An entire cluster may be set to read-only with the following dynamic setting:

Don’t rely on this setting to prevent changes to your cluster. Any user with access to the cluster-update-settings API can make the cluster read-write again.

In a Elasticsearch 7.0 and later, there will be a soft limit on the number of shards in a cluster, based on the number of nodes in the cluster. This is intended to prevent operations which may unintentionally destabilize the cluster. Prior to 7.0, actions which would result in the cluster going over the limit will issue a deprecation warning.

You can set the system property es.enforce_max_shards_per_node to true to opt in to strict enforcement of the shard limit. If this system property is set, actions which would result in the cluster going over the limit will result in an error, rather than a deprecation warning. This property will be removed in Elasticsearch 7.0, as strict enforcement of the limit will be the default and only behavior.

If an operation, such as creating a new index, restoring a snapshot of an index, or opening a closed index would lead to the number of shards in the cluster going over this limit, the operation will issue a deprecation warning.

If the cluster is already over the limit, due to changes in node membership or setting changes, all operations that create or open indices will issue warnings until either the limit is increased as described below, or some indices are closed or deleted to bring the number of shards below the limit.

Replicas count towards this limit, but closed indexes do not. An index with 5 primary shards and 2 replicas will be counted as 15 shards. Any closed index is counted as 0, no matter how many shards and replicas it contains.

The limit defaults to 1,000 shards per node, and be dynamically adjusted using the following property:

For example, a 3-node cluster with the default setting would allow 3,000 shards total, across all open indexes. If the above setting is changed to 1,500, then the cluster would allow 4,500 shards total.

User-defined metadata can be stored and retrieved using the Cluster Settings API. This can be used to store arbitrary, infrequently-changing data about the cluster without the need to create an index to store it. This data may be stored using any key prefixed with cluster.metadata.. For example, to store the email address of the administrator of a cluster under the key cluster.metadata.administrator, issue this request:

PUT /_cluster/settings
{
  "persistent": {
    "cluster.metadata.administrator": "sysadmin@example.com"
  }
}

User-defined cluster metadata is not intended to store sensitive or confidential information. Any information stored in user-defined cluster metadata will be viewable by anyone with access to the Cluster Get Settings API, and is recorded in the Elasticsearch logs.

The cluster state maintains index tombstones to explicitly denote indices that have been deleted. The number of tombstones maintained in the cluster state is controlled by the following property, which cannot be updated dynamically:

The settings which control logging can be updated dynamically with the logger. prefix. For instance, to increase the logging level of the indices.recovery module to DEBUG, issue this request:

PUT /_cluster/settings
{
  "transient": {
    "logger.org.elasticsearch.indices.recovery": "DEBUG"
  }
}

Plugins can create a kind of tasks called persistent tasks. Those tasks are usually long-live tasks and are stored in the cluster state, allowing the tasks to be revived after a full cluster restart.

Every time a persistent task is created, the master nodes takes care of assigning the task to a node of the cluster, and the assigned node will then pick up the task and execute it locally. The process of assigning persistent tasks to nodes is controlled by the following property, which can be updated dynamically: