AWS: Comparing Multi-AZ Deployments & Read Replicas

From the AWS Documentation -

Read replicas are available in Amazon RDS for MySQL, MariaDB, and PostgreSQL as well as Amazon Aurora.

Read replicas in Amazon RDS for MySQL, MariaDB, and PostgreSQL provide a complementary availability mechanism to Amazon RDS Multi-AZ Deployments. 

You can combine Multi-AZ deployments and read replicas to enjoy the benefits of each. For example, you can configure a source database as Multi-AZ for high availability and create a read replica (in Single-AZ) for read scalability.

Multi-AZ Deployments	Read Replicas
Synchronous replication – highly durable	Asynchronous replication – highly scalable
Only database engine on primary instance is active	All read replicas are accessible and can be used for read scaling
Automated backups are taken from standby	No backups configured by default
Always span two Availability Zones within a single Region	Can be within an Availability Zone, Cross-AZ, or Cross-Region
Database engine version upgrades happen on primary	Database engine version upgrade is independent from source instance
Automatic failover to standby when a problem is detected	Can be manually promoted to a standalone database instance