Node

Any time that you start an instance of Elasticsearch, you are starting a node. A collection of connected nodes is called a 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 HTTP and Transport traffic by default. The transport layer is used exclusively for communication between nodes and the 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.

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

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 may be elected to become the master node by the 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 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 dedicated master-eligible node in the default distribution, set:

node.master: true 
node.voting_only: false 
node.data: false 
node.ingest: false 
node.ml: false 
xpack.ml.enabled: true 
cluster.remote.connect: false 

To create a dedicated master-eligible node in the OSS-only distribution, set:

node.master: true 
node.data: false 
node.ingest: false 
cluster.remote.connect: false 

A voting-only master-eligible node is a node that participates in 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, set the following setting:

node.voting_only: true 

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 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 distribution, set:

node.master: true 
node.voting_only: true 
node.data: false 
node.ingest: false 
node.ml: false 
xpack.ml.enabled: true 
cluster.remote.connect: false 

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 in the default distribution, set:

node.master: false 
node.voting_only: false 
node.data: true 
node.ingest: false 
node.ml: false 
cluster.remote.connect: false 

To create a dedicated data node in the OSS-only distribution, set:

node.master: false 
node.data: true 
node.ingest: false 
cluster.remote.connect: false 

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 in the default distribution, set:

node.master: false 
node.voting_only: false 
node.data: false 
node.ingest: true 
node.ml: false 
cluster.remote.connect: false 

To create a dedicated ingest node in the OSS-only distribution, set:

node.master: false 
node.data: false 
node.ingest: true 
cluster.remote.connect: false 

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, like every other node, and they use the cluster state to route requests directly to the appropriate place(s).

To create a dedicated coordinating node in the default distribution, set:

node.master: false 
node.voting_only: false 
node.data: false 
node.ingest: false 
node.ml: false 
cluster.remote.connect: false 

To create a dedicated coordinating node in the OSS-only distribution, set:

node.master: false 
node.data: false 
node.ingest: false 
cluster.remote.connect: false 

The machine learning features provide machine learning nodes, which run jobs and handle machine learning API requests. If xpack.ml.enabled is set to true and node.ml is set to false, the node can service API requests but it cannot run jobs.

If you want to use machine learning features in your cluster, you must enable machine learning (set xpack.ml.enabled to true) on all master-eligible nodes. If you have the OSS-only distribution, do not use these settings.

For more information about these settings, see Machine learning settings.

To create a dedicated machine learning node in the default distribution, set:

node.master: false 
node.voting_only: false 
node.data: false 
node.ingest: false 
node.ml: true 
xpack.ml.enabled: true 
cluster.remote.connect: false 

Each data node maintains the following data on disk:

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

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 dangling indices which can lead to a red cluster health. To be more precise, nodes with node.data: false will refuse to start if they find any shard data on disk at startup, and nodes with both node.master: false and node.data: false 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 setting its node.data or node.master roles to false:

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

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:

path.data:  /var/elasticsearch/data

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

./bin/elasticsearch -Epath.data=/var/elasticsearch/data

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

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.

More node settings can be found in Modules. Of particular note are the cluster.name, the node.name and the network settings.