Had a Zookeeper cluster go down and after things came back up NiFi seemed stuck and wouldn't ever reestablish the cluster. The following error was repeated on the node that wouldn't rejoin. Googling the first message mentions a bug in the curator library that causes it to never reconnect to Zookeeper after an issue. See https://issues.apache.org/jira/browse/CURATOR-405 for an example. This is on 1.14.0 against Zookeeper 3.6.3
2021-09-15 18:03:20,644 WARN [Curator-ConnectionStateManager-0] o.a.c.f.state.ConnectionStateManager Session timeout has elapsed while SUSPENDED. Injecting a session expiration. Elapsed ms: 10000. Adjusted session timeout ms: 10000 2021-09-15 18:03:25,201 WARN [Clustering Tasks Thread-2] o.apache.nifi.controller.FlowController Failed to send heartbeat due to: org.apache.nifi.cluster.protocol.ProtocolException: Cannot send heartbeat because there is no Cluster Coordinator currently elected
