0down votefavorite <http://stackoverflow.com/questions/34763773/modifying-cassandras-threadpool-queues?noredirect=1#>
I've been meddling with Cassandra's (v 2.2.4) threadpool executors (namely SEPExecutor.java module) and trying to change the queues used for storing pending reads (that have no immediately available workers to serve). By default, Cassandra uses a ConcurrentLinkedQueue (which is a non-blocking queue variant). I'm currently trying to override this with a MultiQueue setup in order to schedule requests in non-FIFO order. Lets assume for simplicity that my MultiQueue implementation is an extension of AbstractQueue that simply overrides the offer and poll functions and randomly (de)queues requests to any of the enclosed ConcurrentLinkedQueues. For polling, if one queue returns null, we basically keep going through all the queues until we find a non-null element (otherwise we return null). There's no locking mechanism in place since my intention is to utilize the properties of the enclosed ConcurrentLinkedQueues (which are non-blocking). The main problem is that it seems I'm running into some sort of race condition, where some of the assigned workers can't poll an item that supposedly exists in the queue. In other words, the MultiQueue structure appears to be non-linearizable. More specifically, I'm encountering a NullPointerException on this line: SEPWorker.java [line 105] <https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/concurrent/SEPWorker.java#L105> Any clue as to what could be causing this, or how should I go about maintaining the properties of a single ConcurrentLinkedQueue in a MultiQueue setup?
