[[modules-discovery-bootstrap-cluster]]
=== Bootstrapping a cluster

Starting an Elasticsearch cluster for the very first time requires the initial
set of <<master-node,master-eligible nodes>> 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 first time a cluster starts up: nodes
that have already joined a cluster store this information in their data folder
for use in a <<restart-upgrade,full cluster restart>>, and freshly-started nodes
that are joining a running cluster obtain this information from the cluster's
elected master.

The initial set of master-eligible nodes is defined in the
<<initial_master_nodes,`cluster.initial_master_nodes` setting>>. This should be
set to a list containing one of the following items for each master-eligible
node:

- The <<node-name,node name>> of the node.
- The node's hostname if `node.name` is not set, because `node.name` defaults
  to the node's hostname. You must use either the fully-qualified hostname or
  the bare hostname <<modules-discovery-bootstrap-cluster-fqdns,depending on
  your system configuration>>.
- The IP address of the node's <<modules-network-binding-publishing,transport
  publish address>>, if it is not possible to use the `node.name` of the node.
  This is normally the IP address to which
  <<common-network-settings,`network.host`>> resolves but
  <<advanced-network-settings,this can be overridden>>.
- The IP address and port of the node's publish address, in the form `IP:PORT`,
  if it is not possible to use the `node.name` of the node and there are
  multiple nodes sharing a single IP address.

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. It should not be set for master-ineligible
nodes, master-eligible nodes joining an existing cluster, or cluster restarts.

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 <<node-name,node names>>
`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
--------------------------------------------------

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
--------------------------------------------------

[NOTE]
==================================================

[[modules-discovery-bootstrap-cluster-fqdns]] The node names used in the
`cluster.initial_master_nodes` list must exactly match the `node.name`
properties of the nodes. By default the node name is set to the machine's
hostname which may or may not be fully-qualified depending on your system
configuration. If each node name is a fully-qualified domain name such as
`master-a.example.com` then you must use fully-qualified domain names in the
`cluster.initial_master_nodes` list too; conversely if your node names are bare
hostnames (without the `.example.com` suffix) then you must use bare hostnames
in the `cluster.initial_master_nodes` list. If you use a mix of fully-qualifed
and bare hostnames, or there is some other mismatch between `node.name` and
`cluster.initial_master_nodes`, then the cluster will not form successfully and
you will see log messages like the following.

[source,text]
--------------------------------------------------
[master-a.example.com] master not discovered yet, this node has
not previously joined a bootstrapped (v7+) cluster, and this
node must discover master-eligible nodes [master-a, master-b] to
bootstrap a cluster: have discovered [{master-b.example.com}{...
--------------------------------------------------

This message shows the node names `master-a.example.com` and
`master-b.example.com` as well as the `cluster.initial_master_nodes` entries
`master-a` and `master-b`, and it is clear from this message that they do not
match exactly.

==================================================

[discrete]
==== Choosing a cluster name

The <<cluster-name,`cluster.name`>> 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.

[discrete]
==== 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 <<modules-discovery-bootstrap-cluster,section on cluster bootstrapping>>:

* `discovery.seed_providers`
* `discovery.seed_hosts`
* `cluster.initial_master_nodes`

[NOTE]
==================================================

[[modules-discovery-bootstrap-cluster-joining]] If you start an {es} node
without configuring these settings then it will start up in development mode and
auto-bootstrap itself into a new cluster. If you start some {es} nodes on
different hosts then by default they will not discover each other and will form
a different cluster on each host. {es} will not merge separate clusters together
after they have formed, even if you subsequently try and configure all the nodes
into a single cluster. This is because there is no way to merge these separate
clusters together without a risk of data loss. You can tell that you have formed
separate clusters by checking the cluster UUID reported by `GET /` on each node.
If you intended to form a single cluster then you should start again:

* Shut down all the nodes.
* Completely wipe each node by deleting the contents of their
  <<data-path,data folders>>.
* Configure `cluster.initial_master_nodes` as described above.
* Restart all the nodes and verify that they have formed a single cluster.

==================================================
