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There are a few concepts that are core to machine learning in X-Pack. Understanding these concepts from the outset will tremendously help ease the learning process.
Machine learning nodes
A machine learning node is a node that has xpack.ml.enabled and node.ml set to true,
which is the default behavior. If you set node.ml to false, the node can
service API requests but it cannot run jobs. If you want to use X-Pack machine learning
features, there must be at least one machine learning node in your cluster. For more
information about this setting, see X-Pack Settings.