[[modules-node]] === Node Any time that you start an instance of {es}, you are starting a _node_. A collection of connected nodes is called a <>. If you are running a single node of {es}, then you have a cluster of one node. Every node in the cluster can handle <> traffic by default. The transport layer is used exclusively for communication between nodes; the HTTP layer is used by REST clients. [[modules-node-description]] // tag::modules-node-description-tag[] All nodes know about all the other nodes in the cluster and can forward client requests to the appropriate node. // end::modules-node-description-tag[] [[node-roles]] ==== Node roles You 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` [IMPORTANT] ==== If you set `node.roles`, ensure you specify every node role your cluster needs. Every cluster requires the following node roles: * `master` * {blank} + -- `data_content` and `data_hot` + OR + `data` -- Some {stack} features also require specific node roles: - {ccs-cap} and {ccr} require the `remote_cluster_client` role. - {stack-monitor-app} and ingest pipelines require the `ingest` role. - {fleet}, the {security-app}, and {transforms} require the `transform` role. The `remote_cluster_client` role is also required to use {ccs} with these features. - {ml-cap} features, such as {anomaly-detect}, require the `ml` role. ==== As the cluster grows and in particular if you have large {ml} jobs or {ctransforms}, consider separating dedicated master-eligible nodes from dedicated data nodes, {ml} nodes, and {transform} nodes. <>:: A node that has the `master` role, which makes it eligible to be <>, which controls the cluster. <>:: 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 the `data` role can fill any of the specialised data node roles. <>:: A node that has the `ingest` role. Ingest nodes are able to apply an <> 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 the `ingest` role from nodes that have the `master` or `data` roles. <>:: A node that has the `remote_cluster_client` role, which makes it eligible to act as a remote client. <>:: A node that has the `ml` role. If you want to use {ml-features}, there must be at least one {ml} node in your cluster. For more information, see <> and {ml-docs}/index.html[Machine learning in the {stack}]. <>:: 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 <> and <>. [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 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-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 that is not a <> may be elected to become the master node by the <>. IMPORTANT: 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-node]] ===== Dedicated master-eligible node It 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 <> 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: [source,yaml] ------------------- node.roles: [ master ] ------------------- [[voting-only-node]] ===== Voting-only master-eligible node A voting-only master-eligible node is a node that participates in <> 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: [source,yaml] ------------------- node.roles: [ data, master, voting_only ] ------------------- IMPORTANT: 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 <>. 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: [source,yaml] ------------------- node.roles: [ master, voting_only ] ------------------- [[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.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. [role="xpack"] [[data-content-node]] ==== Content data node Content data nodes accommodate user-created content. They enable operations like CRUD, search and aggregations. To create a dedicated content node, set: [source,yaml] ---- node.roles: [ data_content ] ---- [role="xpack"] [[data-hot-node]] ==== Hot data node Hot data nodes store time series data as it enters {es}. 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: [source,yaml] ---- node.roles: [ data_hot ] ---- [role="xpack"] [[data-warm-node]] ==== Warm data node Warm 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: [source,yaml] ---- node.roles: [ data_warm ] ---- [role="xpack"] [[data-cold-node]] ==== Cold data node Cold 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: [source,yaml] ---- node.roles: [ data_cold ] ---- [role="xpack"] [[data-frozen-node]] ==== Frozen data node The frozen tier stores <> exclusively. We recommend you use dedicated nodes in the frozen tier. To create a dedicated frozen node, set: [source,yaml] ---- node.roles: [ data_frozen ] ---- [[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.roles: [ ingest ] ---- [[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 <>, 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 dedicated coordinating node, set: [source,yaml] ---- node.roles: [ ] ---- [[remote-node]] ==== Remote-eligible node A remote-eligible node acts as a cross-cluster client and connects to <>. Once connected, you can search remote clusters using <>. You can also sync data between clusters using <>. [source,yaml] ---- node.roles: [ remote_cluster_client ] ---- [[ml-node]] ==== [xpack]#Machine learning node# {ml-cap} nodes run jobs and handle {ml} API requests. For more information, see <>. To create a dedicated {ml} node, set: [source,yaml] ---- node.roles: [ ml, remote_cluster_client] ---- The `remote_cluster_client` role is optional but strongly recommended. Otherwise, {ccs} fails when used in {ml} jobs or {dfeeds}. If you use {ccs} in your {anomaly-jobs}, the `remote_cluster_client` role is also required on all master-eligible nodes. Otherwise, the {dfeed} cannot start. See <>. [[transform-node]] ==== [xpack]#{transform-cap} node# {transform-cap} nodes run {transforms} and handle {transform} API requests. For more information, see <>. To create a dedicated {transform} node, set: [source,yaml] ---- node.roles: [ transform, remote_cluster_client ] ---- The `remote_cluster_client` role is optional but strongly recommended. Otherwise, {ccs} fails when used in {transforms}. See <>. [[change-node-role]] ==== Changing the role of a node Each 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 <> 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 <> 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` nor `master` 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 <> 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 <> tool to delete any excess data that prevents a node from starting. [discrete] === Node data path settings [[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 ---- The contents of the `path.data` directory must persist across restarts, because this is where your data is stored. {es} requires the filesystem to act as if it were backed by a local disk, but this means that it will work correctly on properly-configured remote block devices (e.g. a SAN) and remote filesystems (e.g. NFS) as long the remote storage behaves no differently from local storage. You can run multiple {es} nodes on the same filesystem, but each {es} node must have its own data path. The performance of an {es} cluster is often limited by the performance of the underlying storage, so you must ensure that your storage supports acceptable performance. Some remote storage performs very poorly, especially under the kind of load that {es} imposes, so make sure to benchmark your system carefully before committing to a particular storage architecture. TIP: When using the `.zip` or `.tar.gz` distributions, the `path.data` setting should be configured to locate the data directory outside the {es} home directory, so that the home directory can be deleted without deleting your data! The RPM and Debian distributions do this for you already. // tag::modules-node-data-path-warning-tag[] WARNING: Don't modify anything within the data directory or run processes that might interfere with its contents. If something other than {es} modifies the contents of the data directory, then {es} may fail, reporting corruption or other data inconsistencies, or may appear to work correctly having silently lost some of your data. Don't attempt to take filesystem backups of the data directory; there is no supported way to restore such a backup. Instead, use <> to take backups safely. Don't run virus scanners on the data directory. A virus scanner can prevent {es} from working correctly and may modify the contents of the data directory. The data directory contains no executables so a virus scan will only find false positives. // end::modules-node-data-path-warning-tag[] [discrete] [[other-node-settings]] === Other node settings More node settings can be found in <> and <>, including: * <> * <> * <>