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https://issues.apache.org/jira/browse/CAMEL-10272?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Peter Keller updated CAMEL-10272:
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Summary: Unexpected behaviour in aggregator if recipient list is processed
in parallel (was: Aggregation is broken due to race condition in
ParallelAggregateTask.doAggregateInternal())
> Unexpected behaviour in aggregator if recipient list is processed in parallel
> -----------------------------------------------------------------------------
>
> Key: CAMEL-10272
> URL: https://issues.apache.org/jira/browse/CAMEL-10272
> Project: Camel
> Issue Type: Bug
> Components: camel-core
> Affects Versions: 2.16.3, 2.17.3
> Environment: MacOS 10.11.6, JRE 1.7.0_79
> Reporter: Peter Keller
>
> Unfortunately, I am not able to provide a (simple) unit test for
> comprehending the problem. Furthermore our (complex) unit tests are not
> deterministic due to the root cause of the problem.
> However I tried to analyze the Camel Java code, to work out the problem.
> Please find below my findings.
> h3. Problem
> The {{oldExchange}} is {{null}} more than once in the aggregator if a
> recipient list is processed in parallel.
> h3. Camel route
> In my Camel route, a recipient list is worked of in parallel:
> {code}
> from("direct:start")
> .to("direct:pre")
> .recipientList().method(new MyRecipientListBuilder())
> .stopOnException()
> .aggregationStrategy(new MyAggregationStrategy())
> .parallelProcessing()
> .end()
> .bean(new MyPostProcessor());
> {code}
> Snippet of {{MyAggregationStrategy}}:
> {code}
> @Override
> @SuppressWarnings("unchecked")
> public Exchange aggregate(final Exchange oldExchange, final Exchange
> newExchange) {
> if (oldExchange == null) {
> // this is the case more than once which is not expected!
> }
> // ...
> {code}
> {{oldExchange}} is null more than once which is not expected and which
> contradicts the contract with Camel.
> h3. Analysis
> During the processing, Camel invokes {{MulticastProcessor.process()}}. Here
> the result object {{AtomicExchange}} is created which is shared during the
> whole processing.
> If the processing should be done in parallel (as it is the case for our
> route) then {{MulticastProcessor.doProcessParallel()}} is invoked. Here one
> instance of {{AggregateOnTheFlyTask}} is initialized and
> {{aggregateOnTheFly()}} is invoked -*asynchronously* via {{run()}} for
> *every* target in the recipient list-. via
> {{aggregateExecutorService.submit}} ({{aggregationTaskSubmitted}} guarantees
> that this is only be done once)
> In {{aggregateOnTheFly()}}, a new instance of {{ParallelAggregateTask}} is
> generated, and if aggregation is not done in parallel (as it is the case in
> our route), {{ParallelAggregateTask.run()}},
> {{ParallelAggregateTask.doAggregate()}} (this method is synchronized), and
> {{ParallelAggregateTask.doAggregateInternal()}} is invoked synchronously:
> {code}
> protected void doAggregateInternal(AggregationStrategy strategy,
> AtomicExchange result, Exchange exchange) {
> if (strategy != null) {
> // prepare the exchanges for aggregation
> Exchange oldExchange = result.get();
> ExchangeHelper.prepareAggregation(oldExchange, exchange);
> result.set(strategy.aggregate(oldExchange, exchange));
> }
> }
> {code}
> However, in {{ParallelAggregateTask.doAggregateInternal()}} there may occur a
> race condition as {{result}} is shared -by every instance of
> {{AggregateOnTheFlyTask}}- such that {{oldExchange = result.get()}} may be
> {{null}} more than once!
> Note: As a new instance of {{ParallelAggregateTask}} for every target in
> recipient list is created, the {{synchronized}} method
> {{ParallelAggregateTask.doAggregate()}} does not prevent the race condition!
> Does this sounds reasonably?
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