20_Add_failover.asciidoc

Add failover

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. A new node will join the cluster automatically as long as it has the same cluster name set in its config file, and it can talk to the other nodes.

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

Figure 1. 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, 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 6 shards (all 3 primary shards and all 3 replica shards) are active:

{
   "cluster_name":          "elasticsearch",
   "status":                "green", (1)
   "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
}

1. Cluster status is green.

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