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.

Starting a Second Node

To test what happens when you add a second node, you can start a new node in exactly the same way as you started the first one (see [running-elasticsearch]), and from the same directory. Multiple nodes can share the same directory.

When you run a second node on the same machine, it automatically discovers and joins the cluster as long as it has the same cluster.name as the first node. However, for nodes running on different machines to join the same cluster, you need to configure a list of unicast hosts the nodes can contact to join the cluster. For more information, see Prefer Unicast over Multicast.

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

A two-node cluster
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, 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", (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,
  "delayed_unassigned_shards": 0,
  "number_of_pending_tasks": 0,
  "number_of_in_flight_fetch": 0,
  "task_max_waiting_in_queue_millis": 0,
  "active_shards_percent_as_number": 100
}

  1. Cluster status is green.

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

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