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 would want to control this behavior. In this section, we will look at how to avoid cluster rebalance and how to control this process' behavior in depth.

Imagine a situation where you know that your network can handle a very high amount of traffic, or the opposite; 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.