[[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 Elasticsearch,
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 and the
{javaclient}/transport-client.html[Java `TransportClient`]; the HTTP layer is
used only by external REST clients.

All nodes know about all the other nodes in the cluster and can forward client
requests to the appropriate node. Besides that, each node serves one or more
purpose:

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

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

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

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

<<ingest,Ingest node>>::

A node that has `node.ingest` set to `true` (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 mark the master and data nodes as
`node.ingest: false`.

<<modules-tribe,Tribe node>>::

A tribe node, configured via the `tribe.*` settings, is a special type  of
coordinating only node that can connect to multiple clusters and perform
search and other operations across all connected clusters.

By default a node is a master-eligible node and a data node, plus it can
pre-process documents through ingest pipelines. This is very convenient for
small clusters but, as the cluster grows, it becomes important to consider
separating dedicated master-eligible nodes from dedicated data nodes.

[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
all three `node.master`, `node.data` and `node.ingest` set to `false` 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.

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

[float]
[[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 (all nodes by default) may be elected to become the
master node by the <<modules-discovery-zen,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.

Indexing and searching your data is CPU-, memory-, and I/O-intensive work
which can put pressure on a node's resources. To ensure that your master
node is stable and not under pressure, it is a good idea in a bigger
cluster to split the roles between dedicated master-eligible nodes and
dedicated data nodes.

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. It is important
for the stability of the cluster that master-eligible nodes do as little work
as possible.

To create a standalone master-eligible node, set:

[source,yaml]
-------------------
node.master: true <1>
node.data: false <2>
node.ingest: false <3>
-------------------
<1> The `node.master` role is enabled by default.
<2> Disable the `node.data` role (enabled by default).
<3> Disable the `node.ingest` role (enabled by default).

[float]
[[split-brain]]
==== Avoiding split brain with `minimum_master_nodes`

To prevent data loss, it is vital to configure the
`discovery.zen.minimum_master_nodes` setting (which defaults to `1`) so that
each master-eligible node knows the _minimum number of master-eligible nodes_
that must be visible in order to form a cluster.

To explain, imagine that you have a cluster consisting of two master-eligible
nodes. A network failure breaks communication between these two nodes.  Each
node sees one master-eligible node... itself. With `minimum_master_nodes` set
to the default of `1`,  this is sufficient to form a cluster. Each node elects
itself as the new master (thinking that the other master-eligible node has
died) and the result is two clusters, or a _split brain_.  These two nodes
will never rejoin until one node is restarted.  Any data that has been written
to the restarted node will be lost.

Now imagine that you have a cluster with three master-eligible nodes, and
`minimum_master_nodes` set to `2`.  If a network split separates one node from
the other two nodes, the side with one node cannot see enough master-eligible
nodes and will realise that it cannot elect itself as master.  The side with
two nodes will elect a new master (if needed) and continue functioning
correctly.  As soon as the network split is resolved, the single node will
rejoin the cluster and start serving requests again.

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 <1>
----------------------------
<1> Defaults to `1`.

This setting can also be changed dynamically on a live cluster with the
<<cluster-update-settings,cluster update settings API>>:

[source,js]
----------------------------
PUT _cluster/settings
{
  "transient": {
    "discovery.zen.minimum_master_nodes": 2
  }
}
----------------------------
// CONSOLE
// TEST[catch:/cannot set discovery.zen.minimum_master_nodes to more than the current master nodes/]

TIP: An advantage of splitting the master and data roles between dedicated
nodes is that you can have just three master-eligible nodes and set
`minimum_master_nodes` to `2`. You never have to change this setting, no
matter how many dedicated data nodes you add to the cluster.


[float]
[[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.master: false <1>
node.data: true <2>
node.ingest: false <3>
-------------------
<1> Disable the `node.master` role (enabled by default).
<2> The `node.data` role is enabled by default.
<3> Disable the `node.ingest` role (enabled by default).

[float]
[[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.master: false <1>
node.data: false <2>
node.ingest: true <3>
-------------------
<1> Disable the `node.master` role (enabled by default).
<2> Disable the `node.data` role (enabled by default).
<3> The `node.ingest` role is enabled by default.

[float]
[[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 coordinating only node, set:

[source,yaml]
-------------------
node.master: false <1>
node.data: false <2>
node.ingest: false <3>
-------------------
<1> Disable the `node.master` role (enabled by default).
<2> Disable the `node.data` role (enabled by default).
<3> Disable the `node.ingest` role (enabled by default).

[float]
== Node data path settings

[float]
[[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 Elasticsearch
home directory, so that the home directory can be deleted without deleting
your data! The RPM and Debian distributions do this for you already.


[float]
[[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. This is very useful for testing failover and different configurations on your development
machine. In production, however, it is recommended to run only one node of Elasticsearch per server.

By default, Elasticsearch 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.

[float]
== Other node settings

More node settings can be found in <<modules,Modules>>.  Of particular note are
the <<cluster.name,`cluster.name`>>, the <<node.name,`node.name`>> and the
<<modules-network,network settings>>.
