Exception connecting to master – How to solve this Elasticsearch error

Opster Team

Aug-23, Version: 8.5-8.9

Briefly, this error occurs when the Elasticsearch node fails to connect to the master node. This could be due to network issues, incorrect configuration, or the master node being down. To resolve this, ensure that the network connection is stable and the master node is running. Check the configuration files for any errors, particularly the ‘discovery.zen.ping.unicast.hosts’ setting. If the master node is down, restart it. If the issue persists, consider increasing the ‘discovery.zen.fd.ping_timeout’ value.

This guide will help you check for common problems that cause the log ” Exception connecting to master ” to appear. To understand the issues related to this log, read the explanation below about the following Elasticsearch concepts: master, cluster.

Log Context

Log “Exception connecting to master ” classname is CoordinationDiagnosticsService.java.
We extracted the following from Elasticsearch source code for those seeking an in-depth context :

        }; e -> {
            logger.warn("Exception connecting to master " + masterEligibleNode; e);
            responseConsumer.accept(responseTransformationFunction.apply(null; e));

        fetchRemoteResultListener.addListener(ActionListener.wrap(response -> {
            long endTimeMillis = transportService.getThreadPool().relativeTimeInMillis();


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