聊聊spring cloud gateway的RetryGatewayFilter

序

本文主要研究一下spring cloud gateway的RetryGatewayFilter

GatewayAutoConfiguration

spring-cloud-gateway-core-2.0.0.RC2-sources.jar!/org/springframework/cloud/gateway/config/GatewayAutoConfiguration.java

@Configuration
@ConditionalOnProperty(name = "spring.cloud.gateway.enabled", matchIfMissing = true)
@EnableConfigurationProperties
@AutoConfigureBefore(HttpHandlerAutoConfiguration.class)
@AutoConfigureAfter({GatewayLoadBalancerClientAutoConfiguration.class, GatewayClassPathWarningAutoConfiguration.class})
@ConditionalOnClass(DispatcherHandler.class)
public class GatewayAutoConfiguration {
 //......
	@Bean
	public RetryGatewayFilterFactory retryGatewayFilterFactory() {
		return new RetryGatewayFilterFactory();
	}
 //......
}

默认启用了RetryGatewayFilterFactory

RetryGatewayFilterFactory

spring-cloud-gateway-core-2.0.0.RC2-sources.jar!/org/springframework/cloud/gateway/filter/factory/RetryGatewayFilterFactory.java

public class RetryGatewayFilterFactory extends AbstractGatewayFilterFactory<RetryGatewayFilterFactory.RetryConfig> {
	private static final Log log = LogFactory.getLog(RetryGatewayFilterFactory.class);

	public RetryGatewayFilterFactory() {
		super(RetryConfig.class);
	}

	@Override
	public GatewayFilter apply(RetryConfig retryConfig) {
		retryConfig.validate();

		Predicate<? super RepeatContext<ServerWebExchange>> predicate = context -> {
			ServerWebExchange exchange = context.applicationContext();
			if (exceedsMaxIterations(exchange, retryConfig)) {
				return false;
			}

			HttpStatus statusCode = exchange.getResponse().getStatusCode();
			HttpMethod httpMethod = exchange.getRequest().getMethod();

			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));
			}

			boolean retryableMethod = retryConfig.getMethods().contains(httpMethod);
			return retryableMethod && retryableStatusCode;
		};

		Repeat<ServerWebExchange> repeat = Repeat.onlyIf(predicate)
				.doOnRepeat(context -> reset(context.applicationContext()));

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

		Predicate<RetryContext<ServerWebExchange>> retryContextPredicate = context -> {
			if (exceedsMaxIterations(context.applicationContext(), retryConfig)) {
				return false;
			}

			for (Class<? extends Throwable> clazz : retryConfig.getExceptions()) {
				if (clazz.isInstance(context.exception())) {
					return true;
				}
			}
 return false;
		};

		Retry<ServerWebExchange> reactorRetry = Retry.onlyIf(retryContextPredicate)
				.doOnRetry(context -> reset(context.applicationContext()))
				.retryMax(retryConfig.getRetries());
		return apply(repeat, reactorRetry);
	}

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

		//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);
	}

	@Deprecated
	public GatewayFilter apply(Repeat<ServerWebExchange> repeat) {
		return apply(repeat, Retry.onlyIf(ctxt -> false));
	}

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

			int iteration = exchange.getAttributeOrDefault("retry_iteration", -1);
			exchange.getAttributes().put("retry_iteration", iteration + 1);

			return Mono.fromDirect(chain.filter(exchange)
					.log("retry-filter", Level.INFO)
					.retryWhen(retry.withApplicationContext(exchange))
					.repeatWhen(repeat.withApplicationContext(exchange)));
		};
	}
 //......
}

• 可以看到这个filter使用了reactor的Retry组件，同时往exchange的attribues添加retry_iteration，用来记录重试次数，该值默认从-1开始，第一次执行的时候，retry_iteration+1为0。之后每重试一次，就添加1。
• filter的apply接收两个参数，一个是Repeat<ServerWebExchange>，一个是Retry<ServerWebExchange>。
• repeat与retry的区别是repeat是在onCompleted的时候会重试，而retry是在onError的时候会重试。这里由于不一定是异常的时候才可能重试，所以加了repeat。

RetryConfig

spring-cloud-gateway-core-2.0.0.RC2-sources.jar!/org/springframework/cloud/gateway/filter/factory/RetryGatewayFilterFactory.java

	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);

		//......

		public void validate() {
			Assert.isTrue(this.retries > 0, "retries must be greater than 0");
			Assert.isTrue(!this.series.isEmpty() || !this.statuses.isEmpty(),
					"series and status may not both be empty");
			Assert.notEmpty(this.methods, "methods may not be empty");
		}
 //......

	}

可以看到配置文件有5个属性，如下：

• retries，默认为3，用来标识重试次数
• series，用来指定哪些段的状态码需要重试，默认SERVER_ERROR即5xx
• statuses，用于指定哪些状态需要重试，默认为空，它跟series至少得指定一个
• methods，用于指定那些方法的请求需要重试，默认为GET
• exceptions，用于指定哪些异常需要重试，默认为java.io.IOException

实例

spring:
 cloud:
 gateway:
 routes:
 - id: retry-demo
 uri: http://localhost:9090
 predicates:
 - Path=/retry/**
 filters:
 - name: Retry
 args:
 retries: 15
 series:
 - SERVER_ERROR
 - CLIENT_ERROR
 methods:
 - GET
 - POST
 exceptions:
 - java.io.IOException
 - java.util.concurrent.TimeoutException

这里指定了针对4xx及5xx的GET或POST方法或者IOException或TimeoutException的时候进行重试，重试次数为15次。

小结

RetryGatewayFilter借助了reactor-addons的retry组件进行了重试，主要使用了Mono的repeatWhen及retryWhen方法，前者在onCompleted的时候触发，后者在onError的时候触发。

doc

• reactor-extra-retry
• 聊聊reactor extra的retry
• RxJava's repeatWhen and retryWhen, explained