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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 and 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:
- Master-eligible node
-
A node that has
node.master
set totrue
(default), which makes it eligible to be elected as the master node, which controls the cluster. - Data node
-
A node that has
node.data
set totrue
(default). Data nodes hold data and perform data related operations such as CRUD, search, and aggregations. - Client node
-
A client node has both
node.master
andnode.data
set tofalse
. 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. - 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.
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.
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 (all nodes by default) 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 standalone master-eligible node, set:
Avoiding split brain with minimum_master_nodes
editTo 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
:
This setting can also be changed dynamically on a live cluster with the cluster update settings API:
PUT _cluster/settings { "transient": { "discovery.zen.minimum_master_nodes": 2 } }
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.
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:
Client Node
editIf 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, like every other node, and they use the cluster state to route requests directly to the appropriate place(s).
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:
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 --path.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. 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:
node.max_local_storage_nodes: 1
Never run different node types (i.e. master, data, client) from the same data directory. This can lead to unexpected data loss.
Other node settings
editMore node settings can be found in Modules. Of particular note are
the cluster.name
, the node.name
and the
network settings.