This looks like an error on the internal transport but I'm not sure why it's happening. The cluster is green and otherwise happy, ingesting 6-700GB a day across 20 data nodes (20 shards per index). We are having some issues with this 2.3.1 cluster not indexing all documents versus an old 1.7.2 cluster (both being shipped to from the same Heka instances) but I'm not sure that is related to this exception.
No indexes are closed so I assume this is some kind of timeout in the internal communication? Is this a problem?
[2016-05-09 18:20:15,887][DEBUG][action.admin.cluster.node.stats] [ip-10-10-10-10] failed to execute on node [huiodfhg78ytdfghg89]
RemoteTransportException[[ip-10-10-10-10][10.10.10.10:9300][cluster:monitor/nodes/stats[n]]]; nested: AlreadyClosedException[this IndexReader is closed];
Caused by: org.apache.lucene.store.AlreadyClosedException: this IndexReader is closed
at org.apache.lucene.index.IndexReader.ensureOpen(IndexReader.java:274)
at org.apache.lucene.index.CompositeReader.getContext(CompositeReader.java:101)
at org.apache.lucene.index.CompositeReader.getContext(CompositeReader.java:55)
at org.apache.lucene.index.IndexReader.leaves(IndexReader.java:438)
at org.elasticsearch.search.suggest.completion.Completion090PostingsFormat.completionStats(Completion090PostingsFormat.java:330)
at org.elasticsearch.index.shard.IndexShard.completionStats(IndexShard.java:765)
at org.elasticsearch.action.admin.indices.stats.CommonStats.(CommonStats.java:164)
at org.elasticsearch.indices.IndicesService.stats(IndicesService.java:253)
at org.elasticsearch.node.service.NodeService.stats(NodeService.java:158)
at org.elasticsearch.action.admin.cluster.node.stats.TransportNodesStatsAction.nodeOperation(TransportNodesStatsAction.java:82)
at org.elasticsearch.action.admin.cluster.node.stats.TransportNodesStatsAction.nodeOperation(TransportNodesStatsAction.java:44)
at org.elasticsearch.action.support.nodes.TransportNodesAction.nodeOperation(TransportNodesAction.java:92)
at org.elasticsearch.action.support.nodes.TransportNodesAction$NodeTransportHandler.messageReceived(TransportNodesAction.java:230)
at org.elasticsearch.action.support.nodes.TransportNodesAction$NodeTransportHandler.messageReceived(TransportNodesAction.java:226)
at org.elasticsearch.transport.RequestHandlerRegistry.processMessageReceived(RequestHandlerRegistry.java:75)
at org.elasticsearch.transport.TransportService$4.doRun(TransportService.java:376)
at org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:37)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)