Node
editNode
editAny 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; 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.
Node roles
editYou define a node’s roles by setting node.roles
in elasticsearch.yml
. If you
set node.roles
, the node is only assigned the roles you specify. If you don’t
set node.roles
, the node is assigned the following roles:
-
master
-
data
-
data_content
-
data_hot
-
data_warm
-
data_cold
-
data_frozen
-
ingest
-
ml
-
remote_cluster_client
-
transform
If you set node.roles
, ensure you specify every node role your cluster needs.
Every cluster requires the following node roles:
-
master
-
data_content
anddata_hot
OR
data
Some Elastic Stack features also require specific node roles:
-
Cross-cluster search and cross-cluster replication require the
remote_cluster_client
role. -
Stack Monitoring and ingest pipelines require the
ingest
role. -
Fleet, the Elastic Security app, and transforms require the
transform
role. Theremote_cluster_client
role is also required to use cross-cluster search with these features. -
Machine learning features, such as anomaly detection, require the
ml
role.
As the cluster grows and in particular if you have large machine learning jobs or continuous transforms, consider separating dedicated master-eligible nodes from dedicated data nodes, machine learning nodes, and transform nodes.
- Master-eligible node
-
A node that has the
master
role, which makes it eligible to be elected as the master node, which controls the cluster. - Data node
-
A node that has the
data
role. Data nodes hold data and perform data related operations such as CRUD, search, and aggregations. A node with thedata
role can fill any of the specialised data node roles. - Ingest node
-
A node that has the
ingest
role. Ingest nodes are able to apply an 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 theingest
role from nodes that have themaster
ordata
roles. - Remote-eligible node
-
A node that has the
remote_cluster_client
role, which makes it eligible to act as a remote client. - Machine learning node
-
A node that has the
ml
role. If you want to use machine learning features, there must be at least one machine learning node in your cluster. For more information, see Machine learning settings and Machine learning in the Elastic Stack. - Transform node
-
A node that has the
transform
role. If you want to use transforms, there must be at least one transform node in your cluster. For more information, see Transforms settings and Transforming data.
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 result set.
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-eligible node
editThe 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.
Master nodes must have a path.data
directory whose contents
persist across restarts, just like data nodes, because this is where the
cluster metadata is stored. The cluster metadata describes how to read the data
stored on the data nodes, so if it is lost then the data stored on the data
nodes cannot be read.
Dedicated master-eligible node
editIt 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 will not operate well. The
most reliable way to avoid overloading the master with other tasks is to
configure all the master-eligible nodes to be dedicated master-eligible nodes
which only have the master
role, allowing them to focus on managing the
cluster. Master-eligible nodes will still also behave as
coordinating nodes that route requests from clients to
the other nodes in the cluster, but you should not use dedicated master nodes
for this purpose.
A small or lightly-loaded cluster may operate well if its master-eligible nodes have other roles and responsibilities, but once your cluster comprises more than a handful of nodes it usually makes sense to use dedicated master-eligible nodes.
To create a dedicated master-eligible node, set:
node.roles: [ master ]
Voting-only master-eligible node
editA 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, include master
and
voting_only
in the list of roles. For example to create a voting-only data
node:
node.roles: [ data, master, voting_only ]
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 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, set:
node.roles: [ master, voting_only ]
Data node
editData 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.roles: [ data ]
In a multi-tier deployment architecture, you use specialized data roles to
assign data nodes to specific tiers: data_content
,data_hot
, data_warm
,
data_cold
, or data_frozen
. A node can belong to multiple tiers, but a node
that has one of the specialized data roles cannot have the generic data
role.
Content data node
editContent data nodes accommodate user-created content. They enable operations like CRUD, search and aggregations.
To create a dedicated content node, set:
node.roles: [ data_content ]
Hot data node
editHot data nodes store time series data as it enters Elasticsearch. The hot tier must be fast for both reads and writes, and requires more hardware resources (such as SSD drives).
To create a dedicated hot node, set:
node.roles: [ data_hot ]
Warm data node
editWarm data nodes store indices that are no longer being regularly updated, but are still being queried. Query volume is usually at a lower frequency than it was while the index was in the hot tier. Less performant hardware can usually be used for nodes in this tier.
To create a dedicated warm node, set:
node.roles: [ data_warm ]
Cold data node
editCold data nodes store read-only indices that are accessed less frequently. This tier uses less performant hardware and may leverage searchable snapshot indices to minimize the resources required.
To create a dedicated cold node, set:
node.roles: [ data_cold ]
Frozen data node
editThe frozen tier stores partially mounted indices exclusively. We recommend you use dedicated nodes in the frozen tier.
To create a dedicated frozen node, set:
node.roles: [ data_frozen ]
Ingest node
editIngest 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.roles: [ ingest ]
Coordinating only node
editIf 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.roles: [ ]
Remote-eligible node
editA remote-eligible node acts as a cross-cluster client and connects to remote clusters. Once connected, you can search remote clusters using cross-cluster search. You can also sync data between clusters using cross-cluster replication.
node.roles: [ remote_cluster_client ]
Machine learning node
editMachine learning nodes run jobs and handle machine learning API requests. For more information, see Machine learning settings.
To create a dedicated machine learning node, set:
node.roles: [ ml, remote_cluster_client]
The remote_cluster_client
role is optional but strongly recommended.
Otherwise, cross-cluster search fails when used in machine learning jobs or datafeeds. If you use cross-cluster search in
your anomaly detection jobs, the remote_cluster_client
role is also required on all
master-eligible nodes. Otherwise, the datafeed cannot start. See Remote-eligible node.
Transform node
editTransform nodes run transforms and handle transform API requests. For more information, see Transforms settings.
To create a dedicated transform node, set:
node.roles: [ transform, remote_cluster_client ]
The remote_cluster_client
role is optional but strongly recommended.
Otherwise, cross-cluster search fails when used in transforms. See Remote-eligible node.
Changing the role of a node
editEach 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 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
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 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
normaster
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 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 elasticsearch-node repurpose
tool to delete any
excess data that prevents a node from starting.
Node data path settings
editpath.data
editEvery 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
editThe 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.
Never run different node types (i.e. master, data) from the same data directory. This can lead to unexpected data loss.
Other node settings
editMore node settings can be found in Configuring Elasticsearch and Important Elasticsearch configuration, including: