Running a single node means that you have a single point of failure—​there is no redundancy. Fortunately, all we need to do to protect ourselves from data loss is to start another node.

If we start a second node, our cluster would look like Figure 3, “A two-node cluster—​all primary and replica shards are allocated”.

Figure 3. A two-node cluster—​all primary and replica shards are allocated

The second node has joined the cluster, and three replica shards have been allocated to it—​one for each primary shard. That means that we can lose either node, and all of our data will be intact.

Any newly indexed document will first be stored on a primary shard, and then copied in parallel to the associated replica shard(s). This ensures that our document can be retrieved from a primary shard or from any of its replicas.

The cluster-health now shows a status of green, which means that all six shards (all three primary shards and all three replica shards) are active:

{
   "cluster_name":          "elasticsearch",
   "status":                "green", 
   "timed_out":             false,
   "number_of_nodes":       2,
   "number_of_data_nodes":  2,
   "active_primary_shards": 3,
   "active_shards":         6,
   "relocating_shards":     0,
   "initializing_shards":   0,
   "unassigned_shards":     0
}

Our cluster is not only fully functional, but also always available.