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. Besides that, each node serves one or more purpose:

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.

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 master election process.

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 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, set:

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

These settings apply only when X-Pack is not installed. To create a dedicated master-eligible node when X-Pack is installed, see X-Pack node settings.

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:

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

These settings apply only when X-Pack is not installed. To create a dedicated data node when X-Pack is installed, see X-Pack node settings.

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:

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

These settings apply only when X-Pack is not installed. To create a dedicated ingest node when X-Pack is installed, see X-Pack node settings.

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

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:

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

These settings apply only when X-Pack is not installed. To create a dedicated coordinating node when X-Pack is installed, see X-Pack node settings.

Changing the role of a node

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.

Node data path settings

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:

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

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.

node.max_local_storage_nodes

The 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.

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

Other node settings

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

X-Pack node settings

If X-Pack is installed, there is an additional node type:

Do not set use the node.ml setting unless X-Pack is installed. Otherwise, the node fails to start.

If X-Pack is installed, nodes are master-eligible, data, ingest, and machine learning nodes by default. As the cluster grows and in particular if you have large machine learning jobs, consider separating dedicated master-eligible nodes from dedicated data nodes and dedicated machine learning nodes.

To create a dedicated master-eligible node when X-Pack is installed, set:

node.master: true 
node.data: false 
node.ingest: false 
node.ml: false 
xpack.ml.enabled: true 

To create a dedicated data node when X-Pack is installed, set:

node.master: false 
node.data: true 
node.ingest: false 
node.ml: false 

To create a dedicated ingest node when X-Pack is installed, set:

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

To create a dedicated coordinating node when X-Pack is installed, set:

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

Machine learning node

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. Do not use these settings if you do not have X-Pack installed.

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

To create a dedicated machine learning node, set:

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