Gossip protocol is a communication protocol that allows state sharing in distributed systems. This protocol is used in a decentralised system that does not have any central system to manage/track all the nodes in the system and to know if a node is down or not.
The protocol enables each node to keep track of state information about the other nodes in the cluster, such as which nodes are reachable, what key ranges they are responsible for, and so on (this is basically a copy of the hash ring). Nodes share state information to stay in sync. Gossip protocol is a peer-to-peer communication mechanism in which nodes periodically exchange state information about themselves and other nodes they know about. Each node initiates a gossip round every second to exchange state information about itself and other nodes with one other random node. This means that any new event eventually propagates through the system, and all nodes quickly learn about all other nodes in a cluster.
The gossip protocol can result in a logical partition of the cluster in a particular scenario. Let’s understand this with an example:
An administrator joins node A to the ring and then joins node B to the ring. Nodes A and B consider themselves part of the ring, yet neither would be immediately aware of each other. To prevent these logical partitions, some distributed systems use the concept of seed nodes. Seed nodes are fully functional nodes and can be obtained either from a static configuration or a configuration service. This way, all nodes are aware of seed nodes. Each node communicates with seed nodes through gossip protocol to reconcile membership changes. Therefore, logical partitions are highly unlikely.