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Elastic Docs ›Elasticsearch Guide [6.6] ›Modules

Remote clusters

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Remote clusters

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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 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.

Configuring Remote Clusters

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Remote clusters can be specified globally using 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 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:

cluster:
    remote:
        cluster_one: 
            seeds: 127.0.0.1:9300
        cluster_two: 
            seeds: 127.0.0.1:9301

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 settings API to add remote clusters to all nodes in the cluster would look like the following:

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"
          ]
        }
      }
    }
  }
}

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

PUT _cluster/settings
{
  "persistent": {
    "cluster": {
      "remote": {
        "cluster_three": {
          "seeds": null 
        }
      }
    }
  }
}

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

Remote cluster settings

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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.

Retrieving remote clusters info

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The 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.

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