UPDATE: Further analyses have shown that the EventJobManager was not the cause of this. See the 2nd comment for a proper description of this problem.
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When bombarding the enhancer with multiple concurrent EnhancementJobs the EvenJobManager might not correctly process all requests due to changes that do not correctly apply a writeLock on the EnhancementJob.
As fixing those things is not an easy thing I implemented already an new Integration-Test that allows to send long abstracts from dbpedia as content to the enhancer. The integration-test includes enough data for 10.000 requests. It uses "java.util.concurrent.ExecutorService" for async Requests and the "PoolingClientConnectionManager" of apache http commons for sending multiple parallel requests.
Setting this to 1000 requests with 10 threads lets easily to reproduce the problem.