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Briefly, this error occurs when Elasticsearch tries to perform an operation on a node that it doesn’t recognize or that doesn’t exist in the cluster. This could be due to a misconfiguration, a node being down, or a network issue. To resolve this, you can check the cluster’s health and configuration, ensure all nodes are up and running, and verify network connectivity. If the node is no longer part of the cluster, you may need to update your configuration or scripts to remove references to the unknown node.
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This guide will help you check for common problems that cause the log ” unknown node [” + nodeId + “] ” to appear. To understand the issues related to this log, read the explanation below about the following Elasticsearch concepts: node, replication.
Overview
To put it simply, a node is a single server that is part of a cluster. Each node is assigned one or more roles, which describe the node’s responsibility and operations. Data nodes store the data, and participate in the cluster’s indexing and search capabilities, while master nodes are responsible for managing the cluster’s activities and storing the cluster state, including the metadata.
While it is possible to run several node instances of Elasticsearch on the same hardware, it’s considered a best practice to limit a server to a single running instance of Elasticsearch.
Nodes connect to each other and form a cluster by using a discovery method.
Roles
Master node
Master nodes are in charge of cluster-wide settings and changes – deleting or creating indices and fields, adding or removing nodes and allocating shards to nodes. Each cluster has a single master node that is elected from the master eligible nodes using a distributed consensus algorithm and is reelected if the current master node fails.
Coordinating (client) node
There is some confusion in the use of coordinating node terminology. Client nodes were removed from Elasticsearch after version 2.4 and became coordinating nodes.
Coordinating nodes are nodes that do not hold any configured role. They don’t hold data and are not part of the master eligible group nor execute ingest pipelines. Coordinating nodes serve incoming search requests and act as the query coordinator running query and fetch phases, sending requests to every node that holds a shard being queried. The coordinating node also distributes bulk indexing operations and route queries to shards based on the node’s responsiveness.
Overview
Replication refers to storing a redundant copy of the data. Starting from version 7.x, Elasticsearch creates one primary shard with a replication factor set to 1. Replicas never get assigned to the same node on which primary shards are assigned, which means you should have at least two nodes in the cluster to assign the replicas. If a primary shard goes down, the replica automatically acts as a primary shard.
What it is used for
Replicas are used to provide high availability and failover. A higher number of replicas is also helpful for faster searches.
Examples
Update replica count
PUT /api-logs/_settings?pretty
{
"index" : {
"number_of_replicas" : 2
}
}Common problems
- By default, new replicas are not assigned to nodes with more than 85% disk usage. Instead, Elasticsearch throws a warning.
- Creating too many replicas may cause a problem if there are not enough resources available in the cluster.
Log Context
Log “unknown node [” + nodeId + “]” class name is TransportReplicationAction.java. We extracted the following from Elasticsearch source code for those seeking an in-depth context :
final ActionListenerlistener ) { String nodeId = replica.currentNodeId(); final DiscoveryNode node = clusterService.state().nodes().get(nodeId); if (node == null) { listener.onFailure(new NoNodeAvailableException("unknown node [" + nodeId + "]")); return; } final ConcreteReplicaRequest replicaRequest = new ConcreteReplicaRequest<>( request; replica.allocationId().getId();
