When you start an instance of Elasticsearch, you are starting a node. An Elasticsearch cluster is a group of nodes that have the same cluster.name attribute. As nodes join or leave a cluster, the cluster automatically reorganizes itself to evenly distribute the data across the available nodes.

If you are running a single instance of Elasticsearch, you have a cluster of one node. All primary shards reside on the single node. No replica shards can be allocated, therefore the cluster state remains yellow. The cluster is fully functional but is at risk of data loss in the event of a failure.

You add nodes to a cluster to increase its capacity and reliability. By default, a node is both a data node and eligible to be elected as the master node that controls the cluster. You can also configure a new node for a specific purpose, such as handling ingest requests. For more information, see Nodes.

When you add more nodes to a cluster, it automatically allocates replica shards. When all primary and replica shards are active, the cluster state changes to green.

You can run multiple nodes on your local machine in order to experiment with how an Elasticsearch cluster of multiple nodes behaves. To add a node to a cluster running on your local machine:

To add a node to a cluster running on multiple machines, you must also set discovery.seed_hosts so that the new node can discover the rest of its cluster.

For more information about discovery and shard allocation, see Discovery and cluster formation and Cluster-level shard allocation and routing settings.

Master-eligible nodesedit

As nodes are added or removed Elasticsearch maintains an optimal level of fault tolerance by automatically updating the cluster’s voting configuration, which is the set of master-eligible nodes whose responses are counted when making decisions such as electing a new master or committing a new cluster state.

It is recommended to have a small and fixed number of master-eligible nodes in a cluster, and to scale the cluster up and down by adding and removing master-ineligible nodes only. However there are situations in which it may be desirable to add or remove some master-eligible nodes to or from a cluster.

Adding master-eligible nodesedit

If you wish to add some nodes to your cluster, simply configure the new nodes to find the existing cluster and start them up. Elasticsearch adds the new nodes to the voting configuration if it is appropriate to do so.

During master election or when joining an existing formed cluster, a node sends a join request to the master in order to be officially added to the cluster. You can use the cluster.join.timeout setting to configure how long a node waits after sending a request to join a cluster. Its default value is 30s. See Discovery and cluster formation settings.

Removing master-eligible nodesedit

When removing master-eligible nodes, it is important not to remove too many all at the same time. For instance, if there are currently seven master-eligible nodes and you wish to reduce this to three, it is not possible simply to stop four of the nodes at once: to do so would leave only three nodes remaining, which is less than half of the voting configuration, which means the cluster cannot take any further actions.

More precisely, if you shut down half or more of the master-eligible nodes all at the same time then the cluster will normally become unavailable. If this happens then you can bring the cluster back online by starting the removed nodes again.

As long as there are at least three master-eligible nodes in the cluster, as a general rule it is best to remove nodes one-at-a-time, allowing enough time for the cluster to automatically adjust the voting configuration and adapt the fault tolerance level to the new set of nodes.

If there are only two master-eligible nodes remaining then neither node can be safely removed since both are required to reliably make progress. To remove one of these nodes you must first inform Elasticsearch that it should not be part of the voting configuration, and that the voting power should instead be given to the other node. You can then take the excluded node offline without preventing the other node from making progress. A node which is added to a voting configuration exclusion list still works normally, but Elasticsearch tries to remove it from the voting configuration so its vote is no longer required. Importantly, Elasticsearch will never automatically move a node on the voting exclusions list back into the voting configuration. Once an excluded node has been successfully auto-reconfigured out of the voting configuration, it is safe to shut it down without affecting the cluster’s master-level availability. A node can be added to the voting configuration exclusion list using the Voting configuration exclusions API. For example:

# Add node to voting configuration exclusions list and wait for the system
# to auto-reconfigure the node out of the voting configuration up to the
# default timeout of 30 seconds
POST /_cluster/voting_config_exclusions/node_name

# Add node to voting configuration exclusions list and wait for
# auto-reconfiguration up to one minute
POST /_cluster/voting_config_exclusions/node_name?timeout=1m

The node that should be added to the exclusions list is specified using node filters in place of node_name here. If a call to the voting configuration exclusions API fails, you can safely retry it. Only a successful response guarantees that the node has actually been removed from the voting configuration and will not be reinstated. If it’s the active master that was removed from the voting configuration, then it will abdicate to another master-eligible node that’s still in the voting configuration, if such a node is available.

Although the voting configuration exclusions API is most useful for down-scaling a two-node to a one-node cluster, it is also possible to use it to remove multiple master-eligible nodes all at the same time. Adding multiple nodes to the exclusions list has the system try to auto-reconfigure all of these nodes out of the voting configuration, allowing them to be safely shut down while keeping the cluster available. In the example described above, shrinking a seven-master-node cluster down to only have three master nodes, you could add four nodes to the exclusions list, wait for confirmation, and then shut them down simultaneously.

Adding an exclusion for a node creates an entry for that node in the voting configuration exclusions list, which has the system automatically try to reconfigure the voting configuration to remove that node and prevents it from returning to the voting configuration once it has removed. The current list of exclusions is stored in the cluster state and can be inspected as follows:

GET /_cluster/state?filter_path=metadata.cluster_coordination.voting_config_exclusions

This list is limited in size by the cluster.max_voting_config_exclusions setting, which defaults to 10. See Discovery and cluster formation settings. Since voting configuration exclusions are persistent and limited in number, they must be cleaned up. Normally an exclusion is added when performing some maintenance on the cluster, and the exclusions should be cleaned up when the maintenance is complete. Clusters should have no voting configuration exclusions in normal operation.

If a node is excluded from the voting configuration because it is to be shut down permanently, its exclusion can be removed after it is shut down and removed from the cluster. Exclusions can also be cleared if they were created in error or were only required temporarily:

# Wait for all the nodes with voting configuration exclusions to be removed from
# the cluster and then remove all the exclusions, allowing any node to return to
# the voting configuration in the future.
DELETE /_cluster/voting_config_exclusions

# Immediately remove all the voting configuration exclusions, allowing any node
# to return to the voting configuration in the future.
DELETE /_cluster/voting_config_exclusions?wait_for_removal=false