Unicast discovery is a mechanism that allows Elasticsearch nodes to discover other nodes in a cluster by specifying their IP addresses or hostnames. Unicast discovery is used when multicast discovery is not available or when networking restrictions prevent multicast communication.
In unicast discovery, each node in the cluster maintains a list of other nodes that it should try to connect to. The list can be specified in the Elasticsearch configuration file or passed as a command-line argument when starting the node.
When a node starts up, it tries to connect to each node in its unicast list. If a connection can be established, the node exchanges information with the other node to determine the state of the cluster and its topology. If the connection fails, the node tries to connect to the next node in the list.
Unicast discovery is often used in environments where multicast is not available or is not allowed due to security or network restrictions. It is also useful in cloud-based environments, where nodes may be dynamically provisioned and their IP addresses may change frequently.
To configure unicast discovery in Elasticsearch, you can specify a list of IP addresses or hostnames in the `discovery.zen.ping.unicast.hosts` setting in the Elasticsearch configuration file. For example, to specify a list of three nodes with IP addresses `192.168.1.10`, `192.168.1.11`, and `192.168.1.12`, you can add the following line to the configuration file:
discovery.zen.ping.unicast.hosts: ["192.168.1.10", "192.168.1.11", "192.168.1.12"]
Overall, unicast discovery is a key mechanism for discovering and communicating with nodes in an Elasticsearch cluster, and it is used in a wide range of environments where multicast is not available or practical.