Amazon Aurora now allows you to assign a promotion precedence tier to each instance on your database cluster. The precedence tiers give you more control over replica promotion during failovers. When the primary instance fails,Amazon RDS will promote the replica with the highest precedence to primary. You can assign lower precedence tiers to replicas that you don’t want promoted to the primary instance. For more information on promotion tiers and failover logic, read the Amazon Aurora User Guide. To know more about Amazon Aurora, or visit our product page.
Source: amazon.com