How To Solve Issues Related to Log – Failed to connect to

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Updated: Jan-20

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Troubleshooting background

To troubleshoot Elasticsearch log “Failed to connect to” it’s important to understand common problems related to Elasticsearch concepts: cluster, node. See detailed explanations below complete with common problems, examples and useful tips.

Nodes in Elasticsearch

What it is

Simply explained a node is a single server that is part of a cluster. Each node is assigned with one or more roles, which describes the node responsibility and operations – Data nodes stores the data, and participates in the cluster’s indexing and search capabilities, while master nodes are responsible for managing the cluster activities and storing the cluster state, including the metadata.

While it’s 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.

Coordinator Node (aka client node)

Coordinator Node – is a node that does not hold any configured role. It doesn’t hold data, not part of the master eligible group nor execute ingest pipelines. Coordinator node serves incoming search requests and is acting as the query coordinator – running the query and fetch phases, sending requests to every node which holds a shard being queried. The client node also distributes bulk indexing operations and route queries to shards copies based on the nodes responsiveness.


To help troubleshoot related issues we have gathered selected Q&A from the community and issues from Github , please review the following for further information :

1 Elasticsearch: Failed to connect to localhost port 9200 – Connection refused 162.35 K 93

2AWS ElasticSearch as a Service fails to connect via standalone KIBANA 0.85 K  1

Curl 7 Failed To Connect/120073


Log Context

Log ”Failed to connect to {}” classname is NodeConnectionsService.java
We have extracted the following from Elasticsearch source code to get an in-depth context :

                     public void onFailure(Exception e) {
                        // both errors and rejections are logged here. the service
                        // will try again after `cluster.nodes.reconnect_interval` on all nodes but the current master.
                        // On the master; node fault detection will remove these nodes from the cluster as their are not
                        // connected. Note that it is very rare that we end up here on the master.
                        logger.warn(() -> new ParameterizedMessage("failed to connect to {}"; node); e);
                    }

                    
Override
                    protected void doRun() {
                        try (Releasable ignored = nodeLocks.acquire(node)) {





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