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[[modules-discovery]]
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== Discovery and cluster formation
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The discovery and cluster formation processes are responsible for discovering
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nodes, electing a master, forming a cluster, and publishing the cluster state
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each time it changes. All communication between nodes is done using the
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<<modules-transport,transport>> layer.
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The following processes and settings are part of discovery and cluster
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formation:
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<<modules-discovery-hosts-providers>>::
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Discovery is the process where nodes find each other when the master is
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unknown, such as when a node has just started up or when the previous
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master has failed.
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<<modules-discovery-quorums>>::
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How {es} uses a quorum-based voting mechanism to
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make decisions even if some nodes are unavailable.
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<<modules-discovery-voting>>::
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How {es} automatically updates voting configurations as nodes leave and join
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a cluster.
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<<modules-discovery-bootstrap-cluster>>::
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Bootstrapping a cluster is required when an {es} cluster starts up
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for the very first time. In <<dev-vs-prod-mode,development mode>>, with no
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discovery settings configured, this is automatically performed by the nodes
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themselves. As this auto-bootstrapping is
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<<modules-discovery-quorums,inherently unsafe>>, running a node in
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<<dev-vs-prod-mode,production mode>> requires bootstrapping to be
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explicitly configured via the
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<<modules-discovery-bootstrap-cluster,`cluster.initial_master_nodes`
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setting>>.
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<<modules-discovery-adding-removing-nodes,Adding and removing master-eligible nodes>>::
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It is recommended to have a small and fixed number of master-eligible nodes
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in a cluster, and to scale the cluster up and down by adding and removing
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master-ineligible nodes only. However there are situations in which it may
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be desirable to add or remove some master-eligible nodes to or from a
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cluster. This section describes the process for adding or removing
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master-eligible nodes, including the extra steps that need to be performed
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when removing more than half of the master-eligible nodes at the same time.
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<<cluster-state-publishing>>::
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Cluster state publishing is the process by which the elected master node
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updates the cluster state on all the other nodes in the cluster.
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<<cluster-fault-detection>>::
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{es} performs health checks to detect and remove faulty nodes.
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<<modules-discovery-settings,Settings>>::
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There are settings that enable users to influence the discovery, cluster
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formation, master election and fault detection processes.
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include::discovery/discovery.asciidoc[]
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2019-01-07 12:11:14 -05:00
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include::discovery/quorums.asciidoc[]
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include::discovery/voting.asciidoc[]
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include::discovery/bootstrapping.asciidoc[]
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include::discovery/publishing.asciidoc[]
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include::discovery/fault-detection.asciidoc[]
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