Elasticsearch is often deployed as a cluster of nodes. There are a variety of APIs that let you manage and monitor the cluster itself, rather than interact with the data stored within the cluster.
As with most functionality in Elasticsearch, there is an over-arching design goal that tasks should be performed through an API rather than by modifying static configuration files. This becomes especially important as your cluster scales. Even with a provisioning system (puppet, chef, ansible, etc), a single HTTP API call is often simpler than pushing new configurations to hundreds of physical machines.
To that end, this chapter will be discussing the various APIs that allow you to dynamically tweak, tune and configure your cluster. We will also cover a host of APIs that provide statistics about the cluster itself so that you can monitor for health and performance.