How Primary and Replica Shards Interact

For explanation purposes, let’s imagine that we have a cluster consisting of three nodes. It contains one index called blogs that has two primary shards. Each primary shard has two replicas. Copies of the same shard are never allocated to the same node, so our cluster looks something like Figure 8, “A cluster with three nodes and one index”.

Figure 8. A cluster with three nodes and one index

We can send our requests to any node in the cluster. Every node is fully capable of serving any request. Every node knows the location of every document in the cluster and so can forward requests directly to the required node. In the following examples, we will send all of our requests to Node 1, which we will refer to as the coordinating node.