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Apache NiFi 1.15.3 - Load balancing error on one cluster node due to java.net.UnknownHostException: null

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I have a 11 nodes NiFi cluster. Each node can correctly communicate to each other.
However when the other nodes try to connect to a node 021 for load balancing, the following error occurs (x put instead of real name):

 

ERROR [Load-Balanced Client Thread-8] o.a.n.c.q.c.c.a.n.NioAsyncLoadBalanceClient Unable to connect to xx021xxx.xxxxxxxx.xx:18090 for load balancing
java.net.UnknownHostException: null
at sun.nio.ch.Net.translateException(Net.java:154)
at sun.nio.ch.SocketAdaptor.connect(SocketAdaptor.java:127)
at sun.nio.ch.SocketAdaptor.connect(SocketAdaptor.java:80)
at org.apache.nifi.controller.queue.clustered.client.async.nio.NioAsyncLoadBalanceClient.createChannel(NioAsyncLoadBalanceClient.java:496)
at org.apache.nifi.controller.queue.clustered.client.async.nio.NioAsyncLoadBalanceClient.establishConnection(NioAsyncLoadBalanceClient.java:439)
at org.apache.nifi.controller.queue.clustered.client.async.nio.NioAsyncLoadBalanceClient.communicate(NioAsyncLoadBalanceClient.java:234)
at org.apache.nifi.controller.queue.clustered.client.async.nio.NioAsyncLoadBalanceClientTask.run(NioAsyncLoadBalanceClientTask.java:81)
at org.apache.nifi.engine.FlowEngine$2.run(FlowEngine.java:110)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)

The error can be seen on all nodes and only towards the node 021.

 

Load balancing properties are the following on ALL nodes (actual ip address instead of "ipv4_address"):

nifi.cluster.load.balance.host=ipv4_address
nifi.cluster.load.balance.port=6342
nifi.cluster.load.balance.connections.per.node=4
nifi.cluster.load.balance.max.thread.count=8
nifi.cluster.load.balance.comms.timeout=30 sec

While 18090, which is the port appearing in the error, is the HTTPS port.

Any idea on why does the error raise? And why does it appear just towards one node?

Thanks,
Emanuele


 

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