[[important-settings]]
== Important Elasticsearch configuration

While Elasticsearch requires very little configuration, there are a number of
settings which need to be configured manually and should definitely be
configured before going into production.

* <<path-settings,`path.data` and `path.logs`>>
* <<cluster.name,`cluster.name`>>
* <<node.name,`node.name`>>
* <<bootstrap.memory_lock,`bootstrap.memory_lock`>>
* <<network.host,`network.host`>>
* <<unicast.hosts,`discovery.zen.ping.unicast.hosts`>>
* <<minimum_master_nodes,`discovery.zen.minimum_master_nodes`>>
* <<node.max_local_storage_nodes,`node.max_local_storage_nodes`>>

[float]
[[path-settings]]
=== `path.data` and `path.logs`

If you are using the `.zip` or `.tar.gz` archives, the `data` and `logs`
directories are sub-folders of `$ES_HOME`.  If these important folders are
left in their default locations, there is a high risk of them being deleted
while  upgrading Elasticsearch to a new version.

In production use, you will almost certainly want to change the locations of
the data and log folder:

[source,yaml]
--------------------------------------------------
path:
  logs: /var/log/elasticsearch
  data: /var/data/elasticsearch
--------------------------------------------------

The RPM and Debian distributions already use custom paths for `data` and
`logs`.

The `path.data` settings can be set to multiple paths, in which case all paths
will be used to store data (although the files belonging to a single shard
will all be stored on the same data path):

[source,yaml]
--------------------------------------------------
path:
  data:
    - /mnt/elasticsearch_1
    - /mnt/elasticsearch_2
    - /mnt/elasticsearch_3
--------------------------------------------------

[float]
[[cluster.name]]
=== `cluster.name`

A node can only join a cluster when it shares its `cluster.name` with all the
other nodes in the cluster. The default name is `elasticsearch`, but you
should change it to an appropriate name which describes the purpose of the
cluster.

[source,yaml]
--------------------------------------------------
cluster.name: logging-prod
--------------------------------------------------

Make sure that you don't reuse the same cluster names in different
environments, otherwise you might end up with nodes joining the wrong cluster.

[float]
[[node.name]]
=== `node.name`

By default, Elasticsearch will randomly pick a descriptive `node.name` from a
list of around 3000 Marvel characters when your node starts up, but this also
means that the `node.name` will change the next time the node restarts.

It is worth configuring a more meaningful name which will also have the
advantage of persisting after restarting the node:

[source,yaml]
--------------------------------------------------
node.name: prod-data-2
--------------------------------------------------

The `node.name` can also be set to the server's HOSTNAME as follows:

[source,yaml]
--------------------------------------------------
node.name: ${HOSTNAME}
--------------------------------------------------

[float]
[[bootstrap.memory_lock]]
=== `bootstrap.memory_lock`

It is vitally important to the health of your node that none of the JVM is
ever swapped out to disk.  One way of achieving that is set the
`bootstrap.memory_lock` setting to `true`.

For this setting to have effect, other system settings need to be configured
first. See <<mlockall>> for more details about how to set up memory locking
correctly.

[float]
[[network.host]]
=== `network.host`

By default, Elasticsearch binds to loopback addresses only -- e.g. `127.0.0.1`
and `[::1]`. This is sufficient to run a single development node on a server.

TIP: In fact, more than one node can be started from the same `$ES_HOME` location
on a single node.  This can be useful for testing Elasticsearch's ability to
form clusters, but it is not a configuration recommended for production.

In order to communicate and to form a cluster with nodes on other servers,
your node will need to bind to a non-loopback address.  While there are many
<<modules-network,network settings>>, usually all you need to configure is
`network.host`:

[source,yaml]
--------------------------------------------------
network.host: 192.168.1.10
--------------------------------------------------

The `network.host` setting also understands some special values such as
`_local_`, `_site_`, `_global_` and modifiers like `:ip4` and `:ip6`, details
of which can be found in <<network-interface-values>>.

IMPORTANT: As soon you provide a custom setting for `network.host`,
Elasticsearch assumes that you are moving from development mode to production
mode, and upgrades a number of system startup checks from warnings to
exceptions.  See <<dev-vs-prod>> for more information.

[float]
[[unicast.hosts]]
=== `discovery.zen.ping.unicast.hosts`

Out of the box, without any network configuration, Elasticsearch will bind to
the available loopback addresses and will scan ports 9300 to 9305 to try to
connect to other nodes running on the same server. This provides an auto-
clustering experience without having to do any configuration.

When the moment comes to form a cluster with nodes on other servers, you have
to provide a seed list of other nodes in the cluster that are likely to be
live and contactable.  This can be specified as follows:

[source,yaml]
--------------------------------------------------
discovery.zen.ping.unicast.hosts:
   - 192.168.1.10:9300
   - 192.168.1.11 <1>
   - seeds.mydomain.com <2>
--------------------------------------------------
<1> The port will default to 9300 if not specified.
<2> A hostname that resolves to multiple IP addresses will try all resolved addresses.

[float]
[[minimum_master_nodes]]
=== `discovery.zen.minimum_master_nodes`

To prevent data loss, it is vital to configure the
`discovery.zen.minimum_master_nodes setting` so that each master-eligible node
knows the _minimum number of master-eligible nodes_ that must be visible in
order to form a cluster.

Without this setting, a cluster that suffers a network failure is at risk of
having the cluster split into two independent clusters -- a split brain --
which will lead to data loss.  A more detailed explanation is provided
in <<split-brain>>.


To avoid a split brain, this setting should be set to a _quorum_ of master-
eligible nodes:

    (master_eligible_nodes / 2) + 1

In other words, if there are three master-eligible nodes, then minimum master
nodes should be set to `(3 / 2) + 1` or `2`:

[source,yaml]
--------------------------------------------------
discovery.zen.minimum_master_nodes: 2
--------------------------------------------------

IMPORTANT: If `discovery.zen.minimum_master_nodes` is not set when
Elasticsearch is running in <<dev-vs-prod,production mode>>, an exception will
be thrown which will prevent the node from starting.

[float]
[[node.max_local_storage_nodes]]
=== `node.max_local_storage_nodes`

It is possible to start more than one node on the same server from the same
`$ES_HOME`, just by doing the following:

[source,sh]
--------------------------------------------------
./bin/elasticsearch -d
./bin/elasticsearch -d
--------------------------------------------------

This works just fine: the data directory structure is designed to let multiple
nodes coexist.  However, a single instance of Elasticsearch is able to use all
of the resources of a single server and it seldom makes sense to run multiple
nodes on the same server in production.

It is, however, possible to start more than one node on the same server by
mistake and to be completely unaware that this problem exists. To prevent more
than one node from sharing the same data directory, it is advisable to add the
following setting:

[source,yaml]
--------------------------------------------------
node.max_local_storage_nodes: 1
--------------------------------------------------

