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

You can connect a local cluster to other {es} clusters, known as _remote
clusters_. Once connected, you can search remote clusters using
<<modules-cross-cluster-search,{ccs}>>. You can also sync data between clusters
using <<xpack-ccr,{ccr}>>.

To register a remote cluster, connect the local cluster to nodes in the
remote cluster using one of two connection modes:

* <<sniff-mode,Sniff mode>>
* <<proxy-mode,Proxy mode>>

Your local cluster uses the <<modules-transport,transport layer>> to establish
communication with remote clusters. The coordinating nodes in the local cluster
establish <<long-lived-connections,long-lived>> TCP connections with specific
nodes in the remote cluster. {es} requires these connections to remain open,
even if the connections are idle for an extended period.

You can use the <<cluster-remote-info, remote cluster info API>> to get
information about registered remote clusters.

[[sniff-mode]]
[discrete]
==== Sniff mode

In sniff mode, a cluster is created using 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 as part of remote
cluster requests. This mode requires that the gateway node's publish addresses
are accessible by the local cluster.

Sniff mode is the default connection mode.

[[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, similar to the rules for
<<rolling-upgrades>>:
** Any node can communicate with another node on the same
major version. For example, 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. 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 following table depicts version compatibility between
local and remote nodes.
+
[%collapsible]
.Version compatibility table
====
// tag::remote-cluster-compatibility-matrix[]
[cols="^,^,^,^,^,^,^,^"]
|====
| 7+^h| Local cluster
h| Remote cluster | 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-icon} | {yes-icon} | {no-icon} | {no-icon} | {no-icon}  | {no-icon} | {no-icon}
| 5.6           | {yes-icon} | {yes-icon} | {yes-icon} | {yes-icon} | {yes-icon} | {no-icon} | {no-icon}
| 6.0->6.6      | {no-icon} | {yes-icon} | {yes-icon} | {yes-icon} | {yes-icon} | {no-icon} | {no-icon}
| 6.7           | {no-icon} | {yes-icon} | {yes-icon} | {yes-icon} | {yes-icon} | {yes-icon} | {no-icon}
| 6.8           | {no-icon} | {yes-icon} |  {yes-icon} | {yes-icon} | {yes-icon} | {yes-icon} | {yes-icon}
| 7.0           | {no-icon} | {no-icon} | {no-icon} | {yes-icon} | {yes-icon} | {yes-icon} | {yes-icon}
| 7.1->7.x      | {no-icon} | {no-icon} | {no-icon} | {no-icon} | {yes-icon} | {yes-icon} | {yes-icon}
|====
// end::remote-cluster-compatibility-matrix[]
====

* *role*: Dedicated master nodes are never selected as gateway nodes.
* *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.

[[proxy-mode]]
[discrete]
==== Proxy mode

In proxy mode, a cluster is created using a name and a single proxy address.
When you register a remote cluster, a configurable number of socket connections
are opened to the proxy address. The proxy is required to route those
connections to the remote cluster. Proxy mode does not require remote cluster
nodes to have accessible publish addresses.

The proxy mode is not the default connection mode and must be configured. Similar
to the sniff <<gateway-nodes-selection,gateway nodes>>, the remote
connections are subject to the same version compatibility rules as
<<rolling-upgrades>>.

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

You can configure remote clusters settings <<configure-remote-clusters-dynamic,globally>>, or configure
settings <<configure-remote-clusters-static,on individual nodes>> in the
`elasticsearch.yml` file.

[discrete]
[[configure-remote-clusters-dynamic]]
===== Dynamically configure remote clusters
Use the <<cluster-update-settings,cluster update settings API>> to dynamically
configure remote settings 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": {
          "mode": "sniff",
          "seeds": [
            "127.0.0.1:9301"
          ],
          "transport.compress": true,
          "skip_unavailable": true
        },
        "cluster_three": {
          "mode": "proxy",
          "proxy_address": "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 or `proxy_address` 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": {
          "mode": "sniff",
          "seeds": [
            "127.0.0.1:9301"
          ],
          "transport.compress": false
        },
        "cluster_three": {
          "mode": "proxy",
          "proxy_address": "127.0.0.1:9302",
          "transport.compress": true
        }
      }
    }
  }
}
--------------------------------
// 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.

You can delete a remote cluster from the cluster settings by passing `null`
values for each remote cluster setting:

[source,console]
--------------------------------
PUT _cluster/settings
{
  "persistent": {
    "cluster": {
      "remote": {
        "cluster_two": { <1>
          "mode": null,
          "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.

[discrete]
[[configure-remote-clusters-static]]
===== Statically configure remote clusters
If you specify settings in `elasticsearch.yml` files, only the nodes with
those settings can connect to the remote cluster and serve remote cluster requests. For example:

[source,yaml]
--------------------------------
cluster:
    remote:
        cluster_one: <1>
            seeds: 127.0.0.1:9300 <2>
            transport.ping_schedule: 30s <3>
        cluster_two: <1>
            mode: sniff <4>
            seeds: 127.0.0.1:9301 <2>
            transport.compress: true <5>
            skip_unavailable: true <6>
        cluster_three: <1>
            mode: proxy <4>
            proxy_address: 127.0.0.1:9302 <7>

--------------------------------
<1> `cluster_one`, `cluster_two`, and `cluster_three` 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> The configured connection mode. By default, this is <<sniff-mode,`sniff`>>, so
the mode is implicit for `cluster_one`. However, it can be explicitly configured
as demonstrated by `cluster_two` and must be explicitly configured for
<<proxy-mode,proxy mode>> as demonstrated by `cluster_three`.
<5> Compression is explicitly enabled for requests to `cluster_two`.
<6> Disconnected remote clusters are optional for `cluster_two`.
<7> The address for the proxy endpoint used to connect to `cluster_three`.

[discrete]
[[remote-cluster-settings]]
=== Global remote cluster settings

These settings apply to both <<sniff-mode,sniff mode>> and
<<proxy-mode,proxy mode>>. <<remote-cluster-sniff-settings,Sniff mode settings>>
and <<remote-cluster-proxy-settings,proxy mode settings>> are described
separately.

`cluster.remote.<cluster_alias>.mode`::
  The mode used for a remote cluster connection. The only supported modes are
  `sniff` and `proxy`.

`cluster.remote.initial_connect_timeout`::

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

`node.remote_cluster_client`::

  By default, any node in the cluster can act as a cross-cluster client and
  connect to remote clusters. The `node.remote_cluster_client` 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 target of 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.

[discrete]
[[remote-cluster-sniff-settings]]
=== Sniff mode remote cluster settings

`cluster.remote.<cluster_alias>.seeds`::

  The list of seed nodes used to sniff the remote cluster state.

`cluster.remote.<cluster_alias>.node_connections`::

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

`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`.

[discrete]
[[remote-cluster-proxy-settings]]
=== Proxy mode remote cluster settings

`cluster.remote.<cluster_alias>.proxy_address`::

  The address used for all remote connections.

`cluster.remote.<cluster_alias>.proxy_socket_connections`::

  The number of socket connections to open per remote cluster. The default is
  `18`.

[role="xpack"]
`cluster.remote.<cluster_alias>.server_name`::

  An optional hostname string which is sent in the `server_name` field of
  the TLS Server Name Indication extension if
  <<configuring-tls,TLS is enabled>>. The TLS transport will fail to open
  remote connections if this field is not a valid hostname as defined by the
  TLS SNI specification.
