Separate remote clusters docs from CCS (#34612)

With remote clusters taking on a larger role, we have make the
infrastructure more generic than being tied to cross-cluster search
(CCS). We want to refer to the remote clusters configuration in the
cross-cluster replication (CCR) docs. Yet, these docs are still tied to
CCS. This commit extracts the remote clusters docs from CCS (with some
wording changes to make them more general) so that we can refer to them
in the CCR docs.

docs/reference/modules.asciidoc

@@ -73,6 +73,8 @@ The modules in this section are:
     Configure the transport networking layer, used internally by Elasticsearch
     to communicate between nodes.
 
+<<modules-remote-cluster, Remote cluster>>::
+    Remote clusters are used in features that work by connecting across clusters on the transport layer.
 
 <<modules-cross-cluster-search, Cross cluster Search>>::
 
@@ -106,4 +108,6 @@ include::modules/threadpool.asciidoc[]
 
 include::modules/transport.asciidoc[]
 
+include::modules/remote-clusters.asciidoc[]
+
 include::modules/cross-cluster-search.asciidoc[]

docs/reference/modules/cross-cluster-search.asciidoc

@@ -6,52 +6,10 @@ multiple clusters. A cross cluster search node won't join the remote cluster, in
 it connects to a remote cluster in a light fashion in order to execute
 federated search requests.
 
-Cross cluster search works by configuring a remote cluster in the cluster state 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 up to 3
-_gateway nodes_ are selected to be connected to as part of upcoming cross cluster search requests.
-Cross cluster search requests 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 <<cross-cluster-search-settings>>).
-
-Each node in a cluster that has remote clusters configured connects to one or more _gateway nodes_ and uses
-them to federate search requests to the remote cluster.
-
 [float]
-=== Configuring Cross Cluster Search
-
-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, federated search requests will have to be sent specifically to those
-nodes. Remote clusters set via the <<cluster-update-settings,cluster settings API>>
-will be available on every node in the cluster.
-
-[WARNING]
-This feature was added as Beta in Elasticsearch `v5.3` with further improvements made in 5.4 and 5.5. It requires gateway eligible nodes to be on `v5.5` onwards.
-
-The `elasticsearch.yml` config file for a _cross cluster search_ node just 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.
+=== Using cross cluster search
 
-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:
+Cross-cluster search requires <<modules-remote-clusters,configuring remote clusters>>.
 
 [source,js]
 --------------------------------
@@ -84,85 +42,6 @@ PUT _cluster/settings
 // TEST[setup:host]
 // TEST[s/127.0.0.1:9300/\${transport_host}/]
 
-//////////////////////////
-
-We want to be sure that settings have been updated,
-because we'll use them later.
-
-[source,js]
---------------------------------------------------
-{
-    "acknowledged" : true,
-    "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"
-              ]
-            }
-          }
-        }
-    },
-    "transient" : {}
-}
---------------------------------------------------
-// TESTRESPONSE[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.
-
-//////////////////////////
-
-We want to be sure that settings have been updated,
-because we'll use them later.
-
-[source,js]
---------------------------------------------------
-{
-    "acknowledged" : true,
-    "persistent" : {},
-    "transient" : {}
-}
---------------------------------------------------
-// TESTRESPONSE
-
-//////////////////////////
-
-[float]
-=== Using cross cluster search
-
 To search the `twitter` index on remote cluster `cluster_one` the index name
 must be prefixed with the cluster alias separated by a `:` character:
 
@@ -385,47 +264,3 @@ GET /cluster_one:twitter,cluster_two:twitter,twitter/_search <1>
 // TESTRESPONSE[s/"_score": 1/"_score": "$body.hits.hits.0._score"/]
 // TESTRESPONSE[s/"_score": 2/"_score": "$body.hits.hits.1._score"/]
 <1> The `clusters` section indicates that one cluster was unavailable and got skipped
-
-
-[float]
-[[cross-cluster-search-settings]]
-=== Cross cluster search settings
-
-`cluster.remote.connections_per_cluster`::
-
-  The number of 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. Cross-cluster search 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 searched as part of a
-  cross cluster search 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 Cross Cluster Search node is connected to.

docs/reference/modules/remote-clusters.asciidoc

@@ -0,0 +1,161 @@
+[[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.