OpenSearch/docs/reference/modules/node.asciidoc

[[modules-node]]
=== Node

Any time that you start an instance of Elasticsearch, you are starting a _node_.
A collection of connected nodes is called a <<modules-cluster,cluster>>. If you
are running a single node of {es}, then you have a cluster of one node.

Every node in the cluster can handle <<modules-http,HTTP>> and
<<modules-transport,Transport>> traffic by default. The transport layer is used
exclusively for communication between nodes; the HTTP layer is used by REST
clients.

All nodes know about all the other nodes in the cluster and can forward client
requests to the appropriate node.

By default, a node is all of the following types: master-eligible, data, ingest,
and (if available) machine learning and transform.

TIP: As the cluster grows and in particular if you have large {ml} jobs or
{ctransforms}, consider separating dedicated master-eligible nodes from
dedicated data nodes, {ml} nodes, and {transform} nodes.

<<master-node,Master-eligible node>>::

A node that has the `master` role (default), which makes it eligible to be
<<modules-discovery,elected as the _master_ node>>, which controls the cluster.

<<data-node,Data node>>::

A node that has the `data` role (default). Data nodes hold data and perform data
related operations such as CRUD, search, and aggregations.

<<ingest,Ingest node>>::

A node that has the `ingest` role (default). Ingest nodes are able to apply an
<<pipeline,ingest pipeline>> to a document in order to transform and enrich the
document before indexing. With a heavy ingest load, it makes sense to use
dedicated ingest nodes and to not include the `ingest` role from nodes that have
the `master` or `data` roles.

<<ml-node,Machine learning node>>::

A node that has `xpack.ml.enabled` and the `ml` role, which is the default
behavior in the {es} {default-dist}. If you want to use {ml-features}, there
must be at least one {ml} node in your cluster. For more information about
{ml-features}, see {ml-docs}/index.html[Machine learning in the {stack}].
+
IMPORTANT: If you use the {oss-dist}, do not add the `ml` role. Otherwise, the
node fails to start.

<<transform-node,{transform-cap} node>>::

A node that has the `transform` role. If you want to use {transforms}, there
be at least one {transform} node in your cluster. For more information, see
<<transform-settings>> and <<transforms>>.

[NOTE]
[[coordinating-node]]
.Coordinating node
===============================================

Requests like search requests or bulk-indexing requests may involve data held
on different data nodes. A search request, for example, is executed in two
phases which are coordinated by the node which receives the client request --
the _coordinating node_.

In the _scatter_ phase, the coordinating node forwards the request to the data
nodes which hold the data.  Each data node executes the request locally and
returns its results to the coordinating node. In the _gather_  phase, the
coordinating node reduces each data node's results into a single global
resultset.

Every node is implicitly a coordinating node. This means that a node that has
an explicit empty list of roles via `node.roles` will only act as a coordinating
node, which cannot be disabled. As a result, such a node needs to have enough
memory and CPU in order to deal with the gather phase.

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

[[master-node]]
==== Master-eligible node

The master node is responsible for lightweight cluster-wide actions such as
creating or deleting an index, tracking which nodes are part of the cluster,
and deciding which shards to allocate to which nodes. It is important for
cluster health to have a stable master node.

Any master-eligible node that is not a <<voting-only-node,voting-only node>> may
be elected to become the master node by the <<modules-discovery,master election
process>>.

IMPORTANT: Master nodes must have access to the `data/` directory (just like
`data` nodes) as this is where the cluster state is persisted between node
restarts.

[[dedicated-master-node]]
===== Dedicated master-eligible node

It is important for the health of the cluster that the elected master node has
the resources it needs to fulfill its responsibilities. If the elected master
node is overloaded with other tasks then the cluster may not operate well. In
particular, indexing and searching your data can be very resource-intensive, so
in large or high-throughput clusters it is a good idea to avoid using the
master-eligible nodes for tasks such as indexing and searching. You can do this
by configuring three of your nodes to be dedicated master-eligible nodes.
Dedicated master-eligible nodes only have the `master` role, allowing them to
focus on managing the cluster. While master nodes can also behave as
<<coordinating-node,coordinating nodes>> and route search and indexing requests
from clients to data nodes, it is better _not_ to use dedicated master nodes for
this purpose.

To create a dedicated master-eligible node, set:

[source,yaml]
-------------------
node.roles: [ master ]
-------------------

[[voting-only-node]]
===== Voting-only master-eligible node

A voting-only master-eligible node is a node that participates in
<<modules-discovery,master elections>> but which will not act as the cluster's
elected master node. In particular, a voting-only node can serve as a tiebreaker
in elections.

It may seem confusing to use the term "master-eligible" to describe a
voting-only node since such a node is not actually eligible to become the master
at all. This terminology is an unfortunate consequence of history:
master-eligible nodes are those nodes that participate in elections and perform
certain tasks during cluster state publications, and voting-only nodes have the
same responsibilities even if they can never become the elected master.

To configure a master-eligible node as a voting-only node, include `master` and
`voting_only` in the list of roles. For example to create a voting-only data
node:

[source,yaml]
-------------------
node.roles: [ data, master, voting_only ]
-------------------

IMPORTANT: The `voting_only` role requires the {default-dist} of {es} and is not
supported in the {oss-dist}. If you use the {oss-dist} and add the `voting_only`
role then the node will fail to start.  Also note that only nodes with the
`master` role can be marked as having the `voting_only` role.

High availability (HA) clusters require at least three master-eligible nodes, at
least two of which are not voting-only nodes. Such a cluster will be able to
elect a master node even if one of the nodes fails.

Since voting-only nodes never act as the cluster's elected master, they may
require require less heap and a less powerful CPU than the true master nodes.
However all master-eligible nodes, including voting-only nodes, require
reasonably fast persistent storage and a reliable and low-latency network
connection to the rest of the cluster, since they are on the critical path for
<<cluster-state-publishing,publishing cluster state updates>>.

Voting-only master-eligible nodes may also fill other roles in your cluster.
For instance, a node may be both a data node and a voting-only master-eligible
node. A _dedicated_ voting-only master-eligible nodes is a voting-only
master-eligible node that fills no other roles in the cluster. To create a
dedicated voting-only master-eligible node in the {default-dist}, set:

[source,yaml]
-------------------
node.roles: [ master, voting_only ]
-------------------

[[data-node]]
==== Data node

Data nodes hold the shards that contain the documents you have indexed. Data
nodes handle data related operations like CRUD, search, and aggregations.
These operations are I/O-, memory-, and CPU-intensive. It is important to
monitor these resources and to add more data nodes if they are overloaded.

The main benefit of having dedicated data nodes is the separation of the master
and data roles.

To create a dedicated data node, set:
[source,yaml]
-------------------
node.roles: [ data ]
-------------------

[[node-ingest-node]]
==== Ingest node

Ingest nodes can execute pre-processing pipelines, composed of one or more
ingest processors. Depending on the type of operations performed by the ingest
processors and the required resources, it may make sense to have dedicated
ingest nodes, that will only perform this specific task.

To create a dedicated ingest node, set:

[source,yaml]
-------------------
node.roles: [ ingest ]
-------------------

[[coordinating-only-node]]
==== Coordinating only node

If you take away the ability to be able to handle master duties, to hold data,
and pre-process documents, then you are left with a _coordinating_ node that
can only route requests, handle the search reduce phase, and distribute bulk
indexing. Essentially, coordinating only nodes behave as smart load balancers.

Coordinating only nodes can benefit large clusters by offloading the
coordinating node role from data and master-eligible nodes.  They join the
cluster and receive the full <<cluster-state,cluster state>>, like every other
node, and they use the cluster state to route requests directly to the
appropriate place(s).

WARNING: Adding too many coordinating only nodes to a cluster can increase the
burden on the entire cluster because the elected master node must await
acknowledgement of cluster state updates from every node! The benefit of
coordinating only nodes should not be overstated -- data nodes can happily
serve the same purpose.

To create a dedicated coordinating node, set:

[source,yaml]
-------------------
node.roles: [ ]
-------------------

[[ml-node]]
==== [xpack]#Machine learning node#

The {ml-features} provide {ml} nodes, which run jobs and handle {ml} API
requests. If `xpack.ml.enabled` is set to `true` and the node does not have the
`ml` role, the node can service API requests but it cannot run jobs.

If you want to use {ml-features} in your cluster, you must enable {ml}
(set `xpack.ml.enabled` to `true`) on all master-eligible nodes. If you want to
use {ml-features} in clients (including {kib}), it must also be enabled on all
coordinating nodes. If you have the {oss-dist}, do not use these settings.

For more information about these settings, see <<ml-settings>>.

To create a dedicated {ml} node in the {default-dist}, set:

[source,yaml]
-------------------
node.roles: [ ml ]
xpack.ml.enabled: true <1>
-------------------
<1> The `xpack.ml.enabled` setting is enabled by default.

[[transform-node]]
==== [xpack]#{transform-cap} node#

{transform-cap} nodes run {transforms} and handle {transform} API requests.

If you want to use {transforms} in clients (including {kib}), it must also be
enabled on all coordinating nodes. You must also have at least one node with the
`transform` role. This is the default behavior. If you have the {oss-dist}, do
not use these settings. For more information, see <<transform-settings>>.

To create a dedicated {transform} node in the {default-dist}, set:

[source,yaml]
-------------------
node.roles: [ transform ]
-------------------

[[change-node-role]]
==== Changing the role of a node

Each data node maintains the following data on disk:

* the shard data for every shard allocated to that node,
* the index metadata corresponding with every shard allocated to that node, and
* the cluster-wide metadata, such as settings and index templates.

Similarly, each master-eligible node maintains the following data on disk:

* the index metadata for every index in the cluster, and
* the cluster-wide metadata, such as settings and index templates.

Each node checks the contents of its data path at startup. If it discovers
unexpected data then it will refuse to start. This is to avoid importing
unwanted <<modules-gateway-dangling-indices,dangling indices>> which can lead
to a red cluster health. To be more precise, nodes without the `data` role will
refuse to start if they find any shard data on disk at startup, and nodes with
without both the `master` and `data` roles will refuse to start if they have any index metadata on disk at startup.

It is possible to change the roles of a node by adjusting its
`elasticsearch.yml` file and restarting it. This is known as _repurposing_ a
node. In order to satisfy the checks for unexpected data described above, you
must perform some extra steps to prepare a node for repurposing when starting
the node without the `data` or `master` roles.

* If you want to repurpose a data node by removing the `data` role then you
  should first use an <<allocation-filtering,allocation filter>> to safely
  migrate all the shard data onto other nodes in the cluster.

* If you want to repurpose a node to have neither the `data` nor `master` roles
  then it is simplest to start a brand-new node with an empty data path and the
  desired roles. You may find it safest to use an
  <<allocation-filtering,allocation filter>> to migrate the shard data elsewhere
  in the cluster first.

If it is not possible to follow these extra steps then you may be able to use
the <<node-tool-repurpose,`elasticsearch-node repurpose`>> tool to delete any
excess data that prevents a node from starting.

[discrete]
=== Node data path settings

[[data-path]]
==== `path.data`

Every data and master-eligible node requires access to a data directory where
shards and index and cluster metadata will be stored. The `path.data` defaults
to `$ES_HOME/data` but can be configured in the `elasticsearch.yml` config
file an absolute path or a path relative to `$ES_HOME` as follows:

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

Like all node settings, it can also be specified on the command line as:

[source,sh]
-----------------------
./bin/elasticsearch -Epath.data=/var/elasticsearch/data
-----------------------

TIP: When using the `.zip` or `.tar.gz` distributions, the `path.data` setting
should be configured to locate the data directory outside the {es} home
directory, so that the home directory can be deleted without deleting your data!
The RPM and Debian distributions do this for you already.

[discrete]
[[max-local-storage-nodes]]
=== `node.max_local_storage_nodes`

The <<data-path,data path>> can be shared by multiple nodes, even by nodes from
different clusters. It is recommended however to only run one node of {es} using
the same data path. This setting is deprecated in 7.x and will be removed in
version 8.0.

By default, {es} is configured to prevent more than one node from sharing the
same data path. To allow for more than one node (e.g., on your development
machine), use the setting `node.max_local_storage_nodes` and set this to a
positive integer larger than one.

WARNING: Never run different node types (i.e. master, data) from the same data
directory. This can lead to unexpected data loss.

[discrete]
[[other-node-settings]]
=== Other node settings

More node settings can be found in <<settings>> and <<important-settings>>,
including:

* <<cluster.name,`cluster.name`>>
* <<node.name,`node.name`>>
* <<modules-network,network settings>>