一个Camel Multicast组件聚合策略问题的解决过程

摘要：本文通过案例，发现了一个Camel Multicast组件聚合策略相关的问题。通过查看Camel源代码，找到了问题原因并给出了解决方案。希望本文可以帮助到遇到同样问题的Camel用户。

本文分享自华为云社区《使用Apache Camel Multicast组件遇到的一个问题》，作者：中间件小哥。

1 前言

本文翻译自华为加拿大研究所的Reji Mathews发表于Apache Camel社区的《ROUTING MULTICAST OUTPUT AFTER ENCOUNTERING PARTIAL FAILURES》一文。在征得原作者同意后，本文对原文的部分内容作了少许修改。

2 Multicast组件简介

Multicast是Apache Camel（以下简称“Camel”）中一个功能强大的EIP组件，可以将消息发送至多条子路径，然后并行地执行它们。

参考官网文档，我们可以使用两种方式配置Multicast组件：

独立执行所有子路径，并将最后响应的子路径的结果作为最终输出。这也是Multicast组件的默认配置。
通过实现Camel的聚合策略（Aggregation Strategy），使用自定义的聚合器来处理所有子路径的输出。

3 问题描述

本文使用案例如下：使用Jetty组件发布一个API，调用该API后，消息会分别发送至"direct:A"和"direct:B"两条子路径。在使用自定义的聚合策略处理后，继续执行后续步骤。其中在"direct:A"中抛出一个异常，来模拟运行失败；"direct:B"正常运行。同时在onException中定义了异常处理策略。

本文使用的Camel版本为3.8.0

@Override
public void configure() throws Exception {onException(Exception.class).useOriginalMessage().handled(true).log("Exception handler invoked").transform().constant("{\"data\" : \"err\"}").end();from("jetty:http://localhost:8081/myapi?httpMethodRestrict=GET").log("received request").log("Entering multicast").multicast(new SimpleFlowMergeAggregator()).parallelProcessing().to("direct:A", "direct:B").end().log("Aggregated results ${body}").log("Another log").transform(simple("{\"result\" : \"success\"}")).end();from("direct:A").log("Executing PATH_1 - exception path").transform(constant("DATA_FROM_PATH_1")).log("Starting exception throw").throwException(new Exception("USER INITIATED EXCEPTION")).log("PATH_1").end();from("direct:B").log("Executing PATH_2 - success path").delayer(1000).transform(constant("DATA_FROM_PATH_2")).log("PATH_2").end();
}

自定义聚合器SimpleFlowMergeAggregator定义如下，其中我们将所有子路径的结果放入一个list对象。

public class SimpleFlowMergeAggregator implements AggregationStrategy {private static final Logger LOGGER = LoggerFactory.getLogger(SimpleFlowMergeAggregator.class.getName());@Overridepublic Exchange aggregate(Exchange oldExchange, Exchange newExchange) {LOGGER.info("Inside aggregator " + newExchange.getIn().getBody());if(oldExchange == null) {String data = newExchange.getIn().getBody(String.class);List<String> aggregatedDataList = new ArrayList<>();aggregatedDataList.add(data);newExchange.getIn().setBody(aggregatedDataList);return newExchange;}List<String> oldData = oldExchange.getIn().getBody(List.class);oldData.add(newExchange.getIn().getBody(String.class));oldExchange.getIn().setBody(oldData);return oldExchange;}
}

基于对Multicast组件执行逻辑的理解，我们认为存在多个子路径时，其运行结果应该为：如果其中有一条子路径能运行成功，则使用聚合的结果继续执行后续步骤；如果所有子路径都运行失败，则停止整个路由（route）。本案例中，由于子路径"direct:A"运行异常，子路径"direct:B"运行正常，则应该正常执行后续两个步骤日志（log）和转换（transform）。

运行上述案例，日志信息如下：

2021-05-06 12:43:18.565 INFO 13956 --- [qtp916897446-42] route1 : received request
2021-05-06 12:43:18.566 INFO 13956 --- [qtp916897446-42] route1 : Entering multicast
2021-05-06 12:43:18.575 INFO 13956 --- [ #4 - Multicast] route2 : Executing PATH_1 - exception path
2021-05-06 12:43:18.575 INFO 13956 --- [ #4 - Multicast] route2 : Starting exception throw
2021-05-06 12:43:18.578 INFO 13956 --- [ #4 - Multicast] route2 : Exception handler invoked
2021-05-06 12:43:18.579 INFO 13956 --- [ #4 - Multicast] c.e.d.m.SimpleFlowMergeAggregator : Inside aggregator {"data" : "err"}
2021-05-06 12:43:19.575 INFO 13956 --- [ #3 - Multicast] route3 : Executing PATH_2 - success path
2021-05-06 12:43:21.576 INFO 13956 --- [ #3 - Multicast] route3 : PATH_2
2021-05-06 12:43:21.576 INFO 13956 --- [ #3 - Multicast] c.e.d.m.SimpleFlowMergeAggregator : Inside aggregator DATA_FROM_PATH_2

观察上述日志，我们发现完成两条子路径结果的聚合后，后续的两个步骤日志（log）和转换（transform）并未执行。这并不符合我们期望的结果。

经过多次测试，我们还发现，只有当到达聚合器SimpleFlowMergeAggregator的第一个子路径（"direct:A"）执行异常时，便会发生这种后续步骤未执行的情况；而如果第一个子路径（"direct:A"）执行成功，即使另一个子路径（"direct:B"）执行失败，也会继续执行后续的步骤。

4 问题分析

接下来，我们通过查看Camel源代码，来找出上述现象的原因。

在camel-core-processors模块的Pipeline.java 中，其run()方法中有这样一段代码：

@Override
public void run() {boolean stop = exchange.isRouteStop();int num = index;boolean more = num < size;boolean first = num == 0;if (!stop && more && (first || continueProcessing(exchange, "so breaking out of pipeline", LOG))) {// prepare for next runif (exchange.hasOut()) {exchange.setIn(exchange.getOut());exchange.setOut(null);}// get the next processorAsyncProcessor processor = processors.get(index++);processor.process(exchange, this);} else {// copyResults is needed in case MEP is OUT and the message is not an OUT messageExchangeHelper.copyResults(exchange, exchange);// logging nextExchange as it contains the exchange that might have altered the payload and since// we are logging the completion if will be confusing if we log the original instead// we could also consider logging the original and the nextExchange then we have *before* and *after* snapshotsif (LOG.isTraceEnabled()) {LOG.trace("Processing complete for exchangeId: {} >>> {}", exchange.getExchangeId(), exchange);}AsyncCallback cb = callback;taskFactory.release(this);reactiveExecutor.schedule(cb);}
}

其中，这个if判断决定了是否继续执行后续步骤：

if (!stop && more && (first || continueProcessing(exchange, "so breaking out of pipeline", LOG)))

可以看出，在如下三种情况下，后续步骤将不会被执行：

1. 之前的步骤已经将exchange 对象标记为停止状态。

boolean stop = exchange.isRouteStop();

2. 后续没有步骤可执行。

boolean more = num < size;

3. continueProcessing()方法返回false。

我们来看看continueProcessing()方法的代码。

public final class PipelineHelper {public static boolean continueProcessing(Exchange exchange, String message, Logger log) {ExtendedExchange ee = (ExtendedExchange) exchange;boolean stop = ee.isFailed() || ee.isRollbackOnly() || ee.isRollbackOnlyLast()|| (ee.isErrorHandlerHandledSet() && ee.isErrorHandlerHandled());if (stop) {if (log.isDebugEnabled()) {StringBuilder sb = new StringBuilder();sb.append("Message exchange has failed: ").append(message).append(" for exchange: ").append(exchange);if (exchange.isRollbackOnly() || exchange.isRollbackOnlyLast()) {sb.append(" Marked as rollback only.");}if (exchange.getException() != null) {sb.append(" Exception: ").append(exchange.getException());}if (ee.isErrorHandlerHandledSet() && ee.isErrorHandlerHandled()) {sb.append(" Handled by the error handler.");}log.debug(sb.toString());}return false;}if (ee.isRouteStop()) {if (log.isDebugEnabled()) {log.debug("ExchangeId: {} is marked to stop routing: {}", exchange.getExchangeId(), exchange);}return false;}return true;}
}

可以看出，当执行过程发生异常并且被异常处理器捕获时，continueProcessing()方法将返回false。

再回到我们的案例，第一个到达聚合器SimpleFlowMergeAggregator的子路径（"direct:A"），会作为后续聚合的基础，其它子路径（"direct:B"）会在此基础上追加各自的body数据。实际上，很多Camel用户都会采用这种方式来实现自定义聚合策略。但这样做存在一个问题：在异常处理时，子路径"direct:A"的exchange对象会被设置一个状态标识，而此状态标识会被传递到下游，用于判断是否继续执行后续步骤。由于作为聚合基础的"direct:A"子路径的exchange对象状态为“异常”，最终continueProcessing()方法将返回false，后续的步骤也就不会再执行。

5 解决方案

对于上述问题，用户可以使用多种方式来设置异常处理时exchange对象的状态。本文采用如下解决方案：如果第一个子路径执行正常，则继续执行后续步骤；如果第一个子路径执行异常，则将其与其它执行成功的子路径交换，然后继续执行后续步骤。

更新后的自定义聚合器SimpleFlowMergeAggregator如下：

public class SimpleFlowMergeAggregator implements AggregationStrategy {private static final Logger LOGGER = LoggerFactory.getLogger(SimpleFlowMergeAggregator.class.getName());@Overridepublic Exchange aggregate(Exchange oldExchange, Exchange newExchange) {LOGGER.info("Inside aggregator " + newExchange.getIn().getBody());if(oldExchange == null) {String data = newExchange.getIn().getBody(String.class);List<String> aggregatedDataList = new ArrayList<>();aggregatedDataList.add(data);newExchange.getIn().setBody(aggregatedDataList);return newExchange;}if(hadException(oldExchange)) {if(!hadException(newExchange)) {// aggregate and swap the baseLOGGER.info("Found new exchange with success. swapping the base exchange");List<String> oldData = oldExchange.getIn().getBody(List.class);oldData.add(newExchange.getIn().getBody(String.class));// swapped the base herenewExchange.getIn().setBody(oldData);                 return newExchange;}}List<String> oldData = oldExchange.getIn().getBody(List.class);oldData.add(newExchange.getIn().getBody(String.class));oldExchange.getIn().setBody(oldData);return oldExchange;}private boolean hadException(Exchange exchange) {if(exchange.isFailed()) {return true;}if(exchange.isRollbackOnly()) {return true;}if(exchange.isRollbackOnlyLast()) {return true;}if(((ExtendedExchange)exchange).isErrorHandlerHandledSet()&& ((ExtendedExchange)exchange).isErrorHandlerHandled()) {return true;}return false;}
}

再次运行上述案例，日志信息如下：

2021-05-06 12:46:19.122 INFO 2576 --- [qtp174245837-45] route1 : received request
2021-05-06 12:46:19.123 INFO 2576 --- [qtp174245837-45] route1 : Entering multicast
2021-05-06 12:46:19.130 INFO 2576 --- [ #3 - Multicast] route2 : Executing PATH_1 - exception path
2021-05-06 12:46:19.130 INFO 2576 --- [ #3 - Multicast] route2 : Starting exception throw
2021-05-06 12:46:19.134 INFO 2576 --- [ #3 - Multicast] route2 : Exception handler invoked
2021-05-06 12:46:19.135 INFO 2576 --- [ #3 - Multicast] c.e.d.m.SimpleFlowMergeAggregator : Inside aggregator {"data" : "err"}
2021-05-06 12:46:20.130 INFO 2576 --- [ #4 - Multicast] route3 : Executing PATH_2 - success path
2021-05-06 12:46:22.132 INFO 2576 --- [ #4 - Multicast] route3 : PATH_2
2021-05-06 12:46:22.132 INFO 2576 --- [ #4 - Multicast] c.e.d.m.SimpleFlowMergeAggregator : Inside aggregator DATA_FROM_PATH_2
2021-05-06 12:46:22.132 INFO 2576 --- [ #4 - Multicast] c.e.d.m.SimpleFlowMergeAggregator : Found new exchange with success. swapping the base exchange
2021-05-06 12:46:22.133 INFO 2576 --- [ #4 - Multicast] route1 : Aggregated results {"data" : "err"},DATA_FROM_PATH_2
2021-05-06 12:46:22.133 INFO 2576 --- [ #4 - Multicast] route1 : Another log

可以看出，使用新的自定义聚合策略后，后续的日志（log）和转换（transform）步骤都成功执行。

6 结语

本文通过案例，发现了一个Camel Multicast组件聚合策略相关的问题。通过查看Camel源代码，找到了问题原因并给出了解决方案。

希望本文可以帮助到遇到同样问题的Camel用户。

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