[fizx]

Given that this is based on Elasticsearch, there will be a master node, which should be chosen explicitly in configuration to be away from the data nodes, because ES responds poorly to garbage collection on the master node. So you want to keep the load down on your master.

So you can run in a sort-of-shared-nothing configuration, but its not recommended.

[spanktar]

True. There is an elected master node and you can configure a cluster to have master-only (non-data) nodes, data-only nodes (non-master eligible), or both. Even still, and even if you choose to set up a cluster this way, every node can perform the same functions, it's more a matter of what responsibility a node has. And even though there is a master node, that master can disappear without consequence, another will be elected in its stead. There is nothing really "special" about a master node.