VCS global clustering monitors and manages the replication jobs and clusters at each site. In the event of a site outage, global clustering controls the shift of replication roles to the Secondary site, bring up the critical applications and redirects client traffic from one cluster to the other.
Before Release 5.1SP1, if there was a disaster at the Primary site or a network disruption, the applications were taken offline on the original Primary and failed over to the Secondary. When the original Primary returned or the network disruption was corrected, you had the following options:
Manually resynchronize the original Primary with the data from the new Primary, once the original Primary comes back up. The applications are only active on the new Primary site.
Automatically resynchronize the original Primary with the data from the new Primary, once the original Primary comes back up. The applications are only active on the new Primary site.
Beginning in Release 5.1SP1, you have a third option. Applications can be active on both the original Primary and Secondary sites. After the original Primary returns or the network disruption is corrected, you have the option of specifying which site is the Primary going forward. This option is called the primary-elect feature, and it is enabled through the VCS global clustering.
The key difference between the primary-elect feature and the other options is that if a network disruption occurs, applications continue to run on the Primary site and they are also failed over to the Secondary. This feature lets you maintain application availability on both sites while the network is down.
See Overview of how to configure the primary-elect feature in a VCS globally clustered environment. for detailed information on configuring and using the primary-elect feature.