}

• statuses：状态码配置，和series不同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需要进行重试逻辑，默认值是GET方法，取值如下：

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

• exceptions：指定哪些异常需要进行重试逻辑，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，然后抛出一个异常模拟500，通过网关访问这个接口，如果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，证明重试生效了。

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");
}

更多Spring Cloud代码尽在：https://github.com/yinjihuan/spring-cloud

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);
                        // 增加重试次数
                        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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