Alex Dettinger commented on CAMEL-10272:

Peter, thanks for your inputs.

  As you don't use _parallelAggregate()_, it may not be the cause of this issue.
However, it's interesting to note that I could also produce _oldExchange_ to be 
null more than once with _parallelAggregate()_, even with a thread safe 
aggregation strategy.
It may be an itch to scratch in another ticket.

  Back to this issue, I will propose a PR to log such exceptions in DEBUG level 
so one could have more details when facing such a situation.
We could make a great step forward if you could reproduce the behavior once 
while logging/unwinding throwables from your aggregation strategy.
Or you could also test against master with debug log level if the PR is 

> Aggregation is broken due to race condition in 
> ParallelAggregateTask.doAggregateInternal()
> ------------------------------------------------------------------------------------------
>                 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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