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:
- Set up a new Elasticsearch instance.
Specify the name of the cluster with the
elasticsearch.yml. For example, to add a node to the
logging-prodcluster, add the line
- Start Elasticsearch. The node automatically discovers and joins the specified cluster.
To add a node to a cluster running on multiple machines, you must also
discovery.seed_hosts so that the new node can discover
the rest of its cluster.