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The Failover Process for Amazon RDS

In the event of a planned or unplanned outage of your DB instance, Amazon RDS automatically switches to a standby replica in another Availability Zone if you have enabled Multi-AZ. The automatic failover mechanism simply changes the DNS record of the DB instance to point to the standby DB instance. As a result, you will need to re-establish any existing connections to your DB instance.
High availability does not mean that Amazon keeps, for example, two databases running in parallel; it is the data on disk that is replicated. If the primary DB instance becomes unavailable, a failover begins and the database software is started on the standby replica. The time it takes for the failover to complete depends on the database activity and other conditions at the time the primary DB instance became unavailable. When the failover is complete, it can take additional time for the RDS console UI to reflect the new Availability Zone.
Amazon RDS handles failovers automatically so you can resume database operations as quickly as possible without administrative intervention. The primary DB instance switches over automatically to the standby replica if any of the following conditions occur:
  • An Availability Zone outage
  • The primary DB instance fails
  • The DB instance's server type is changed
  • The DB instance is undergoing software patching
  • A manual failover of the DB instance was initiated using Reboot with failover
There are several ways to determine if your Multi-AZ DB instance has failed over:
  • DB event subscriptions can be setup to notify you via email or SMS that a failover has been initiated. For more information about events, see Using Amazon RDS Event Notification
  • You can view your DB events via the Amazon RDS console or APIs.
  • You can view the current state of your Multi-AZ deployment via the Amazon RDS console and APIs.

    source: http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/Concepts.MultiAZ.html
     
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