key

This setting is required when using the allocation action.

The value of this setting should correspond to a node setting on one or more nodes in your cluster.

For example, you might have set

node.tag: myvalue

in your elasticsearch.yml file for one or more of your nodes. To match allocation in this case, set key to tag.

These special attributes are also supported:

attribute	description
`_name`	Match nodes by node name
`_host_ip`	Match nodes by host IP address (IP associated with hostname)
`_publish_ip`	Match nodes by publish IP address
`_ip`	Match either `_host_ip` or `_publish_ip`
`_host`	Match nodes by hostname

There is no default value. This setting must be set by the user or an exception will be raised, and execution will halt.