Database Group Replication HA Behavior in Multi-Member Cluster

Activity	Description
Server startup	When an Oracle Communications Session Delivery Manager server is started it joins the cluster as a replica and an election is held if there is currently no master. If the member ends up as a replica, then it is synchronized with the master during the initialization phase of the database service startup. With the introduction of Oracle Communications Session Delivery Manager, Release 8.0, use the following steps to start an SDM server cluster: Select one server to start in the cluster only. Once the server you selected is started and operational, you can start the other server(s) in the cluster.
Master member failure	When the master fails or becomes partitioned from the rest of the members in the cluster, an election is automatically held by the remaining replicas to elect a new master.
Transactions (Quorum)	Transactions return successfully if a majority of the members in the cluster have replied that they received the replicated datasets. If quorum in replies from replicas is not achieved in a specific time period, the transaction fails.
Network partition	When a network partition exists between the members, only the members that can communicate with a majority of the members may elect a new master database. Members that can not communicate with a majority will enter READ-ONLY mode. Upon re-establishing network connectivity, re-elections take place and a master is elected while the other members revert to replicas.
Elections	In a cluster of three or more members, an election is won by the member with the simple majority of votes. The member that wins an election is always the one with the most recent log records. In the case of a tie, the member with the highest priority wins.
Rejoining cluster after shutdown for an extended period	If the server that needs to rejoin the cluster has been down for more than 24 hours, perform a hot backup on the host running the master database and restore the backup to the server that is down before restarting it. This avoids the potentially high cost synchronizing the server with the master during startup.