I had a queue that built up overnight with several thousand flowfiles. The queue was pointing to a RouteOnAttribute processor, which was running. For some reason, the RouteOnAttribute processor wasn't emptying the queue so the queue just built up. Stopping and starting the processor didn't help. However, simply restarting Nifi got the flow moving again. I can't figure out why the queue was stuck.

What could cause a queue to build up like that? How would restarting Nifi get it going again? There weren't any errors in the log.  I did have backpressure on the relationship to the processor after the RouteOnAttribute set to 1, but that queue was empty. There was no backpressure on the queue heading in to the RouteOnAttribute processor. The RouteOnAttribute processor has a Penalty Duration and a Yield of 1 sec. The relationships all used the PriorityAttributeAnalyzer as the prioritizer.

The interesting thing is I had the exact same problem on two different nifis (one was version 0.2.0 and one 0.3.0). The same type of flow files were going from one to the other using site-to-site (I was doing this for load balancing). On the other Nifi, instead of restarting, I added another RouteOnAttribute processor and rerouted the stuck queue to the new one instead, and that also got things moving again. (I have a backpressure since the processor after the RouteOnAttribute is a ControlRate processor, and I only want to allow 1 flowfile through per second so as not to overwhelm the system downstream. The backpressure is to assist with load balancing, so if one pathway fills up, I have the other pathway which uses the site-to-site to route the flow to the other nifi).

Is there some combination of backpressure, penalty or yield along with prioritization that could cause a kind of deadlock-like situation? Any ideas as to how I can prevent this from occurring?