How To Solve Issues Related to Log – Discovered which is also master but with an older cluster_state. telling to rejoin the cluster ()

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Updated: Feb-20

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Troubleshooting background

To troubleshoot Elasticsearch log “Discovered which is also master but with an older cluster_state. telling to rejoin the cluster ()” it’s important to understand common problems related to Elasticsearch concepts: cluster, discovery, master. See detailed explanations below complete with common problems, examples and useful tips.


Discovery in Elasticsearch

What it is

The process known as discovery occurs when an Elasticsearch node starts, restarts or loses contact with the master node for any reason. In those cases, the node needs to contact other nodes in the cluster to find the existing master node for the cluster or initiate the election of a new master node. 

How It Works

Upon startup, each node looks for other nodes, firstly by contacting those ip addresses of eligible master nodes held in previous cluster state.  If they are not available, it will look for nodes based upon the seed host providers mechanisms available.

Seed host providers may be defined in 3 ways: list based, file based or plugin based.  All of these methods will provide a list of IP addresses or hostnames which the node should contact in order to obtain a list of master eligible nodes.  The node will contact all of these addresses in turn, until either an active master is found, or failing that, until sufficient nodes can be found to elect a new master node.


The simplest form is to define a list of seed host providers in elasticsearch.yml


An alternative way is to refer to a file using the following setting:

discovery.seed_providers: file

The file MUST be placed in the following filepath: $ES_PATH_CONF/unicast_hosts.txt
# an IPv6 address

Note that the use of a port is optional. If not used, then the default port range of 9300-9400 will be used.

If you use AWS or  GCS then you can install and use a PLUGIN to obtain a list of seed hosts from an API.  A plugin also exists for Azure but is deprecated since version 5.

AWS Plugin

A typical configuration could be as follows:

discovery.seed_providers: ec2
discovery.ec2.tag.role: master
discovery.ec2.tag.environment: dev
cloud.node.auto_attributes: true
cluster.routing.allocation.awareness.attributes: aws_availability_zone

The above configuration would look for all nodes with a tag called “environment” set to “dev” and a tag called “role” set to “master”, in the AWS zone us-east-1. The last two lines set up cluster routing allocation awareness based upon aws availability zones. (Not necessary, but nice to have).

GCE Plugin

A typical configuration could be as follows:

discovery.seed_providers: gce
cloud.gce.project_id: <your-google-project-id> <your-zone>
discovery.gce.tags: <my-tag-name>

The above configuration would look for all virtual machines inside your project, zone and with a tag set to the tag name you provide.

Notes and good things to know

Cluster formation depends on correct setup of the settings in elasticsearch.yml.  Make sure that the nodes can reach each other across the network using the IP address / hostname you are using, and are not getting blocked due to firewall settings on the ports required.

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 Whats Happend My Es Cluster Plz Hel  

2Github Issue Number 12415  

Log Context

Log ”discovered [{}] which is also master but with an older cluster_state; telling [{}] to rejoin the cluster ([{}])” classname is
We have extracted the following from Elasticsearch source code to get an in-depth context :

         assert Thread.currentThread().getName().contains(InternalClusterService.UPDATE_THREAD_NAME) : "not called from the cluster state update thread";

        if (otherClusterStateVersion > localClusterState.version()) {
            return rejoin(localClusterState; "zen-disco-discovered another master with a new cluster_state [" + otherMaster + "][" + reason + "]");
        } else {
            logger.warn("discovered [{}] which is also master but with an older cluster_state; telling [{}] to rejoin the cluster ([{}])"; otherMaster; otherMaster; reason);
            try {
                // make sure we're connected to this node (connect to node does nothing if we're already connected)
                // since the network connections are asymmetric; it may be that we received a state but have disconnected from the node
                // in the past (after a master failure; for example)

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