[[modules-discovery-settings]]
=== Discovery and cluster formation settings

Discovery and cluster formation are affected by the following settings:

`discovery.seed_hosts`::

    Provides a list of master-eligible nodes in the cluster. Each value has the
    format `host:port` or `host`, where `port` defaults to the setting
    `transport.profiles.default.port`. Note that IPv6 hosts must be bracketed.
    The default value is `["127.0.0.1", "[::1]"]`. See <<unicast.hosts>>. This
    setting was previously known as `discovery.zen.ping.unicast.hosts`. Its old
    name is deprecated but continues to work in order to preserve backwards
    compatibility. Support for the old name will be removed in a future
    version.

`discovery.seed_providers`::

    Specifies which types of <<built-in-hosts-providers,seed hosts provider>>
    to use to obtain the addresses of the seed nodes used to start the
    discovery process. By default, it is the
    <<settings-based-hosts-provider,settings-based seed hosts provider>>.  This
    setting was previously known as `discovery.zen.hosts_provider`. Its old
    name is deprecated but continues to work in order to preserve backwards
    compatibility. Support for the old name will be removed in a future
    version.
    
`discovery.type`::
     
    Specifies whether {es} should form a multiple-node cluster. By default, {es}
    discovers other nodes when forming a cluster and allows other nodes to join
    the cluster later. If `discovery.type` is set to `single-node`, {es} forms a
    single-node cluster. For more information about when you might use this
    setting, see <<single-node-discovery>>.    

`cluster.initial_master_nodes`::

    Sets the initial set of master-eligible nodes in a brand-new cluster. By
    default this list is empty, meaning that this node expects to join a cluster
    that has already been bootstrapped. See <<initial_master_nodes>>.
            
[float]
==== Expert settings

Discovery and cluster formation are also affected by the following
_expert-level_ settings, although it is not recommended to change any of these
from their default values.

WARNING: If you adjust these settings then your cluster may not form correctly
or may become unstable or intolerant of certain failures.

`discovery.cluster_formation_warning_timeout`::

    Sets how long a node will try to form a cluster before logging a warning
    that the cluster did not form. Defaults to `10s`. If a cluster has not
    formed after `discovery.cluster_formation_warning_timeout` has elapsed then
    the node will log a warning message that starts with the phrase `master not
    discovered` which describes the current state of the discovery process.

`discovery.find_peers_interval`::

    Sets how long a node will wait before attempting another discovery round.
    Defaults to `1s`.

`discovery.probe.connect_timeout`::

    Sets how long to wait when attempting to connect to each address. Defaults
    to `3s`.

`discovery.probe.handshake_timeout`::

    Sets how long to wait when attempting to identify the remote node via a
    handshake. Defaults to `1s`.

`discovery.request_peers_timeout`::

    Sets how long a node will wait after asking its peers again before
    considering the request to have failed. Defaults to `3s`.

`discovery.seed_resolver.max_concurrent_resolvers`::

    Specifies how many concurrent DNS lookups to perform when resolving the
    addresses of seed nodes. Defaults to `10`. This setting was previously
    known as `discovery.zen.ping.unicast.concurrent_connects`. Its old name is
    deprecated but continues to work in order to preserve backwards
    compatibility. Support for the old name will be removed in a future
    version.

`discovery.seed_resolver.timeout`::

    Specifies how long to wait for each DNS lookup performed when resolving the
    addresses of seed nodes. Defaults to `5s`. This setting was previously
    known as `discovery.zen.ping.unicast.hosts.resolve_timeout`. Its old name
    is deprecated but continues to work in order to preserve backwards
    compatibility. Support for the old name will be removed in a future
    version.

`cluster.auto_shrink_voting_configuration`::

    Controls whether the <<modules-discovery-voting,voting configuration>>
    sheds departed nodes automatically, as long as it still contains at least 3
    nodes. The default value is `true`. If set to `false`, the voting
    configuration never shrinks automatically and you must remove departed
    nodes manually with the <<voting-config-exclusions,voting configuration
    exclusions API>>.

[[master-election-settings]]`cluster.election.back_off_time`::

    Sets the amount to increase the upper bound on the wait before an election
    on each election failure. Note that this is _linear_ backoff. This defaults
    to `100ms`. Changing this setting from the default may cause your cluster
    to fail to elect a master node.

`cluster.election.duration`::

    Sets how long each election is allowed to take before a node considers it
    to have failed and schedules a retry. This defaults to `500ms`.  Changing
    this setting from the default may cause your cluster to fail to elect a
    master node.

`cluster.election.initial_timeout`::

    Sets the upper bound on how long a node will wait initially, or after the
    elected master fails, before attempting its first election. This defaults
    to `100ms`. Changing this setting from the default may cause your cluster
    to fail to elect a master node.

`cluster.election.max_timeout`::

    Sets the maximum upper bound on how long a node will wait before attempting
    an first election, so that an network partition that lasts for a long time
    does not result in excessively sparse elections. This defaults to `10s`.
    Changing this setting from the default may cause your cluster to fail to
    elect a master node.

[[fault-detection-settings]]`cluster.fault_detection.follower_check.interval`::

    Sets how long the elected master waits between follower checks to each
    other node in the cluster. Defaults to `1s`. Changing this setting from the
    default may cause your cluster to become unstable.

`cluster.fault_detection.follower_check.timeout`::

    Sets how long the elected master waits for a response to a follower check
    before considering it to have failed. Defaults to `10s`. Changing this
    setting from the default may cause your cluster to become unstable.

`cluster.fault_detection.follower_check.retry_count`::

    Sets how many consecutive follower check failures must occur to each node
    before the elected master considers that node to be faulty and removes it
    from the cluster. Defaults to `3`. Changing this setting from the default
    may cause your cluster to become unstable.

`cluster.fault_detection.leader_check.interval`::

    Sets how long each node waits between checks of the elected master.
    Defaults to `1s`. Changing this setting from the default may cause your
    cluster to become unstable.

`cluster.fault_detection.leader_check.timeout`::

    Sets how long each node waits for a response to a leader check from the
    elected master before considering it to have failed. Defaults to `10s`.
    Changing this setting from the default may cause your cluster to become
    unstable.

`cluster.fault_detection.leader_check.retry_count`::

    Sets how many consecutive leader check failures must occur before a node
    considers the elected master to be faulty and attempts to find or elect a
    new master. Defaults to `3`. Changing this setting from the default may
    cause your cluster to become unstable.

`cluster.follower_lag.timeout`::

    Sets how long the master node waits to receive acknowledgements for cluster
    state updates from lagging nodes. The default value is `90s`. If a node
    does not successfully apply the cluster state update within this period of
    time, it is considered to have failed and is removed from the cluster. See
    <<cluster-state-publishing>>.

`cluster.join.timeout`::

    Sets how long a node will wait after sending a request to join a cluster
    before it considers the request to have failed and retries. Defaults to
    `60s`.

`cluster.max_voting_config_exclusions`::

    Sets a limit on the number of voting configuration exclusions at any one
    time. The default value is `10`. See
    <<modules-discovery-adding-removing-nodes>>.

`cluster.publish.info_timeout`::

    Sets how long the master node waits for each cluster state update to be
    completely published to all nodes before logging a message indicating that
    some nodes are responding slowly. The default value is `10s`.

`cluster.publish.timeout`::

    Sets how long the master node waits for each cluster state update to be
    completely published to all nodes. The default value is `30s`. See
    <<cluster-state-publishing>>.

[[no-master-block]]`cluster.no_master_block`::
Specifies which operations are rejected when there is no active master in a
cluster. This setting has two valid values:
+
--
`all`::: All operations on the node (both read and write operations) are rejected.
This also applies for API cluster state read or write operations, like the get
index settings, put mapping and cluster state API.

`write`::: (default) Write operations are rejected. Read operations succeed,
based on the last known cluster configuration. This situation may result in
partial reads of stale data as this node may be isolated from the rest of the
cluster.

[NOTE]
===============================
* The `cluster.no_master_block` setting doesn't apply to nodes-based APIs
(for example, cluster stats, node info, and node stats APIs). Requests to these
APIs are not be blocked and can run on any available node.

* For the cluster to be fully operational, it must have an active master.
===============================

WARNING: This setting replaces the `discovery.zen.no_master_block` setting in
earlier versions. The `discovery.zen.no_master_block` setting is ignored.

--
