Database Group Replication HA Behavior in a Two Member Cluster

Operation	Description
Server startup	When an Oracle Communications Session Delivery Manager server is started it joins the cluster as a replica and 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 in the two-node cluster.
Master member failure	When the master fails the remaining replica becomes the 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 the master fails the remaining replica becomes the new master.
Elections	An election can be won with a single vote. This allows the replica to be elected master in the case the master fails.
Recovery after a network partition	In a two node cluster it is possible for the network connection between the master and replica to be partitioned or become unresponsive due to network latency. In this situation an election is held and both nodes are elected and act as masters. While in this state, write transactions can occur at both sites. As a result, special handling is required after the partition is resolved and the system recovers from a two master configuration to a single master configuration: Before the partition is resolved both nodes are in the role of master. After the partition is resolved an election is automatically held to elect a master. When the election is complete the node that wins remains the master and the other will become the replica. The node that loses the election and becomes the replica tries to recover itself by restarting automatically if required if it had any write transactions that need to be rolled back to synchronize its database with the new master. Also, if the partition exists for more than 24 hours, Oracle recommends that you to take the cold backup from newly elected master and us it to restore the replica to avoid any data discrepancies.
Rejoining a cluster after graceful shutdown	An election can be won with a single vote. This allows the replica to be elected master in the case the master fails.
Rejoining cluster after shutdown for extended period	Perform a hot backup on the host running the master database before restarting a server that has been down for a long time. This avoids the potentially high cost synchronizing the server with the master during startup.