By default, ElasticSearch tries to keep the shards and their replicas evenly balanced across the cluster. Such behavior is good in most cases, but there are times where we want to control that behavior. In Chapter 7, Administrating Your Cluster, we discussed how to take total control over how shards and replicas are distributed. In this section, we will look at how to avoid cluster rebalance and how to control the behavior of this process in depth.
Imagine a situation where you know that your network can handle a very high amount of traffic or the opposite to that—your network is used extensively and you want to avoid too much stress on it. The other example is that you may want to decrease the pressure that is put on your I/O subsystem after a full-cluster restart and you want to have less shards and replicas being initialized at the same time. These are only two examples where rebalance control may be handy.