Remote clustersedit

The remote clusters module enables you to establish uni-directional connections to a remote cluster. This functionality is used in cross-cluster replication and cross-cluster search.

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 transport layer. Remote cluster connections consist of uni-directional connections from the coordinating node to the selected remote gateway nodes only.

Gateway nodes selectionedit

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.

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.

Configuring remote clustersedit

You can configure remote clusters globally by using 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:

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_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 hostname and transport port (default: 9300) of a seed node in the remote cluster.

A keep-alive ping is configured for cluster_one.

Compression is explicitly enabled for requests to cluster_two.

Disconnected remote clusters are optional for cluster_two.

For more information about the optional transport settings, see Transport.

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

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"
          ]
        }
      }
    }
  }
}

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

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
        }
      }
    }
  }
}
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 :

PUT _cluster/settings
{
  "persistent": {
    "cluster": {
      "remote": {
        "cluster_two": { 
          "seeds": null,
          "skip_unavailable": null,
          "transport": {
            "compress": null
          }
        }
      }
    }
  }
}

cluster_two would be removed from the cluster settings, leaving cluster_one and cluster_three intact.

Remote cluster settingsedit

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.

Retrieving remote clusters infoedit

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