Hi,
We could see that nodes are disconnecting frequently from cluster. Below are the errors from Master and data nodes. We are using ELK version 5.3.1. Could you please help us to prevent the node disconnection issue?
**Error from Master node :**
[2021-02-22T12:38:16,153][INFO ][o.e.c.r.a.AllocationService] [elk-denmod-web] Cluster health status changed from [GREEN] to [YELLOW] (reason: [{elk-denmod-1}{t7JmFe-iSn63CALYD-0lxw}{7aHU8PepTH-PAzPrnr-q8w}{168.124.25.140}{168.124.25.140:9300} transport disconnected]).
[2021-02-22T12:38:16,153][INFO ][o.e.c.s.ClusterService ] [elk-denmod-web] removed {{elk-denmod-1}{t7JmFe-iSn63CALYD-0lxw}{7aHU8PepTH-PAzPrnr-q8w}{168.124.25.140}{168.124.25.140:9300},}, reason: zen-disco-node-failed({elk-denmod-1}{t7JmFe-iSn63CALYD-0lxw}{7aHU8PepTH-PAzPrnr-q8w}{168.124.25.140}{168.124.25.140:9300}), reason(transport disconnected)[{elk-denmod-1}{t7JmFe-iSn63CALYD-0lxw}{7aHU8PepTH-PAzPrnr-q8w}{168.124.25.140}{168.124.25.140:9300} transport disconnected]
[2021-02-22T12:38:18,197][INFO ][o.e.c.r.DelayedAllocationService] [elk-denmod-web] scheduling reroute for delayed shards in [57.9s] (44 delayed shards)
[2021-02-22T12:38:20,318][INFO ][o.e.c.s.ClusterService ] [elk-denmod-web] added {{elk-denmod-1}{t7JmFe-iSn63CALYD-0lxw}{7aHU8PepTH-PAzPrnr-q8w}{168.124.25.140}{168.124.25.140:9300},}, reason: zen-disco-node-join[{elk-denmod-1}{t7JmFe-iSn63CALYD-0lxw}{7aHU8PepTH-PAzPrnr-q8w}{168.124.25.140}{168.124.25.140:9300}]
[2021-02-22T12:39:45,620][INFO ][o.e.c.r.a.AllocationService] [elk-denmod-web] Cluster health status changed from [YELLOW] to [GREEN] (reason: [shards started [[en_m_54][2]] ...]).
[2021-02-22T12:41:57,612][WARN ][o.e.l.LicenseService ] [elk-denmod-web]
**Error from data node:**
[2021-02-22T13:56:31,924][INFO ][o.e.d.z.ZenDiscovery ] [elk-denmod-6] master_left [{elk-denmod-web}{sMFUmPkRQPiJPstaGBObwg}{SY08ahurSG2uH-TWIs4aJA}{xspw10f206w.pharma.aventis.com}{168.124.147.161:9300}], reason [transport disconnected]
[2021-02-22T13:56:31,924][WARN ][o.e.d.z.ZenDiscovery ] [elk-denmod-6] master left (reason = transport disconnected), current nodes: nodes:
{elk-denmod-6}{fU5vCMTTS46Pboz5zMXD0Q}{3Tomsar_ReuXxB8_4Ym0fA}{168.124.25.122}{168.124.25.122:9300}, local
{elk-denmod-5}{yez29U5iQxWl2hh6Yh-_xg}{IKnp6uBtRGKlE9j3a70S9g}{168.124.54.142}{168.124.54.142:9300}
{elk-denmod-4}{rS_6QTYGSiKTyXJnjvSdyw}{cMJFNWhxSG-DqTCuyKLoyQ}{168.124.170.244}{168.124.170.244:9300}
{elk-denmod-1}{t7JmFe-iSn63CALYD-0lxw}{7aHU8PepTH-PAzPrnr-q8w}{168.124.25.140}{168.124.25.140:9300}
{elk-denmod-3}{5w1hFgnvRlOxtQ0QrX3tKQ}{wh63T78fSmWyamXRZje8jg}{168.124.29.126}{168.124.29.126:9300}
{elk-denmod-web}{sMFUmPkRQPiJPstaGBObwg}{SY08ahurSG2uH-TWIs4aJA}{xspw10f206w.pharma.aventis.com}{168.124.147.161:9300}, master
{elk-denmod-2}{VnvJTaKIQIqrdzXSlPUxxQ}{LIW0GTJdTMei5nWYebyszg}{168.124.29.129}{168.124.29.129:9300}
[2021-02-22T13:56:52,376][WARN ][o.e.c.NodeConnectionsService] [elk-denmod-6] failed to connect to node {elk-denmod-web}{sMFUmPkRQPiJPstaGBObwg}{SY08ahurSG2uH-TWIs4aJA}{xspw10f206w.pharma.aventis.com}{168.124.147.161:9300} (tried [1] times)
org.elasticsearch.transport.ConnectTransportException: [elk-denmod-web][168.124.147.161:9300] general node connection failure
at org.elasticsearch.transport.TcpTransport.openConnection(TcpTransport.java:519) ~[elasticsearch-5.3.1.jar:5.3.1]
at org.elasticsearch.transport.TcpTransport.connectToNode(TcpTransport.java:460) ~[elasticsearch-5.3.1.jar:5.3.1]
at org.elasticsearch.transport.TransportService.connectToNode(TransportService.java:314) ~[elasticsearch-5.3.1.jar:5.3.1]
at org.elasticsearch.transport.TransportService.connectToNode(TransportService.java:301) ~[elasticsearch-5.3.1.jar:5.3.1]
at org.elasticsearch.cluster.NodeConnectionsService.validateNodeConnected(NodeConnectionsService.java:121) [elasticsearch-5.3.1.jar:5.3.1]
at org.elasticsearch.cluster.NodeConnectionsService.connectToNodes(NodeConnectionsService.java:87) [elasticsearch-5.3.1.jar:5.3.1]
at org.elasticsearch.cluster.service.ClusterService.publishAndApplyChanges(ClusterService.java:780) [elasticsearch-5.3.1.jar:5.3.1]
at org.elasticsearch.cluster.service.ClusterService.runTasks(ClusterService.java:633) [elasticsearch-5.3.1.jar:5.3.1]
at org.elasticsearch.cluster.service.ClusterService$UpdateTask.run(ClusterService.java:1117) [elasticsearch-5.3.1.jar:5.3.1]
at org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingRunnable.run(ThreadContext.java:569) [elasticsearch-5.3.1.jar:5.3.1]
at org.elasticsearch.common.util.concurrent.PrioritizedEsThreadPoolExecutor$TieBreakingPrioritizedRunnable.runAndClean(PrioritizedEsThreadPoolExecutor.java:238) [elasticsearch-5.3.1.jar:5.3.1]
at org.elasticsearch.common.util.concurrent.PrioritizedEsThreadPoolExecutor$TieBreakingPrioritizedRunnable.run(PrioritizedEsThreadPoolExecutor.java:201) [elasticsearch-5.3.1.jar:5.3.1]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) [?:1.8.0_131]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) [?:1.8.0_131]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_131]
Caused by: java.lang.IllegalStateException: handshake failed, channel already closed
at org.elasticsearch.transport.TcpTransport.executeHandshake(TcpTransport.java:1549) ~[elasticsearch-5.3.1.jar:5.3.1]
at org.elasticsearch.transport.TcpTransport.openConnection(TcpTransport.java:502) ~[elasticsearch-5.3.1.jar:5.3.1]
... 14 more
[2021-02-22T13:56:52,423][INFO ][o.e.g.DanglingIndicesState] [elk-denmod-6] failed to send allocated dangled
org.elasticsearch.discovery.MasterNotDiscoveredException: no master to send allocate dangled request
at org.elasticsearch.gateway.LocalAllocateDangledIndices.allocateDangled(LocalAllocateDangledIndices.java:84) ~[elasticsearch-5.3.1.jar:5.3.1]
at org.elasticsearch.gateway.DanglingIndicesState.allocateDanglingIndices(DanglingIndicesState.java:164) ~[elasticsearch-5.3.1.jar:5.3.1]
at org.elasticsearch.gateway.DanglingIndicesState.processDanglingIndices(DanglingIndicesState.java:82) ~[elasticsearch-5.3.1.jar:5.3.1]
at org.elasticsearch.gateway.DanglingIndicesState.clusterChanged(DanglingIndicesState.java:185) ~[elasticsearch-5.3.1.jar:5.3.1]
at org.elasticsearch.cluster.service.ClusterService.lambda$publishAndApplyChanges$11(ClusterService.java:824) ~[elasticsearch-5.3.1.jar:5.3.1]
at java.util.Spliterators$ArraySpliterator.forEachRemaining(Spliterators.java:948) [?:1.8.0_131]
at java.util.stream.Streams$ConcatSpliterator.forEachRemaining(Streams.java:742) [?:1.8.0_131]
at java.util.stream.ReferencePipeline$Head.forEach(ReferencePipeline.java:580) [?:1.8.0_131]
at org.elasticsearch.cluster.service.ClusterService.publishAndApplyChanges(ClusterService.java:821) [elasticsearch-5.3.1.jar:5.3.1]
at org.elasticsearch.cluster.service.ClusterService.runTasks(ClusterService.java:633) [elasticsearch-5.3.1.jar:5.3.1]
at org.elasticsearch.cluster.service.ClusterService$UpdateTask.run(ClusterService.java:1117) [elasticsearch-5.3.1.jar:5.3.1]
at org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingRunnable.run(ThreadContext.java:569) [elasticsearch-5.3.1.jar:5.3.1]
at org.elasticsearch.common.util.concurrent.PrioritizedEsThreadPoolExecutor$TieBreakingPrioritizedRunnable.runAndClean(PrioritizedEsThreadPoolExecutor.java:238) [elasticsearch-5.3.1.jar:5.3.1]
at org.elasticsearch.common.util.concurrent.PrioritizedEsThreadPoolExecutor$TieBreakingPrioritizedRunnable.run(PrioritizedEsThreadPoolExecutor.java:201) [elasticsearch-5.3.1.jar:5.3.1]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) [?:1.8.0_131]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) [?:1.8.0_131]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_131]
[2021-02-22T13:56:52,454][INFO ][o.e.d.z.ZenDiscovery ] [elk-denmod-6] failed to send join request to master [{elk-denmod-web}{sMFUmPkRQPiJPstaGBObwg}{SY08ahurSG2uH-TWIs4aJA}{xspw10f206w.pharma.aventis.com}{168.124.147.161:9300}], reason [RemoteTransportException[[elk-denmod-web][168.124.147.161:9300][internal:discovery/zen/join]]; nested: IllegalStateException[failure when sending a validation request to node]; nested: NodeDisconnectedException[[elk-denmod-6][168.124.25.122:9300][internal:discovery/zen/join/validate] disconnected]; ]
[2021-02-22T13:56:56,120][INFO ][o.e.c.s.ClusterService ] [elk-denmod-6] detected_master {elk-denmod-web}{sMFUmPkRQPiJPstaGBObwg}{SY08ahurSG2uH-TWIs4aJA}{xspw10f206w.pharma.aventis.com}{168.124.147.161:9300}, reason: zen-disco-receive(from master [master {elk-denmod-web}{sMFUmPkRQPiJPstaGBObwg}{SY08ahurSG2uH-TWIs4aJA}{xspw10f206w.pharma.aventis.com}{168.124.147.161:9300} committed version [68667]])