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Elastic Docs ›Elasticsearch Guide [8.1] ›Set up Elasticsearch ›Discovery and cluster formation

Bootstrapping a cluster

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Bootstrapping a cluster

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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 first time a cluster starts up: nodes that have already joined a cluster store this information in their data folder for use in a full cluster restart, and freshly-started nodes that are joining a running 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 should be set to a list containing one of the following items for each master-eligible node:

  • The node name of the node.
  • The node’s hostname if node.name is not set, because node.name defaults to the node’s hostname. You must use either the fully-qualified hostname or the bare hostname depending on your system configuration.
  • The IP address of the node’s transport publish address, if it is not possible to use the node.name of the node. This is normally the IP address to which network.host resolves but this can be overridden.
  • The IP address and port of the node’s publish address, in the form IP:PORT, if it is not possible to use the node.name of the node and there are multiple nodes sharing a single IP address.

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, remove this setting from each node’s configuration. It should not be set for master-ineligible nodes, master-eligible nodes joining an existing cluster, or when restarting one or more nodes.

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

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 -E cluster.initial_master_nodes=master-a,master-b,master-c

Node name formats must match

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-qualified 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

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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

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By default each node will automatically bootstrap itself into a single-node cluster the first time it starts. If any of the following settings are configured then auto-bootstrapping will not take place:

  • discovery.seed_providers
  • discovery.seed_hosts
  • cluster.initial_master_nodes

To add a new node into an existing cluster, configure discovery.seed_hosts or other relevant discovery settings so that the new node can discover the existing master-eligible nodes in the cluster. To bootstrap a new multi-node cluster, configure cluster.initial_master_nodes as described in the section on cluster bootstrapping as well as discovery.seed_hosts or other relevant discovery settings.

Forming a single cluster

Once an Elasticsearch node has joined an existing cluster, or bootstrapped a new cluster, it will not join a different cluster. Elasticsearch will not merge separate clusters together after they have formed, even if you subsequently try and configure all the nodes into a single cluster. This is because there is no way to merge these separate clusters together without a risk of data loss. You can tell that you have formed separate clusters by checking the cluster UUID reported by GET / on each node.

If you intended to add a node into an existing cluster but instead bootstrapped a separate single-node cluster then you must start again:

  1. Shut down the node.
  2. Completely wipe the node by deleting the contents of its data folder.
  3. Configure discovery.seed_hosts or discovery.seed_providers and other relevant discovery settings.
  4. Restart the node and verify that it joins the existing cluster rather than forming its own one-node cluster.

If you intended to form a new multi-node cluster but instead bootstrapped a collection of single-node clusters then you must start again:

  1. Shut down all the nodes.
  2. Completely wipe each node by deleting the contents of their data folders.
  3. Configure cluster.initial_master_nodes as described above.
  4. Configure discovery.seed_hosts or discovery.seed_providers and other relevant discovery settings.
  5. Restart all the nodes and verify that they have formed a single cluster.
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