elasticsearch/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 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 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.

<<client-node,Client node>>::

A client node has both `node.master` and `node.data` set to `false`. It can
neither hold data nor become the master node.  It behaves as a ``smart
router'' and is used to forward cluster-level requests to the master node and
data-related requests (such as search) to the appropriate data nodes.

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

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

By default a node is both a master-eligible node and a data node. 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.

This means that a _client_ 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>>.

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>
-------------------
<1> The `node.master` role is enabled by default.
<2> Disable the `node.data` 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
  }
}
----------------------------
// AUTOSENSE

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>
-------------------
<1> Disable the `node.master` role (enabled by default).
<2> The `node.data` role is enabled by default.

[float]
[[client-node]]
=== Client Node

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

Standalone client nodes can benefit large clusters by offloading the
coordinating node role from data and master-eligible nodes.  Client nodes 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 client 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 client nodes should
not be overstated -- data nodes can happily serve the same purpose as client
nodes.

To create a deciated client node, set:

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

To prevent more than one node from sharing the same data path, add this
setting to the `elasticsearch.yml` config file:

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

WARNING: Never run different node types (i.e. master, data, client) 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>>.