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.

There is 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.

This limit is intended as a safety net, not a sizing recommendation. The exact number of shards your cluster can safely support depends on your hardware configuration and workload, but should remain well below this limit in almost all cases, as the default limit is set quite high.

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 fail with an error indicating the shard limit.

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 fail 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 data node, and can 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 500, then the cluster would allow 1,500 shards total.

If there are no data nodes in the cluster, the limit will not be enforced. This allows the creation of indices during cluster creation if dedicated master nodes are set up before data nodes.

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 node 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 properties, which can be updated dynamically: