I can see intermittently NPE [1] during failover scenario with EAP undertow used for load balancing using mod_cluster filter. The NPE occurs on the load balancer side.

The failover is caused by having one of workers killed using force kill signal on JVM.

It is intermittent and rather rarely occurring issue.

[1]
2018-02-19 07:35:56,524 ERROR [io.undertow.client] (default I/O-8) UT005001: An exception occurred processing the request: java.lang.NullPointerException
at io.undertow.client.http.HttpClientConnection$ClientReadListener.handleEvent(HttpClientConnection.java:554)
at io.undertow.client.http.HttpClientConnection$ClientReadListener.handleEvent(HttpClientConnection.java:511)
at org.xnio.ChannelListeners.invokeChannelListener(ChannelListeners.java:92)
at org.xnio.conduits.ReadReadyHandler$ChannelListenerHandler.readReady(ReadReadyHandler.java:66)
at io.undertow.protocols.ssl.SslConduit$SslReadReadyHandler.readReady(SslConduit.java:1131)
at org.xnio.nio.NioSocketConduit.handleReady(NioSocketConduit.java:89)
at org.xnio.nio.WorkerThread.run(WorkerThread.java:591)

2018-02-19 07:35:56,526 ERROR [org.xnio.listener] (default I/O-8) XNIO001007: A channel event listener threw an exception: java.lang.NullPointerException
at io.undertow.client.http.HttpClientConnection$ClientReadListener.handleEvent(HttpClientConnection.java:661)
at io.undertow.client.http.HttpClientConnection$ClientReadListener.handleEvent(HttpClientConnection.java:511)
at org.xnio.ChannelListeners.invokeChannelListener(ChannelListeners.java:92)
at org.xnio.conduits.ReadReadyHandler$ChannelListenerHandler.readReady(ReadReadyHandler.java:66)
at io.undertow.protocols.ssl.SslConduit$SslReadReadyHandler.readReady(SslConduit.java:1131)
at org.xnio.nio.NioSocketConduit.handleReady(NioSocketConduit.java:89)
at org.xnio.nio.WorkerThread.run(WorkerThread.java:591)

2018-02-19 07:37:46,510 ERROR [org.xnio.listener] (default I/O-8) XNIO001007: A channel event listener threw an exception: java.lang.NullPointerException
at io.undertow.client.http.HttpRequestConduit.processWrite(HttpRequestConduit.java:102)
at io.undertow.client.http.HttpRequestConduit.flush(HttpRequestConduit.java:660)
at io.undertow.conduits.AbstractFixedLengthStreamSinkConduit.flush(AbstractFixedLengthStreamSinkConduit.java:229)
at org.xnio.conduits.ConduitStreamSinkChannel.flush(ConduitStreamSinkChannel.java:162)
at org.xnio.ChannelListeners$14.handleEvent(ChannelListeners.java:413)
at org.xnio.ChannelListeners$14.handleEvent(ChannelListeners.java:409)
at org.xnio.ChannelListeners.invokeChannelListener(ChannelListeners.java:92)
at org.xnio.conduits.WriteReadyHandler$ChannelListenerHandler.writeReady(WriteReadyHandler.java:65)
at io.undertow.protocols.ssl.SslConduit$SslWriteReadyHandler.writeReady(SslConduit.java:1227)
at io.undertow.protocols.ssl.SslConduit$3.run(SslConduit.java:275)
at org.xnio.nio.WorkerThread.safeRun(WorkerThread.java:612)
at org.xnio.nio.WorkerThread.run(WorkerThread.java:479)

2018-02-19 07:37:46,511 ERROR [org.xnio.listener] (default I/O-8) XNIO001007: A channel event listener threw an exception: java.lang.NullPointerException
at io.undertow.client.http.HttpRequestConduit.processWrite(HttpRequestConduit.java:102)
at io.undertow.client.http.HttpRequestConduit.flush(HttpRequestConduit.java:660)
at io.undertow.conduits.AbstractFixedLengthStreamSinkConduit.flush(AbstractFixedLengthStreamSinkConduit.java:229)
at org.xnio.conduits.ConduitStreamSinkChannel.flush(ConduitStreamSinkChannel.java:162)
at org.xnio.ChannelListeners$14.handleEvent(ChannelListeners.java:413)
at org.xnio.ChannelListeners$14.handleEvent(ChannelListeners.java:409)
at org.xnio.ChannelListeners.invokeChannelListener(ChannelListeners.java:92)
at org.xnio.conduits.WriteReadyHandler$ChannelListenerHandler.writeReady(WriteReadyHandler.java:65)
at io.undertow.protocols.ssl.SslConduit$SslWriteReadyHandler.writeReady(SslConduit.java:1227)
at io.undertow.protocols.ssl.SslConduit$3.run(SslConduit.java:275)
at org.xnio.nio.WorkerThread.safeRun(WorkerThread.java:612)
at org.xnio.nio.WorkerThread.run(WorkerThread.java:479)