[[modules-tribe]]
== Tribe node

The _tribes_ feature allows a _tribe node_ to act as a federated client across
multiple clusters.

The tribe node works by retrieving the cluster state from all connected
clusters and merging them into a global cluster state. With this information
at hand, it is able to perform read and write operations against the nodes in
all clusters as if they were local. Note that a tribe node needs to be able
to connect to each single node in every configured cluster.

The `elasticsearch.yml` config file for a tribe node just needs to list the
clusters that should be joined, for instance:

[source,yaml]
--------------------------------
tribe:
    t1: <1>
        cluster.name:   cluster_one
    t2: <1>
        cluster.name:   cluster_two
--------------------------------
<1> `t1` and `t2` are arbitrary names representing the connection to each
    cluster.

The example above configures connections to two clusters, name `t1` and `t2`
respectively.  The tribe node will create a <<modules-node,node client>> to
connect each cluster using <<unicast,unicast discovery>> by default. Any
other settings for the connection can be configured under `tribe.{name}`, just
like the `cluster.name` in the example.

The merged global cluster state means that almost all operations work in the
same way as a single cluster: distributed search, suggest, percolation,
indexing, etc.

However, there are a few exceptions:

* The merged view cannot handle indices with the same name in multiple
  clusters. By default it will pick one of them, see later for on_conflict options.

* Master level read operations (eg <<cluster-state>>, <<cluster-health>>)
  will automatically execute with a local flag set to true since there is
  no master.

* Master level write operations (eg <<indices-create-index>>) are not
  allowed. These should be performed on a single cluster.

The tribe node can be configured to block all write operations and all
metadata operations with:

[source,yaml]
--------------------------------
tribe:
    blocks:
        write:    true
        metadata: true
--------------------------------

The tribe node can also configure blocks on selected indices:

[source,yaml]
--------------------------------
tribe:
    blocks:
        write.indices:    hk*,ldn*
        metadata.indices: hk*,ldn*
--------------------------------

When there is a conflict and multiple clusters hold the same index, by default
the tribe node will pick one of them. This can be configured using the `tribe.on_conflict`
setting. It defaults to `any`, but can be set to `drop` (drop indices that have
a conflict), or `prefer_[tribeName]` to prefer the index from a specific tribe.