[[modules-discovery-bootstrap-cluster]] === Bootstrapping a cluster Starting an Elasticsearch cluster for the very first time requires the initial set of <> to be explicitly defined on one or more of the master-eligible nodes in the cluster. This is known as _cluster bootstrapping_. This is only required the very first time the cluster starts up: nodes that have already joined a cluster store this information in their data folder and freshly-started nodes that are joining an existing cluster obtain this information from the cluster's elected master. The initial set of master-eligible nodes is defined in the <>. When you start a master-eligible node, you can provide this setting on the command line or in the `elasticsearch.yml` file. After the cluster has formed, this setting is no longer required and is ignored. It need not be set on master-ineligible nodes, nor on master-eligible nodes that are started to join an existing cluster. Note that master-eligible nodes should use storage that persists across restarts. If they do not, and `cluster.initial_master_nodes` is set, and a full cluster restart occurs, then another brand-new cluster will form and this may result in data loss. It is technically sufficient to set `cluster.initial_master_nodes` on a single master-eligible node in the cluster, and only to mention that single node in the setting's value, but this provides no fault tolerance before the cluster has fully formed. It is therefore better to bootstrap using at least three master-eligible nodes, each with a `cluster.initial_master_nodes` setting containing all three nodes. WARNING: You must set `cluster.initial_master_nodes` to the same list of nodes on each node on which it is set in order to be sure that only a single cluster forms during bootstrapping and therefore to avoid the risk of data loss. For a cluster with 3 master-eligible nodes (with <> `master-a`, `master-b` and `master-c`) the configuration will look as follows: [source,yaml] -------------------------------------------------- cluster.initial_master_nodes: - master-a - master-b - master-c -------------------------------------------------- Alternatively the IP addresses or hostnames (<>) can be used. If there is more than one Elasticsearch node with the same IP address or hostname then the transport ports must also be given to specify exactly which node is meant: [source,yaml] -------------------------------------------------- cluster.initial_master_nodes: - 10.0.10.101 - 10.0.10.102:9300 - 10.0.10.102:9301 - master-node-hostname -------------------------------------------------- Like all node settings, it is also possible to specify the initial set of master nodes on the command-line that is used to start Elasticsearch: [source,bash] -------------------------------------------------- $ bin/elasticsearch -Ecluster.initial_master_nodes=master-a,master-b,master-c -------------------------------------------------- [float] ==== Choosing a cluster name The <> setting enables you to create multiple clusters which are separated from each other. Nodes verify that they agree on their cluster name when they first connect to each other, and Elasticsearch will only form a cluster from nodes that all have the same cluster name. The default value for the cluster name is `elasticsearch`, but it is recommended to change this to reflect the logical name of the cluster. [float] ==== Auto-bootstrapping in development mode If the cluster is running with a completely default configuration then it will automatically bootstrap a cluster based on the nodes that could be discovered to be running on the same host within a short time after startup. This means that by default it is possible to start up several nodes on a single machine and have them automatically form a cluster which is very useful for development environments and experimentation. However, since nodes may not always successfully discover each other quickly enough this automatic bootstrapping cannot be relied upon and cannot be used in production deployments. If any of the following settings are configured then auto-bootstrapping will not take place, and you must configure `cluster.initial_master_nodes` as described in the <>: * `discovery.seed_providers` * `discovery.seed_hosts` * `cluster.initial_master_nodes`