RDS Multi-AZ and Read Replicas

Production databases need to survive hardware failures and handle growing read traffic. Amazon RDS provides two complementary features to address these requirements: Multi-AZ deployments for high availability and read replicas for horizontal read scalability.

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Multi-AZ Deployments

How It Works

When you enable Multi-AZ, RDS automatically provisions and maintains a synchronous standby replica in a different Availability Zone within the same Region.

1. Every write to the primary is synchronously replicated to the standby before RDS acknowledges the commit to your application
2. The standby is not accessible for read traffic — it exists solely for failover
3. If the primary fails, RDS performs an automatic failover to the standby, typically completing within 60–120 seconds

What Triggers a Failover?

Event	Automatic Failover?
Primary AZ outage	Yes
Primary host failure	Yes
Primary instance unresponsive	Yes
Storage failure on primary	Yes
Manual reboot with failover	Yes (you initiate)
DB engine patching	Yes (if maintenance window triggers reboot)
Scaling instance class	Brief downtime, not a failover