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

Discovery and cluster formation are affected by the following settings:

[[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`

`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`.
    
`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`.

    
`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`
        
[[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`.

`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 `30s`.

`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`.

`cluster.fault_detection.leader_check.interval`::

    Sets how long each node waits between checks of the elected master.
    Defaults to `1s`.

`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 `30s`.

`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`.

`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.initial_master_nodes`::

    Sets a list of the <<node.name,node names>> or transport addresses of 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>>.

`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.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`. If this
    period of time elapses, the cluster state change is rejected. See
    <<cluster-state-publishing>>.   

`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.zen.hosts_provider`:: 
    Specifies which type of <<built-in-hosts-providers,hosts provider>> provides
    the list of seed nodes. By default, it is the 
    <<settings-based-hosts-provider,settings-based hosts provider>>.

[[no-master-block]]`discovery.zen.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 `discovery.zen.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.
===============================
--

`discovery.zen.ping.unicast.hosts`::

    Provides a list of master-eligible nodes in the cluster. The list contains
    either an array of hosts or a comma-delimited string. 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>>.

`discovery.zen.ping.unicast.hosts.resolve_timeout`::

    Sets the amount of time to wait for DNS lookups on each round of discovery. This is specified as a <<time-units, time unit>> and defaults to `5s`.