External Reference Entity

To prepare for failover situations, you must configure an External Reference Entity (ERE) for the primary and secondary master nodes.

When the communication link between the primary and secondary master nodes fails, both database servers might mistakenly assume that the other one is down. As a result, a dual primary situation can arise and you might lose transactions when databases are later synchronized. To avoid this situation, you can use a network reference device, such as a network router, as an ERE to check the health of the network.

If you configure a secondary master, you must also configure an ERE for the primary and secondary masters. If the primary master loses its connection to the secondary master, it can contact the ERE to determine whether there is a network fault or the secondary master is down.

In a distributed configuration, you can separate the primary and secondary masters into one data center and the tertiary and quaternary masters into another data center. If the data center link fails, the primary and tertiary masters operate in parallel and service requests in their local networks. When the data center link is restored, the tertiary master becomes inactive and reconciles its updates with the primary master.