In the previous recipe, we saw how to create a REST entrypoint, but to execute the action at the cluster level, we will need to create a cluster action.
An Elasticsearch action is generally executed and distributed in the cluster and, in this recipe, we will see how to implement this kind of action. The cluster action will be very bare; we send a string with a value to every shard and the shards echo a result string concatenating the string with the shard number.
