elasticsearch allows to configure a node to either be allowed to store data locally or not. Storing data locally basically means that shards of different indices are allowed to be allocated on that node. By default, each node is considered to be a data node, and it can be turned off by setting to false.

This is a powerful setting allowing to create 2 types of non-data nodes: dedicated master nodes and client nodes.

client nodes are smart load balancers that take part in some of the processing steps. Lets take an example:

We can start a whole cluster of data nodes which do not even start an HTTP transport by setting http.enabled to false. Such nodes will communicate with one another using the transport module. In front of the cluster we can start one or more "client" nodes which will start with HTTP enabled. These client nodes will have the settings false and node.master: false. All HTTP communication will be performed through these client nodes.

These "client" nodes are still part of the cluster, and they can redirect operations exactly to the node that holds the relevant data without having to query all nodes. However, they do not store data and also do not perform cluster management operations. The other benefit is the fact that for scatter / gather based operations (such as search), since the client nodes will start the scatter process, they will perform the actual gather processing. This relieves the data nodes to do the heavy duty of indexing and searching, without needing to process HTTP requests (parsing), overload the network, or perform the gather processing.

dedicated master nodes are nodes with the settings false and node.master: true. We actively promote the use of dedicated master nodes in critical clusters to make sure that there are 3 dedicated nodes whose only role is to be master, a lightweight operational (cluster management) responsibility. By reducing the amount of resource intensive work that these nodes do (in other words, do not send index or search requests to these dedicated master nodes), we greatly reduce the chance of cluster instability.