Rebalancing is the process of moving shards between different nodes in our cluster. As we have already mentioned, it's fine in most situations, but sometimes you may want to completely avoid that. For example, if we define how our shards are placed and we want to keep it that way, we want to avoid rebalancing. However, ElasticSearch, by default, will try to rebalance the cluster whenever the cluster state changes and ElasticSearch thinks rebalancing is needed. This may happen, for example, after each full-cluster restart that can happen during upgrade, plugin installation, or after node failure.

We already know that our indices can be built of shards and replicas. Primary shards or just shards are the ones that are used when new documents are indexed, there is an update, or there is a delete—or just in case of any index change. We also have replicas, which get the data from the primary shards.

You can think of the cluster as being ready to use when all primary shards are assigned to their nodes in your cluster—as soon as the yellow health state is achieved. However, ElasticSearch may still initialize other shards—the replicas. But you can use your cluster and be sure that you can search your whole data set and you can send index change commands. Then those will be processed properly.

ElasticSearch lets us control the rebalancing process with the use of a few properties that can be set in the elasticsearch.yml file or by using ElasticSearch REST API. Since the first method is very simple, let's skip discussing it and look at the second method—the one using the REST API.

In order to set one of the properties described later, we need to use the HTTP PUT method and send a proper request to the _cluster/settings URI. However, we have two options—transient and permanent property settings.

The first one—the transient—will set the property only till the first restart. In order to do that, we send the following command:

curl -XPUT 'localhost:9200/_cluster/settings' -d '{
  "transient" : {
    "PROPERTY_NAME" : "PROPERTY_VALUE"
  }
}'

As you can see, in the preceding command, we used the object named transient and we added our property definition there. This means that the property will be valid only until the next restart.

If we want our property setting to persist between restarts, instead of using the object named transient, we will use one named persistent. So, the sample command will be as follows:

curl -XPUT 'localhost:9200/_cluster/settings' -d '{
  "persistent" : {
    "PROPERTY_NAME" : "PROPERTY_VALUE"
  }
}'

And now let's look at what ElasticSearch allows us to control.