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

The discovery and cluster formation module is responsible for discovering
nodes, electing a master, forming a cluster, and publishing the cluster state
each time it changes. It is integrated with other modules. For example, all
communication between nodes is done using the <<modules-transport,transport>>
module. This module is divided into the following sections:

<<modules-discovery-hosts-providers>>::

    Discovery is the process where nodes find each other when the master is
    unknown, such as when a node has just started up or when the previous
    master has failed.

<<modules-discovery-bootstrap-cluster>>::

    Bootstrapping a cluster is required when an Elasticsearch cluster starts up
    for the very first time. In <<dev-vs-prod-mode,development mode>>, with no
    discovery settings configured, this is automatically performed by the nodes
    themselves. As this auto-bootstrapping is
    <<modules-discovery-quorums,inherently unsafe>>, running a node in
    <<dev-vs-prod-mode,production mode>> requires bootstrapping to be
    explicitly configured via the
    <<modules-discovery-bootstrap-cluster,`cluster.initial_master_nodes`
    setting>>.

<<modules-discovery-adding-removing-nodes,Adding and removing master-eligible nodes>>::

    It is recommended to have a small and fixed number of master-eligible nodes
    in a cluster, and to scale the cluster up and down by adding and removing
    master-ineligible nodes only. However there are situations in which it may
    be desirable to add or remove some master-eligible nodes to or from a
    cluster. This section describes the process for adding or removing
    master-eligible nodes, including the extra steps that need to be performed
    when removing more than half of the master-eligible nodes at the same time.

<<cluster-state-publishing>>::

    Cluster state publishing is the process by which the elected master node
    updates the cluster state on all the other nodes in the cluster.

<<no-master-block>>::

    The no-master block is put in place when there is no known elected master,
    and can be configured to determine which operations should be rejected when
    it is in place.

Advanced settings::

    There are settings that allow advanced users to influence the
    <<master-election-settings,master election>> and
    <<fault-detection-settings,fault detection>> processes.

<<modules-discovery-quorums>>::

    This section describes the detailed design behind the master election and
    auto-reconfiguration logic.

include::discovery/discovery.asciidoc[]

include::discovery/bootstrapping.asciidoc[]

include::discovery/adding-removing-nodes.asciidoc[]

include::discovery/publishing.asciidoc[]

include::discovery/no-master-block.asciidoc[]

include::discovery/master-election.asciidoc[]

include::discovery/fault-detection.asciidoc[]

include::discovery/quorums.asciidoc[]