[[modules-remote-clusters]]
== Remote clusters

ifndef::include-xpack[]
The _remote clusters_ module allows establishing uni-directional connections to
a remote cluster. This functionality is used in
<<modules-cross-cluster-search,cross-cluster search>>.
endif::[]
ifdef::include-xpack[]
The _remote clusters_ module allows establishing uni-directional connections to
a remote cluster. This functionality is used in cross-cluster replication,
<<modules-cross-cluster-search,cross-cluster search>>.
endif::[]

Remote cluster connections work by configuring a remote cluster and connecting
only to a limited number of nodes in the remote cluster. Each remote cluster is
referenced by a name and a list of seed nodes.  When a remote cluster is
registered, its cluster state is retrieved from one of the seed nodes so that by
default up to three _gateway nodes_ are selected to be connected to as part of
remote cluster requests.  Remote cluster connections consist of uni-directional
connections from the coordinating node to the previously selected remote nodes
only. It is possible to tag which nodes should be selected through node
attributes (see <<remote-cluster-settings>>).

Each node in a cluster that has remote clusters configured connects to one or
more _gateway nodes_ and uses them to federate requests to the remote cluster.

[float]
[[configuring-remote-clusters]]
=== Configuring Remote Clusters

Remote clusters can be specified globally using
<<cluster-update-settings,cluster settings>> (which can be updated dynamically),
or local to individual nodes using the `elasticsearch.yml` file.

If a remote cluster is configured via `elasticsearch.yml` only the nodes with
that configuration will be able to connect to the remote cluster. In other
words, functionality that relies on remote cluster requests will have to be
driven specifically from those nodes. Remote clusters set via the
<<cluster-update-settings,cluster settings API>> will be available on every node
in the cluster.

The `elasticsearch.yml` config file for a node that connects to remote clusters
needs to list the remote clusters that should be connected to, for instance:

[source,yaml]
--------------------------------
cluster:
    remote:
        cluster_one: <1>
            seeds: 127.0.0.1:9300
        cluster_two: <1>
            seeds: 127.0.0.1:9301

--------------------------------
<1> `cluster_one` and `cluster_two` are arbitrary _cluster aliases_ representing
the connection to each cluster. These names are subsequently used to distinguish
between local and remote indices.

The equivalent example using the <<cluster-update-settings,cluster settings
API>> to add remote clusters to all nodes in the cluster would look like the
following:

[source,js]
--------------------------------
PUT _cluster/settings
{
  "persistent": {
    "cluster": {
      "remote": {
        "cluster_one": {
          "seeds": [
            "127.0.0.1:9300"
          ]
        },
        "cluster_two": {
          "seeds": [
            "127.0.0.1:9301"
          ]
        },
        "cluster_three": {
          "seeds": [
            "127.0.0.1:9302"
          ]
        }
      }
    }
  }
}
--------------------------------
// CONSOLE
// TEST[setup:host]
// TEST[s/127.0.0.1:9300/\${transport_host}/]

A remote cluster can be deleted from the cluster settings by setting its seeds
to `null`:

[source,js]
--------------------------------
PUT _cluster/settings
{
  "persistent": {
    "cluster": {
      "remote": {
        "cluster_three": {
          "seeds": null <1>
        }
      }
    }
  }
}
--------------------------------
// CONSOLE
// TEST[continued]
<1> `cluster_three` would be removed from the cluster settings, leaving
`cluster_one` and `cluster_two` intact.

[float]
[[remote-cluster-settings]]
=== Remote cluster settings

`cluster.remote.connections_per_cluster`::

  The number of gateway nodes to connect to per remote cluster. The default is
  `3`.

`cluster.remote.initial_connect_timeout`::

  The time to wait for remote connections to be established when the node
  starts. The default is `30s`.

`cluster.remote.node.attr`::

  A node attribute to filter out nodes that are eligible as a gateway node in
  the remote cluster. For instance a node can have a node attribute
  `node.attr.gateway: true` such that only nodes with this attribute will be
  connected to if `cluster.remote.node.attr` is set to `gateway`.

`cluster.remote.connect`::

  By default, any node in the cluster can act as a cross-cluster client and
  connect to remote clusters. The `cluster.remote.connect` setting can be set to
  `false` (defaults to `true`) to prevent certain nodes from connecting to
  remote clusters. Remote cluster requests must be sent to a node that is
  allowed to act as a cross-cluster client.

`cluster.remote.${cluster_alias}.skip_unavailable`::

  Per cluster boolean setting that allows to skip specific clusters when no
  nodes belonging to them are available and they are the targetof a remote
  cluster request. Default is `false`, meaning that all clusters are mandatory
  by default, but they can selectively be made optional by setting this setting
  to `true`.

[float]
[[retrieve-remote-clusters-info]]
=== Retrieving remote clusters info

The <<cluster-remote-info, Remote Cluster Info API>> allows to retrieve
information about the configured remote clusters, as well as the remote nodes
that the node is connected to.