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
second node has joined the cluster, and three replica shards have
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.
shows a status of
green, which means that all six
shards (all three primary shards and all three replica shards) are active:
Our cluster is not only fully functional, but also always available.