Starting an Elasticsearch cluster for the very first time requires the initial set of master-eligible nodes to be explicitly defined on one or more of the master-eligible nodes in the cluster. This is known as cluster bootstrapping. This is only required the very first time the cluster starts up: nodes that have already joined a cluster store this information in their data folder and freshly-started nodes that are joining an existing cluster obtain this information from the cluster’s elected master.

The initial set of master-eligible nodes is defined in the cluster.initial_master_nodes setting. This is a list of the node names or IP addresses of the master-eligible nodes in the new cluster. If you do not configure node.name then it is set to the node’s hostname, so in this case you can use hostnames in cluster.initial_master_nodes too.

When you start a master-eligible node, you can provide this setting on the command line or in the elasticsearch.yml file. After the cluster has formed, this setting is no longer required and is ignored. It need not be set on master-ineligible nodes, nor on master-eligible nodes that are started to join an existing cluster. Note that master-eligible nodes should use storage that persists across restarts. If they do not, and cluster.initial_master_nodes is set, and a full cluster restart occurs, then another brand-new cluster will form and this may result in data loss.

It is technically sufficient to set cluster.initial_master_nodes on a single master-eligible node in the cluster, and only to mention that single node in the setting’s value, but this provides no fault tolerance before the cluster has fully formed. It is therefore better to bootstrap using at least three master-eligible nodes, each with a cluster.initial_master_nodes setting containing all three nodes.

You must set cluster.initial_master_nodes to the same list of nodes on each node on which it is set in order to be sure that only a single cluster forms during bootstrapping and therefore to avoid the risk of data loss.

For a cluster with 3 master-eligible nodes (with node names master-a, master-b and master-c) the configuration will look as follows:

cluster.initial_master_nodes:
  - master-a
  - master-b
  - master-c

You can use a mix of IP addresses and node names too. If there is more than one Elasticsearch node with the same IP address then the transport port must also be given to specify exactly which node is meant:

cluster.initial_master_nodes:
  - 10.0.10.101
  - 10.0.10.102:9300
  - 10.0.10.102:9301
  - master-node-hostname

Like all node settings, it is also possible to specify the initial set of master nodes on the command-line that is used to start Elasticsearch:

$ bin/elasticsearch -Ecluster.initial_master_nodes=master-a,master-b,master-c

The node names used in the cluster.initial_master_nodes list must exactly match the node.name properties of the nodes. By default the node name is set to the machine’s hostname which may or may not be fully-qualified depending on your system configuration. If each node name is a fully-qualified domain name such as master-a.example.com then you must use fully-qualified domain names in the cluster.initial_master_nodes list too; conversely if your node names are bare hostnames (without the .example.com suffix) then you must use bare hostnames in the cluster.initial_master_nodes list. If you use a mix of fully-qualifed and bare hostnames, or there is some other mismatch between node.name and cluster.initial_master_nodes, then the cluster will not form successfully and you will see log messages like the following.

[master-a.example.com] master not discovered yet, this node has
not previously joined a bootstrapped (v7+) cluster, and this
node must discover master-eligible nodes [master-a, master-b] to
bootstrap a cluster: have discovered [{master-b.example.com}{...

This message shows the node names master-a.example.com and master-b.example.com as well as the cluster.initial_master_nodes entries master-a and master-b, and it is clear from this message that they do not match exactly.

Choosing a cluster name

The cluster.name setting enables you to create multiple clusters which are separated from each other. Nodes verify that they agree on their cluster name when they first connect to each other, and Elasticsearch will only form a cluster from nodes that all have the same cluster name. The default value for the cluster name is elasticsearch, but it is recommended to change this to reflect the logical name of the cluster.

Auto-bootstrapping in development mode

If the cluster is running with a completely default configuration then it will automatically bootstrap a cluster based on the nodes that could be discovered to be running on the same host within a short time after startup. This means that by default it is possible to start up several nodes on a single machine and have them automatically form a cluster which is very useful for development environments and experimentation. However, since nodes may not always successfully discover each other quickly enough this automatic bootstrapping cannot be relied upon and cannot be used in production deployments.

If any of the following settings are configured then auto-bootstrapping will not take place, and you must configure cluster.initial_master_nodes as described in the section on cluster bootstrapping: