Both Multi-AZ deployments and Read Replicas are features of Amazon RDS that involve creating additional copies of your database. However, they serve different purposes and are used in different scenarios:

Multi-AZ Deployments (High Availability):

• Purpose: Primarily used for high availability and disaster recovery.
• How it Works: A primary database instance is created in one Availability Zone (AZ), and a synchronous standby replica is created in another AZ within the same region. Data is replicated synchronously from the primary to the standby. If the primary instance fails, RDS automatically fails over to the standby with minimal downtime.
• Use Case: Critical applications that require high availability and cannot tolerate extended downtime.
• Limitations: Does not scale read capacity, as the standby replica is not used for read traffic.

Read Replicas (Read Scalability):

• Purpose: Primarily used for read scalability and offloading read traffic from the primary database instance.
• How it Works: One or more read-only copies of your database are created. Data is replicated asynchronously from the primary to the replicas. You can direct read traffic to the replicas, reducing the load on the primary instance.
• Use Case: Applications with read-heavy workloads where you need to scale read capacity.
• Limitations: Replicas are not used for automatic failover in case of primary instance failure.

Which to Choose?

• If your primary concern is high availability and disaster recovery, then choose Multi-AZ deployments.
• If your primary concern is read scalability and offloading read traffic, then choose Read Replicas.

Can You Use Both?

Yes, you can combine both Multi-AZ deployments and Read Replicas to achieve both high availability and read scalability. This provides a robust solution for critical applications with demanding read workloads. For example, you could have a Multi-AZ deployment for the primary database and then create Read Replicas across different regions for disaster recovery and read scalability.

Important Considerations:

• Cost: Both Multi-AZ deployments and Read Replicas incur additional costs.
• Replication Lag: Read Replicas may have a slight replication lag, so they may not always have the most up-to-date data.
• Cross-Region Replication: Read Replicas can be created in different regions for disaster recovery, but this incurs additional data transfer costs.