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.
Enroll nodes in an existing clusteredit
You can enroll additional nodes on your local machine to experiment with how an Elasticsearch cluster with multiple nodes behaves.
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.
When Elasticsearch starts for the first time, the security auto-configuration process
binds the HTTP layer to
0.0.0.0, but only binds the transport layer to
localhost. This intended behavior ensures that you can start
a single-node cluster with security enabled by default without any additional
Before enrolling a new node, additional actions such as binding to an address
localhost or satisfying bootstrap checks are typically necessary
in production clusters. During that time, an auto-generated enrollment token
could expire, which is why enrollment tokens aren’t generated automatically.
Additionally, only nodes on the same host can join the cluster without
additional configuration. If you want nodes from another host to join your
cluster, you need to set
transport.host to a
(such as uncommenting the suggested value of
0.0.0.0), or an IP address
that’s bound to an interface where other hosts can reach it. Refer to
transport settings for more
To enroll new nodes in your cluster, create an enrollment token with the
elasticsearch-create-enrollment-token tool on any existing node in your
cluster. You can then start a new node with the
so that it joins an existing cluster.
In a separate terminal from where Elasticsearch is running, navigate to the directory where you installed Elasticsearch and run the
elasticsearch-create-enrollment-tokentool to generate an enrollment token for your new nodes.
bin\elasticsearch-create-enrollment-token -s node
Copy the enrollment token, which you’ll use to enroll new nodes with your Elasticsearch cluster.
From the installation directory of your new node, start Elasticsearch and pass the enrollment token with the
bin\elasticsearch --enrollment-token <enrollment-token>
Elasticsearch automatically generates certificates and keys in the following directory:
- Repeat the previous step for any new nodes that you want to enroll.
For more information about discovery and shard allocation, refer to Discovery and cluster formation and Cluster-level shard allocation and routing settings.
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.
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_names=node_name # Add node to voting configuration exclusions list and wait for # auto-reconfiguration up to one minute POST /_cluster/voting_config_exclusions?node_names=node_name&timeout=1m
The nodes that should be added to the exclusions list are specified by name
?node_names query parameter, or by their persistent node IDs using
?node_ids query parameter. 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 the elected master node is excluded from the
voting configuration then it will abdicate to another master-eligible node that
is 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.
Voting exclusions are only required when removing at least half of the master-eligible nodes from a cluster in a short time period. They are not required when removing master-ineligible nodes, nor are they required when removing fewer than half of the master-eligible nodes.
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:
response = client.cluster.state( filter_path: 'metadata.cluster_coordination.voting_config_exclusions' ) puts response
This list is limited in size by the
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
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 by specifying
# 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