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:

Discovery
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.
Quorum-based decision making
This section describes how Elasticsearch uses a quorum-based voting mechanism to make decisions even if some nodes are unavailable.
Voting configurations
This section describes the concept of voting configurations, which Elasticsearch automatically updates as nodes leave and join the cluster.
Bootstrapping a cluster
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.
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.
Publishing the cluster state
Cluster state publishing is the process by which the elected master node updates the cluster state on all the other nodes in the cluster.
Cluster fault detection
Elasticsearch performs health checks to detect and remove faulty nodes.
Settings
There are settings that enable users to influence the discovery, cluster formation, master election and fault detection processes.