code: public BlockingQueue<CollectionDocument> registerQueue(String queueName, int capacity) {
  		Preconditions.checkArgument(!registry.containsKey(queueName), "Queue %s already exists", queueName);
  		ArrayBlockingQueue<CollectionDocument> q = Queues.newArrayBlockingQueue(capacity);
  		registry.put(queueName, q);
  		return q;
  	}
docs: Creates and registers a new {@link ArrayBlockingQueue} with given initial
  capacity.
  
  @param queueName
  The name of the queue
  @param capacity
  The initial capacity of the queue
  @return
  the created and registered queue
  
  @throws IllegalArgumentException if <code>queueName</code> is already registered.