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 transport
module. This module is divided into the following sections:
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Discovery
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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.
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Quorum-based decision making
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This section describes how Elasticsearch uses a quorum-based voting mechanism to
make decisions even if some nodes are unavailable.
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Voting configurations
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This section describes the concept of voting configurations, which Elasticsearch
automatically updates as nodes leave and join the cluster.
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Bootstrapping a cluster
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Bootstrapping a cluster is required when an Elasticsearch cluster starts up
for the very first time. In development mode, with no
discovery settings configured, this is automatically performed by the nodes
themselves. As this auto-bootstrapping is
inherently unsafe, running a node in
production mode requires bootstrapping to be
explicitly configured via the
cluster.initial_master_nodes setting.
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Adding and removing master-eligible nodes
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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.
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Publishing the cluster state
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Cluster state publishing is the process by which the elected master node
updates the cluster state on all the other nodes in the cluster.
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Cluster fault detection
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Elasticsearch performs health checks to detect and remove faulty nodes.
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Settings
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There are settings that enable users to influence the discovery, cluster
formation, master election and fault detection processes.