What is Cluster HA and how is Epsilon utilized to maintain RDB Quorum?

What is RDB Quorum?

RDB Quorum is a precondition for a fully functioning cluster

• Configurations that have greater than 2 nodes in the cluster, the entire cluster maintains quorum using a voting mechanism where each node has a vote that decides which node will maintain the master copy (read/write) of the replicated databases (also known as the RDB Master)
• When a cluster is in quorum, a simple majority of nodes are healthy and can communicate with each other
• When quorum is lost, the cluster loses the ability to accomplish normal cluster operations

What is epsilon?

Epsilon is a tie breaker

• Because there is the possibility of a tie in a cluster that has an even number of nodes, one node has an extra fractional voting weight called epsilon
• If the connectivity between two equal portions of a large cluster fails, the group of nodes containing epsilon maintains quorum, assuming that all of the nodes are healthy

What is Cluster HA?

Cluster HA allows continuous monitoring of a 2 node Cluster's state

• A 2 node cluster presents some unique challenges for maintaining quorum
• 2 node clusters use cluster HA, in which neither node holds epsilon
• Instead, both nodes are continuously polled to ensure that if one node fails, the other has full read-write access to data, as well as access to logical interfaces and management functions
• Quorum is maintained by a successful takeover

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