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：
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
@Overridepublic 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();}
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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()); @Override public 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; }}
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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 request2021-05-06 12:43:18.566 INFO 13956 --- [qtp916897446-42] route1 : Entering multicast2021-05-06 12:43:18.575 INFO 13956 --- [ #4 - Multicast] route2 : Executing PATH_1 - exception path2021-05-06 12:43:18.575 INFO 13956 --- [ #4 - Multicast] route2 : Starting exception throw2021-05-06 12:43:18.578 INFO 13956 --- [ #4 - Multicast] route2 : Exception handler invoked2021-05-06 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 path2021-05-06 12:43:21.576 INFO 13956 --- [ #3 - Multicast] route3 : PATH_22021-05-06 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：
@Overridepublic 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 run if (exchange.hasOut()) { exchange.setIn(exchange.getOut()); exchange.setOut(null); } // get the next processor AsyncProcessor processor = processors.get(index++); processor.process(exchange, this); } else { // copyResults is needed in case MEP is OUT and the message is not an OUT message ExchangeHelper.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* snapshots if (LOG.isTraceEnabled()) { LOG.trace("Processing complete for exchangeId: {} >>> {}", exchange.getExchangeId(), exchange); } AsyncCallback cb = callback; taskFactory.release(this); reactiveExecutor.schedule(cb); }}
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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)))
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可以看出，在如下三种情况下，后续步骤将不会被执行：
之前的步骤已经将 exchange 对象标记为停止状态。
boolean stop = exchange.isRouteStop();
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2. 后续没有步骤可执行。
boolean more = num < size;
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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; }}
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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()); @Override public 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 base LOGGER.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 here newExchange.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; }}
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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 request2021-05-06 12:46:19.123 INFO 2576 --- [qtp174245837-45] route1 : Entering multicast2021-05-06 12:46:19.130 INFO 2576 --- [ #3 - Multicast] route2 : Executing PATH_1 - exception path2021-05-06 12:46:19.130 INFO 2576 --- [ #3 - Multicast] route2 : Starting exception throw2021-05-06 12:46:19.134 INFO 2576 --- [ #3 - Multicast] route2 : Exception handler invoked2021-05-06 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 path2021-05-06 12:46:22.132 INFO 2576 --- [ #4 - Multicast] route3 : PATH_22021-05-06 12:46:22.132 INFO 2576 --- [ #4 - Multicast] c.e.d.m.SimpleFlowMergeAggregator : Inside aggregator DATA_FROM_PATH_22021-05-06 12:46:22.132 INFO 2576 --- [ #4 - Multicast] c.e.d.m.SimpleFlowMergeAggregator : Found new exchange with success. swapping the base exchange2021-05-06 12:46:22.133 INFO 2576 --- [ #4 - Multicast] route1 : Aggregated results {"data" : "err"},DATA_FROM_PATH_22021-05-06 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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