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

ifndef::include-xpack[]
The _remote clusters_ module enables you to establish uni-directional
connections to a remote cluster. This functionality is used in
<<modules-cross-cluster-search,{ccs}>>.
endif::[]
ifdef::include-xpack[]
The _remote clusters_ module enables you to establish uni-directional
connections to a remote cluster. This functionality is used in
{stack-ov}/xpack-ccr.html[{ccr}] and
<<modules-cross-cluster-search,{ccs}>>.
endif::[]

Remote cluster connections work by configuring a remote cluster and connecting
only to a limited number of nodes in that 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 and up
to three _gateway nodes_ are selected to be connected to as part of remote
cluster requests. All the communication required between different clusters
goes through the <<modules-transport,transport layer>>. Remote cluster
connections consist of uni-directional connections from the coordinating
node to the selected remote _gateway nodes_ only.

[float]
[[gateway-nodes-selection]]
=== Gateway nodes selection

The _gateway nodes_ selection depends on the following criteria:

- *version*: Remote nodes must be compatible with the cluster they are
registered to. This is subject to rules that are similar to those for
<<rolling-upgrades>>. Any node can communicate with any other node on the same
major version (e.g. 7.0 can talk to any 7.x node). Only nodes on the last minor
version of a certain major version can communicate with nodes on the following
major version. Note that in the 6.x series, 6.8 can communicate with any 7.x
node, while 6.7 can only communicate with 7.0. Version compatibility  is
symmetric, meaning that if 6.7 can communicate with 7.0, 7.0 can also
communicate with 6.7. The matrix below summarizes compatibility as described above.

[cols="^,^,^,^,^,^,^,^"]
|====
| Compatibility | 5.0->5.5 | 5.6 | 6.0->6.6 | 6.7 | 6.8 | 7.0 | 7.1->7.x
| 5.0->5.5      |    Yes   | Yes |    No    | No  | No  | No  |    No
| 5.6           |    Yes   | Yes |    Yes   | Yes | Yes | No  |    No
| 6.0->6.6      |    No    | Yes |    Yes   | Yes | Yes | No  |    No
| 6.7           |    No    | Yes |    Yes   | Yes | Yes | Yes |    No
| 6.8           |    No    | Yes |    Yes   | Yes | Yes | Yes |    Yes
| 7.0           |    No    | No  |    No    | Yes | Yes | Yes |    Yes
| 7.1->7.x      |    No    | No  |    No    | No  | Yes | Yes |    Yes
|====

- *role*: Dedicated master nodes never get selected.
- *attributes*: You can tag which nodes should be selected
(see <<remote-cluster-settings>>), though such tagged nodes still have
to satisfy the two above requirements.

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

You can configure remote clusters globally by using
<<cluster-update-settings,cluster settings>>, which you can update dynamically.
Alternatively, you can configure them locally on individual nodes by using the
 `elasticsearch.yml` file.

If you specify the settings in `elasticsearch.yml` files, only the nodes with
those settings can connect to the remote cluster. In other words, functionality
that relies on remote cluster requests must be driven specifically from those
nodes. For example:

[source,yaml]
--------------------------------
cluster:
    remote:
        cluster_one: <1>
            seeds: 127.0.0.1:9300 <2>
            transport.ping_schedule: 30s <3>
        cluster_two: 
            seeds: 127.0.0.1:9301
            transport.compress: true <4>
            skip_unavailable: true <5>

--------------------------------
<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.
<2> The hostname and <<modules-transport,transport>> port (default: 9300) of a
seed node in the remote cluster.
<3> A keep-alive ping is configured for `cluster_one`.
<4> Compression is explicitly enabled for requests to `cluster_two`.
<5> Disconnected remote clusters are optional for `cluster_two`.

For more information about the optional transport settings, see
<<modules-transport>>. 


If you use <<cluster-update-settings,cluster settings>>, the remote clusters
are available on every node in the cluster. For example:

[source,console]
--------------------------------
PUT _cluster/settings
{
  "persistent": {
    "cluster": {
      "remote": {
        "cluster_one": {
          "seeds": [
            "127.0.0.1:9300"
          ],
          "transport.ping_schedule": "30s"
        },
        "cluster_two": {
          "seeds": [
            "127.0.0.1:9301"
          ],
          "transport.compress": true,
          "skip_unavailable": true
        },
        "cluster_three": {
          "seeds": [
            "127.0.0.1:9302"
          ]
        }
      }
    }
  }
}
--------------------------------
// TEST[setup:host]
// TEST[s/127.0.0.1:9300/\${transport_host}/]

You can dynamically update the compression and ping schedule settings. However,
you must re-include seeds in the settings update request. For example:

[source,console]
--------------------------------
PUT _cluster/settings
{
  "persistent": {
    "cluster": {
      "remote": {
        "cluster_one": {
          "seeds": [
            "127.0.0.1:9300"
          ],
          "transport.ping_schedule": "60s"
        },
        "cluster_two": {
          "seeds": [
            "127.0.0.1:9301"
          ],
          "transport.compress": false
        }
      }
    }
  }
}
--------------------------------
// TEST[continued]

NOTE: When the compression or ping schedule settings change, all the existing
node connections must close and re-open, which can cause in-flight requests to
fail.

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

[source,console]
--------------------------------
PUT _cluster/settings
{
  "persistent": {
    "cluster": {
      "remote": {
        "cluster_two": { <1>
          "seeds": null,
          "skip_unavailable": null,
          "transport": {
            "compress": null
          }
        }
      }
    }
  }
}
--------------------------------
// TEST[continued]

<1> `cluster_two` would be removed from the cluster settings, leaving
`cluster_one` and `cluster_three` 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`.

`cluster.remote.${cluster_alias}.transport.ping_schedule`::

  Sets the time interval between regular application-level ping messages that
  are sent to ensure that transport connections to nodes belonging to remote
  clusters are kept alive. If set to `-1`, application-level ping messages to
  this remote cluster are not sent. If unset, application-level ping messages
  are sent according to the global `transport.ping_schedule` setting, which
  defaults to `-1` meaning that pings are not sent.

`cluster.remote.${cluster_alias}.transport.compress`::

  Per cluster boolean setting that enables you to configure compression for
  requests to a specific remote cluster. This setting impacts only requests
  sent to the remote cluster. If the inbound request is compressed,
  Elasticsearch compresses the response. If unset, the global
  `transport.compress` is used as the fallback setting.

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

  Sets a proxy address for the specified remote cluster. By default this is not
  set, meaning that Elasticsearch will connect directly to the nodes in the
  remote cluster using their <<advanced-network-settings,publish addresses>>.
  If this setting is set to an IP address or hostname then Elasticsearch will
  connect to the nodes in the remote cluster using this address instead.

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

You can use the <<cluster-remote-info, remote cluster info API>> to retrieve
information about the configured remote clusters, as well as the remote nodes
that the node is connected to.
