I should have said that this was using the embedded option for the coordinator. I managed to find the solution by way of an old mailinglist thread (from 2016) [1] that suggested starting the nodes at the same time, rather than one after the other. The reason is that if you start up one node, it will go into a connection frenzy failing to contact the other nodes, quickly hitting the connection limit (and leaving lots of dangling connections in "close wait" state). [1] http://apache-nifi.1125220.n5.nabble.com/Zookeeper-error-td13355.html ... View more