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 when we want to control this behavior—for example, during rolling restarts. We don't want to rebalance the entire cluster when one or two nodes are restarted. In this section, we will look at how to avoid cluster rebalance and control this process' behavior in depth.
Imagine a situation where you know that your network can handle very high amounts of traffic or the opposite of this— your network is used extensively and you want to avoid too much load 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.